DASH(1) General Commands Manual DASH(1)
NAME
dash — command interpreter (shell)
SYNOPSIS
dash [-aCefnuvxIimqVEbp] [+aCefnuvxIimqVEbp] [-o option_name]
[+o option_name] [command_file [argument ...]]
dash -c [-aCefnuvxIimqVEbp] [+aCefnuvxIimqVEbp] [-o option_name]
[+o option_name] command_string [command_name [argument ...]]
dash -s [-aCefnuvxIimqVEbp] [+aCefnuvxIimqVEbp] [-o option_name]
[+o option_name] [argument ...]
DESCRIPTION
dash is the standard command interpreter for the system. The current
version of dash is in the process of being changed to conform with the
POSIX 1003.2 and 1003.2a specifications for the shell. This version has
many features which make it appear similar in some respects to the Korn
shell, but it is not a Korn shell clone (see ksh(1)). Only features
designated by POSIX, plus a few Berkeley extensions, are being incorpo-
rated into this shell. This man page is not intended to be a tutorial
or a complete specification of the shell.
Overview
The shell is a command that reads lines from either a file or the termi-
nal, interprets them, and generally executes other commands. It is the
program that is running when a user logs into the system (although a
user can select a different shell with the chsh(1) command). The shell
implements a language that has flow control constructs, a macro facility
that provides a variety of features in addition to data storage, along
with built in history and line editing capabilities. It incorporates
many features to aid interactive use and has the advantage that the in-
terpretative language is common to both interactive and non-interactive
use (shell scripts). That is, commands can be typed directly to the
running shell or can be put into a file and the file can be executed di-
rectly by the shell.
Invocation
If no args are present and if the standard input of the shell is con-
nected to a terminal (or if the -i flag is set), and the -c option is
not present, the shell is considered an interactive shell. An interac-
tive shell generally prompts before each command and handles programming
and command errors differently (as described below). When first start-
ing, the shell inspects argument 0, and if it begins with a dash ‘-’,
the shell is also considered a login shell. This is normally done auto-
matically by the system when the user first logs in. A login shell
first reads commands from the files /etc/profile and .profile if they
exist. If the environment variable ENV is set on entry to an interac-
tive shell, or is set in the .profile of a login shell, the shell next
reads commands from the file named in ENV. Therefore, a user should
place commands that are to be executed only at login time in the
.profile file, and commands that are executed for every interactive
shell inside the ENV file. To set the ENV variable to some file, place
the following line in your .profile of your home directory
ENV=$HOME/.shinit; export ENV
substituting for “.shinit” any filename you wish.
If command line arguments besides the options have been specified, then
the shell treats the first argument as the name of a file from which to
read commands (a shell script), and the remaining arguments are set as
the positional parameters of the shell ($1, $2, etc). Otherwise, the
shell reads commands from its standard input.
Argument List Processing
All of the single letter options that have a corresponding name can be
used as an argument to the -o option. The set -o name is provided next
to the single letter option in the description below. Specifying a dash
“-” turns the option on, while using a plus “+” disables the option.
The following options can be set from the command line or with the set
builtin (described later).
-a allexport Export all variables assigned to.
-c Read commands from the command_string operand in-
stead of from the standard input. Special para-
meter 0 will be set from the command_name operand
and the positional parameters ($1, $2, etc.) set
from the remaining argument operands.
-C noclobber Don't overwrite existing files with “>”.
-e errexit If not interactive, exit immediately if any
untested command fails. The exit status of a
command is considered to be explicitly tested if
the command is used to control an if, elif,
while, or until; or if the command is the left
hand operand of an “&&” or “||” operator.
-f noglob Disable pathname expansion.
-n noexec If not interactive, read commands but do not exe-
cute them. This is useful for checking the syn-
tax of shell scripts.
-u nounset Write a message to standard error when attempting
to expand a variable that is not set, and if the
shell is not interactive, exit immediately.
-v verbose The shell writes its input to standard error as
it is read. Useful for debugging.
-x xtrace Write each command to standard error (preceded by
a ‘+ ’) before it is executed. Useful for debug-
ging.
-I ignoreeof Ignore EOF's from input when interactive.
-i interactive Force the shell to behave interactively.
-l Make dash act as if it had been invoked as a lo-
gin shell.
-m monitor Turn on job control (set automatically when in-
teractive).
-s stdin Read commands from standard input (set automati-
cally if no file arguments are present). This
option has no effect when set after the shell has
already started running (i.e. with set).
-V vi Enable the built-in vi(1) command line editor
(disables -E if it has been set).
-E emacs Enable the built-in emacs(1) command line editor
(disables -V if it has been set).
-b notify Enable asynchronous notification of background
job completion. (UNIMPLEMENTED for 4.4alpha)
-p priviliged Do not attempt to reset effective uid if it does
not match uid. This is not set by default to help
avoid incorrect usage by setuid root programs via
system(3) or popen(3).
Lexical Structure
The shell reads input in terms of lines from a file and breaks it up
into words at whitespace (blanks and tabs), and at certain sequences of
characters that are special to the shell called “operators”. There are
two types of operators: control operators and redirection operators
(their meaning is discussed later). Following is a list of operators:
Control operators:
& && ( ) ; ;; | || <newline>
Redirection operators:
< > >| << >> <& >& <<- <>
Quoting
Quoting is used to remove the special meaning of certain characters or
words to the shell, such as operators, whitespace, or keywords. There
are three types of quoting: matched single quotes, matched double
quotes, and backslash.
Backslash
A backslash preserves the literal meaning of the following character,
with the exception of ⟨newline⟩. A backslash preceding a ⟨newline⟩ is
treated as a line continuation.
Single Quotes
Enclosing characters in single quotes preserves the literal meaning of
all the characters (except single quotes, making it impossible to put
single-quotes in a single-quoted string).
Double Quotes
Enclosing characters within double quotes preserves the literal meaning
of all characters except dollarsign ($), backquote (`), and backslash
(\). The backslash inside double quotes is historically weird, and
serves to quote only the following characters:
$ ` " \ <newline>.
Otherwise it remains literal.
Reserved Words
Reserved words are words that have special meaning to the shell and are
recognized at the beginning of a line and after a control operator. The
following are reserved words:
! elif fi while case
else for then { }
do done until if esac
Their meaning is discussed later.
Aliases
An alias is a name and corresponding value set using the alias(1)
builtin command. Whenever a reserved word may occur (see above), and
after checking for reserved words, the shell checks the word to see if
it matches an alias. If it does, it replaces it in the input stream
with its value. For example, if there is an alias called “lf” with the
value “ls -F”, then the input:
lf foobar ⟨return⟩
would become
ls -F foobar ⟨return⟩
Aliases provide a convenient way for naive users to create shorthands
for commands without having to learn how to create functions with argu-
ments. They can also be used to create lexically obscure code. This
use is discouraged.
Commands
The shell interprets the words it reads according to a language, the
specification of which is outside the scope of this man page (refer to
the BNF in the POSIX 1003.2 document). Essentially though, a line is
read and if the first word of the line (or after a control operator) is
not a reserved word, then the shell has recognized a simple command.
Otherwise, a complex command or some other special construct may have
been recognized.
Simple Commands
If a simple command has been recognized, the shell performs the follow-
ing actions:
1. Leading words of the form “name=value” are stripped off and
assigned to the environment of the simple command. Redirec-
tion operators and their arguments (as described below) are
stripped off and saved for processing.
2. The remaining words are expanded as described in the section
called “Expansions”, and the first remaining word is consid-
ered the command name and the command is located. The re-
maining words are considered the arguments of the command.
If no command name resulted, then the “name=value” variable
assignments recognized in item 1 affect the current shell.
3. Redirections are performed as described in the next section.
Redirections
Redirections are used to change where a command reads its input or sends
its output. In general, redirections open, close, or duplicate an ex-
isting reference to a file. The overall format used for redirection is:
[n] redir-op file
where redir-op is one of the redirection operators mentioned previously.
Following is a list of the possible redirections. The [n] is an op-
tional number between 0 and 9, as in ‘3’ (not ‘[3]’), that refers to a
file descriptor.
[n]> file Redirect standard output (or n) to file.
[n]>| file Same, but override the -C option.
[n]>> file Append standard output (or n) to file.
[n]< file Redirect standard input (or n) from file.
[n1]<&n2 Copy file descriptor n2 as stdout (or fd n1). fd n2.
[n]<&- Close standard input (or n).
[n1]>&n2 Copy file descriptor n2 as stdin (or fd n1). fd n2.
[n]>&- Close standard output (or n).
[n]<> file Open file for reading and writing on standard input
(or n).
The following redirection is often called a “here-document”.
[n]<< delimiter
here-doc-text ...
delimiter
All the text on successive lines up to the delimiter is saved away and
made available to the command on standard input, or file descriptor n if
it is specified. If the delimiter as specified on the initial line is
quoted, then the here-doc-text is treated literally, otherwise the text
is subjected to parameter expansion, command substitution, and arith-
metic expansion (as described in the section on “Expansions”). If the
operator is “<<-” instead of “<<”, then leading tabs in the here-doc-
text are stripped.
Search and Execution
There are three types of commands: shell functions, builtin commands,
and normal programs – and the command is searched for (by name) in that
order. They each are executed in a different way.
When a shell function is executed, all of the shell positional parame-
ters (except $0, which remains unchanged) are set to the arguments of
the shell function. The variables which are explicitly placed in the
environment of the command (by placing assignments to them before the
function name) are made local to the function and are set to the values
given. Then the command given in the function definition is executed.
The positional parameters are restored to their original values when the
command completes. This all occurs within the current shell.
Shell builtins are executed internally to the shell, without spawning a
new process.
Otherwise, if the command name doesn't match a function or builtin, the
command is searched for as a normal program in the file system (as de-
scribed in the next section). When a normal program is executed, the
shell runs the program, passing the arguments and the environment to the
program. If the program is not a normal executable file (i.e., if it
does not begin with the "magic number" whose ASCII representation is
"#!", so execve(2) returns ENOEXEC then) the shell will interpret the
program in a subshell. The child shell will reinitialize itself in this
case, so that the effect will be as if a new shell had been invoked to
handle the ad-hoc shell script, except that the location of hashed com-
mands located in the parent shell will be remembered by the child.
Note that previous versions of this document and the source code itself
misleadingly and sporadically refer to a shell script without a magic
number as a "shell procedure".
Path Search
When locating a command, the shell first looks to see if it has a shell
function by that name. Then it looks for a builtin command by that
name. If a builtin command is not found, one of two things happen:
1. Command names containing a slash are simply executed without per-
forming any searches.
2. The shell searches each entry in PATH in turn for the command. The
value of the PATH variable should be a series of entries separated
by colons. Each entry consists of a directory name. The current
directory may be indicated implicitly by an empty directory name,
or explicitly by a single period.
Command Exit Status
Each command has an exit status that can influence the behaviour of
other shell commands. The paradigm is that a command exits with zero
for normal or success, and non-zero for failure, error, or a false indi-
cation. The man page for each command should indicate the various exit
codes and what they mean. Additionally, the builtin commands return
exit codes, as does an executed shell function.
If a command consists entirely of variable assignments then the exit
status of the command is that of the last command substitution if any,
otherwise 0.
Complex Commands
Complex commands are combinations of simple commands with control opera-
tors or reserved words, together creating a larger complex command.
More generally, a command is one of the following:
• simple command
• pipeline
• list or compound-list
• compound command
• function definition
Unless otherwise stated, the exit status of a command is that of the
last simple command executed by the command.
Pipelines
A pipeline is a sequence of one or more commands separated by the con-
trol operator |. The standard output of all but the last command is
connected to the standard input of the next command. The standard out-
put of the last command is inherited from the shell, as usual.
The format for a pipeline is:
[!] command1 [| command2 ...]
The standard output of command1 is connected to the standard input of
command2. The standard input, standard output, or both of a command is
considered to be assigned by the pipeline before any redirection speci-
fied by redirection operators that are part of the command.
If the pipeline is not in the background (discussed later), the shell
waits for all commands to complete.
If the pipefail option was enabled when the shell began execution of the
pipeline, the pipeline's exit status is the exit status of the last com-
mand specified in the pipeline that exited with non-zero status, or zero
if all commands in the pipeline exited with a status of zero. If the
pipefail option was not enabled, the pipeline's exit status is the exit
status of the last command specified in the pipeline; the exit statuses
of any other commands are not used. If the reserved word ! precedes the
pipeline, its exit status is the logical NOT of the exit status de-
scribed above.
Because pipeline assignment of standard input or standard output or both
takes place before redirection, it can be modified by redirection. For
example:
$ command1 2>&1 | command2
sends both the standard output and standard error of command1 to the
standard input of command2.
A ; or ⟨newline⟩ terminator causes the preceding AND-OR-list (described
next) to be executed sequentially; a & causes asynchronous execution of
the preceding AND-OR-list.
Note that unlike some other shells, each process in the pipeline is a
child of the invoking shell (unless it is a shell builtin, in which case
it executes in the current shell – but any effect it has on the environ-
ment is wiped).
Background Commands – &
If a command is terminated by the control operator ampersand (&), the
shell executes the command asynchronously – that is, the shell does not
wait for the command to finish before executing the next command.
The format for running a command in background is:
command1 & [command2 & ...]
If the shell is not interactive, the standard input of an asynchronous
command is set to /dev/null.
Lists – Generally Speaking
A list is a sequence of zero or more commands separated by newlines,
semicolons, or ampersands, and optionally terminated by one of these
three characters. The commands in a list are executed in the order they
are written. If command is followed by an ampersand, the shell starts
the command and immediately proceeds onto the next command; otherwise it
waits for the command to terminate before proceeding to the next one.
Short-Circuit List Operators
“&&” and “||” are AND-OR list operators. “&&” executes the first com-
mand, and then executes the second command if and only if the exit sta-
tus of the first command is zero. “||” is similar, but executes the
second command if and only if the exit status of the first command is
nonzero. “&&” and “||” both have the same priority.
Flow-Control Constructs – if, while, for, case
The syntax of the if command is
if list
then list
[ elif list
then list ] ...
[ else list ]
fi
The syntax of the while command is
while list
do list
done
The two lists are executed repeatedly while the exit status of the first
list is zero. The until command is similar, but has the word until in
place of while, which causes it to repeat until the exit status of the
first list is zero.
The syntax of the for command is
for variable [ in [ word ... ] ]
do list
done
The words following in are expanded, and then the list is executed re-
peatedly with the variable set to each word in turn. Omitting in word
... is equivalent to in "$@".
The syntax of the break and continue command is
break [ num ]
continue [ num ]
Break terminates the num innermost for or while loops. Continue contin-
ues with the next iteration of the innermost loop. These are imple-
mented as builtin commands.
The syntax of the case command is
case word in
[(]pattern) list ;;
...
esac
The pattern can actually be one or more patterns (see “Shell Patterns”
described later), separated by “|” characters. The “(” character before
the pattern is optional.
Grouping Commands Together
Commands may be grouped by writing either
(list)
or
{ list; }
The first of these executes the commands in a subshell. Builtin com-
mands grouped into a (list) will not affect the current shell. The sec-
ond form does not fork another shell so is slightly more efficient.
Grouping commands together this way allows you to redirect their output
as though they were one program:
{ printf " hello " ; printf " world\n" ; } > greeting
Note that “}” must follow a control operator (here, “;”) so that it is
recognized as a reserved word and not as another command argument.
Functions
The syntax of a function definition is
name () command
A function definition is an executable statement; when executed it in-
stalls a function named name and returns an exit status of zero. The
command is normally a list enclosed between “{” and “}”.
Variables may be declared to be local to a function by using a local
command. This should appear as the first statement of a function, and
the syntax is
local [variable | -] ...
Local is implemented as a builtin command.
When a variable is made local, it inherits the initial value and ex-
ported and readonly flags from the variable with the same name in the
surrounding scope, if there is one. Otherwise, the variable is ini-
tially unset. The shell uses dynamic scoping, so that if you make the
variable x local to function f, which then calls function g, references
to the variable x made inside g will refer to the variable x declared
inside f, not to the global variable named x.
The only special parameter that can be made local is “-”. Making “-”
local any shell options that are changed via the set command inside the
function to be restored to their original values when the function re-
turns.
The syntax of the return command is
return [exitstatus]
It terminates the currently executing function. Return is implemented
as a builtin command.
Variables and Parameters
The shell maintains a set of parameters. A parameter denoted by a name
is called a variable. When starting up, the shell turns all the envi-
ronment variables into shell variables. New variables can be set using
the form
name=value
Variables set by the user must have a name consisting solely of alpha-
betics, numerics, and underscores - the first of which must not be nu-
meric. A parameter can also be denoted by a number or a special charac-
ter as explained below.
Positional Parameters
A positional parameter is a parameter denoted by a number (n > 0). The
shell sets these initially to the values of its command line arguments
that follow the name of the shell script. The set builtin can also be
used to set or reset them.
Special Parameters
A special parameter is a parameter denoted by one of the following spe-
cial characters. The value of the parameter is listed next to its char-
acter.
* Expands to the positional parameters, starting from one.
When the expansion occurs within a double-quoted string it
expands to a single field with the value of each parameter
separated by the first character of the IFS variable, or by
a ⟨space⟩ if IFS is unset.
@ Expands to the positional parameters, starting from one.
When the expansion occurs within double-quotes, each posi-
tional parameter expands as a separate argument. If there
are no positional parameters, the expansion of @ generates
zero arguments, even when @ is double-quoted. What this
basically means, for example, is if $1 is “abc” and $2 is
“def ghi”, then "$@" expands to the two arguments:
"abc" "def ghi"
# Expands to the number of positional parameters.
? Expands to the exit status of the most recent pipeline.
- (Hyphen.) Expands to the current option flags (the single-letter op-
tion names concatenated into a string) as specified on in-
vocation, by the set builtin command, or implicitly by the
shell.
$ Expands to the process ID of the invoked shell. A subshell
retains the same value of $ as its parent.
! Expands to the process ID of the most recent background
command executed from the current shell. For a pipeline,
the process ID is that of the last command in the pipeline.
0 (Zero.) Expands to the name of the shell or shell script.
Word Expansions
This clause describes the various expansions that are performed on
words. Not all expansions are performed on every word, as explained
later.
Tilde expansions, parameter expansions, command substitutions, arith-
metic expansions, and quote removals that occur within a single word ex-
pand to a single field. It is only field splitting or pathname expan-
sion that can create multiple fields from a single word. The single ex-
ception to this rule is the expansion of the special parameter @ within
double-quotes, as was described above.
The order of word expansion is:
1. Tilde Expansion, Parameter Expansion, Command Substitution, Arith-
metic Expansion (these all occur at the same time).
2. Field Splitting is performed on fields generated by step (1) unless
the IFS variable is null.
3. Pathname Expansion (unless set -f is in effect).
4. Quote Removal.
The $ character is used to introduce parameter expansion, command sub-
stitution, or arithmetic evaluation.
Tilde Expansion (substituting a user's home directory)
A word beginning with an unquoted tilde character (~) is subjected to
tilde expansion. All the characters up to a slash (/) or the end of the
word are treated as a username and are replaced with the user's home di-
rectory. If the username is missing (as in ~/foobar), the tilde is re-
placed with the value of the HOME variable (the current user's home di-
rectory).
Parameter Expansion
The format for parameter expansion is as follows:
${expression}
where expression consists of all characters until the matching “}”. Any
“}” escaped by a backslash or within a quoted string, and characters in
embedded arithmetic expansions, command substitutions, and variable ex-
pansions, are not examined in determining the matching “}”.
The simplest form for parameter expansion is:
${parameter}
The value, if any, of parameter is substituted.
The parameter name or symbol can be enclosed in braces, which are op-
tional except for positional parameters with more than one digit or when
parameter is followed by a character that could be interpreted as part
of the name. If a parameter expansion occurs inside double-quotes:
1. Pathname expansion is not performed on the results of the expan-
sion.
2. Field splitting is not performed on the results of the expansion,
with the exception of @.
In addition, a parameter expansion can be modified by using one of the
following formats.
${parameter:-word} Use Default Values. If parameter is unset or
null, the expansion of word is substituted; other-
wise, the value of parameter is substituted.
${parameter:=word} Assign Default Values. If parameter is unset or
null, the expansion of word is assigned to parame-
ter. In all cases, the final value of parameter
is substituted. Only variables, not positional
parameters or special parameters, can be assigned
in this way.
${parameter:?[word]} Indicate Error if Null or Unset. If parameter is
unset or null, the expansion of word (or a message
indicating it is unset if word is omitted) is
written to standard error and the shell exits with
a nonzero exit status. Otherwise, the value of
parameter is substituted. An interactive shell
need not exit.
${parameter:+word} Use Alternative Value. If parameter is unset or
null, null is substituted; otherwise, the expan-
sion of word is substituted.
In the parameter expansions shown previously, use of the colon in the
format results in a test for a parameter that is unset or null; omission
of the colon results in a test for a parameter that is only unset.
${#parameter} String Length. The length in characters of the
value of parameter.
The following four varieties of parameter expansion provide for sub-
string processing. In each case, pattern matching notation (see “Shell
Patterns”), rather than regular expression notation, is used to evaluate
the patterns. If parameter is * or @, the result of the expansion is
unspecified. Enclosing the full parameter expansion string in double-
quotes does not cause the following four varieties of pattern characters
to be quoted, whereas quoting characters within the braces has this ef-
fect.
${parameter%word} Remove Smallest Suffix Pattern. The word is ex-
panded to produce a pattern. The parameter expan-
sion then results in parameter, with the smallest
portion of the suffix matched by the pattern
deleted.
${parameter%%word} Remove Largest Suffix Pattern. The word is ex-
panded to produce a pattern. The parameter expan-
sion then results in parameter, with the largest
portion of the suffix matched by the pattern
deleted.
${parameter#word} Remove Smallest Prefix Pattern. The word is ex-
panded to produce a pattern. The parameter expan-
sion then results in parameter, with the smallest
portion of the prefix matched by the pattern
deleted.
${parameter##word} Remove Largest Prefix Pattern. The word is ex-
panded to produce a pattern. The parameter expan-
sion then results in parameter, with the largest
portion of the prefix matched by the pattern
deleted.
Command Substitution
Command substitution allows the output of a command to be substituted in
place of the command name itself. Command substitution occurs when the
command is enclosed as follows:
$(command)
or (“backquoted” version):
`command`
The shell expands the command substitution by executing command in a
subshell environment and replacing the command substitution with the
standard output of the command, removing sequences of one or more
⟨newline⟩s at the end of the substitution. (Embedded ⟨newline⟩s before
the end of the output are not removed; however, during field splitting,
they may be translated into ⟨space⟩s, depending on the value of IFS and
quoting that is in effect.)
Arithmetic Expansion
Arithmetic expansion provides a mechanism for evaluating an arithmetic
expression and substituting its value. The format for arithmetic expan-
sion is as follows:
$((expression))
The expression is treated as if it were in double-quotes, except that a
double-quote inside the expression is not treated specially. The shell
expands all tokens in the expression for parameter expansion, command
substitution, and quote removal.
Next, the shell treats this as an arithmetic expression and substitutes
the value of the expression.
White Space Splitting (Field Splitting)
After parameter expansion, command substitution, and arithmetic expan-
sion the shell scans the results of expansions and substitutions that
did not occur in double-quotes for field splitting and multiple fields
can result.
The shell treats each character of the IFS as a delimiter and uses the
delimiters to split the results of parameter expansion and command sub-
stitution into fields.
Pathname Expansion (File Name Generation)
Unless the -f flag is set, file name generation is performed after word
splitting is complete. Each word is viewed as a series of patterns,
separated by slashes. The process of expansion replaces the word with
the names of all existing files whose names can be formed by replacing
each pattern with a string that matches the specified pattern. There
are two restrictions on this: first, a pattern cannot match a string
containing a slash, and second, a pattern cannot match a string starting
with a period unless the first character of the pattern is a period.
The next section describes the patterns used for both Pathname Expansion
and the case command.
Shell Patterns
A pattern consists of normal characters, which match themselves, and
meta-characters. The meta-characters are “!”, “*”, “?”, and “[”. These
characters lose their special meanings if they are quoted. When command
or variable substitution is performed and the dollar sign or back quotes
are not double quoted, the value of the variable or the output of the
command is scanned for these characters and they are turned into meta-
characters.
An asterisk (“*”) matches any string of characters. A question mark
matches any single character. A left bracket (“[”) introduces a charac-
ter class. The end of the character class is indicated by a (“]”); if
the “]” is missing then the “[” matches a “[” rather than introducing a
character class. A character class matches any of the characters be-
tween the square brackets. A range of characters may be specified using
a minus sign. The character class may be complemented by making an ex-
clamation point the first character of the character class.
To include a “]” in a character class, make it the first character
listed (after the “!”, if any). To include a minus sign, make it the
first or last character listed.
Builtins
This section lists the builtin commands which are builtin because they
need to perform some operation that can't be performed by a separate
process. In addition to these, there are several other commands that
may be builtin for efficiency (e.g. printf(1), echo(1), test(1), etc).
:
true A null command that returns a 0 (true) exit value.
false A null command that returns a 1 (false) exit value.
. file
The commands in the specified file are read and executed by the
shell.
alias [name[=string ...]]
If name=string is specified, the shell defines the alias name
with value string. If just name is specified, the value of the
alias name is printed. With no arguments, the alias builtin
prints the names and values of all defined aliases (see unalias).
bg [job] ...
Continue the specified jobs (or the current job if no jobs are
given) in the background.
command [-p] [-v] [-V] command [arg ...]
Execute the specified command but ignore shell functions when
searching for it. (This is useful when you have a shell function
with the same name as a builtin command.)
-p search for command using a PATH that guarantees to find
all the standard utilities.
-V Do not execute the command but search for the command and
print the resolution of the command search. This is the
same as the type builtin.
-v Do not execute the command but search for the command and
print the absolute pathname of utilities, the name for
builtins or the expansion of aliases.
cd|chdir -
cd|chdir [-LP] [directory]
Switch to the specified directory (default HOME). If an entry
for CDPATH appears in the environment of the cd command or the
shell variable CDPATH is set and the directory name does not be-
gin with a slash, then the directories listed in CDPATH will be
searched for the specified directory. The format of CDPATH is
the same as that of PATH. If a single dash is specified as the
argument, it will be replaced by the value of OLDPWD. The cd
command will print out the name of the directory that it actually
switched to if this is different from the name that the user
gave. These may be different either because the CDPATH mechanism
was used or because the argument is a single dash. The -P option
causes the physical directory structure to be used, that is, all
symbolic links are resolved to their respective values. The -L
option turns off the effect of any preceding -P options.
echo [-n] args...
Print the arguments on the standard output, separated by spaces.
Unless the -n option is present, a newline is output following
the arguments.
If any of the following sequences of characters is encountered
during output, the sequence is not output. Instead, the speci-
fied action is performed:
\b A backspace character is output.
\c Subsequent output is suppressed. This is normally used
at the end of the last argument to suppress the trailing
newline that echo would otherwise output.
\e Outputs an escape character (ESC).
\f Output a form feed.
\n Output a newline character.
\r Output a carriage return.
\t Output a (horizontal) tab character.
\v Output a vertical tab.
\0digits
Output the character whose value is given by zero to
three octal digits. If there are zero digits, a nul
character is output.
\\ Output a backslash.
All other backslash sequences elicit undefined behaviour.
eval string ...
Concatenate all the arguments with spaces. Then re-parse and ex-
ecute the command.
exec [command arg ...]
Unless command is omitted, the shell process is replaced with the
specified program (which must be a real program, not a shell
builtin or function). Any redirections on the exec command are
marked as permanent, so that they are not undone when the exec
command finishes.
exit [exitstatus]
Terminate the shell process. If exitstatus is given it is used
as the exit status of the shell; otherwise the exit status of the
preceding command is used.
export name ...
export -p
The specified names are exported so that they will appear in the
environment of subsequent commands. The only way to un-export a
variable is to unset it. The shell allows the value of a vari-
able to be set at the same time it is exported by writing
export name=value
With no arguments the export command lists the names of all ex-
ported variables. With the -p option specified the output will
be formatted suitably for non-interactive use.
fc [-e editor] [first [last]]
fc -l [-nr] [first [last]]
fc -s [old=new] [first]
The fc builtin lists, or edits and re-executes, commands previ-
ously entered to an interactive shell.
-e editor
Use the editor named by editor to edit the commands. The
editor string is a command name, subject to search via the
PATH variable. The value in the FCEDIT variable is used
as a default when -e is not specified. If FCEDIT is null
or unset, the value of the EDITOR variable is used. If
EDITOR is null or unset, ed(1) is used as the editor.
-l (ell)
List the commands rather than invoking an editor on them.
The commands are written in the sequence indicated by the
first and last operands, as affected by -r, with each com-
mand preceded by the command number.
-n Suppress command numbers when listing with -l.
-r Reverse the order of the commands listed (with -l) or
edited (with neither -l nor -s).
-s Re-execute the command without invoking an editor.
first
last Select the commands to list or edit. The number of previ-
ous commands that can be accessed are determined by the
value of the HISTSIZE variable. The value of first or
last or both are one of the following:
[+]number
A positive number representing a command number;
command numbers can be displayed with the -l op-
tion.
-number
A negative decimal number representing the command
that was executed number of commands previously.
For example, -1 is the immediately previous com-
mand.
string
A string indicating the most recently entered command that
begins with that string. If the old=new operand is not
also specified with -s, the string form of the first
operand cannot contain an embedded equal sign.
The following environment variables affect the execution of fc:
FCEDIT Name of the editor to use.
HISTSIZE The number of previous commands that are accessible.
fg [job]
Move the specified job or the current job to the foreground.
getopts optstring var [arg ...]
The POSIX getopts command, not to be confused with the Bell
Labs-derived getopt(1).
The first argument should be a series of letters, each of which
may be optionally followed by a colon to indicate that the option
requires an argument. The variable specified is set to the
parsed option.
The getopts command deprecates the older getopt(1) utility due to
its handling of arguments containing whitespace.
The getopts builtin may be used to obtain options and their argu-
ments from a list of parameters. When invoked, getopts places
the value of the next option from the option string in the list
in the shell variable specified by var and its index in the shell
variable OPTIND. When the shell is invoked, OPTIND is initial-
ized to 1. For each option that requires an argument, the
getopts builtin will place it in the shell variable OPTARG. If
an option is not allowed for in the optstring, then OPTARG will
be unset.
By default, the variables $1, ..., $n are inspected; if args are
specified, they'll be parsed instead.
optstring is a string of recognized option letters (see
getopt(3)). If a letter is followed by a colon, the option is
expected to have an argument which may or may not be separated
from it by white space. If an option character is not found
where expected, getopts will set the variable var to a “?”;
getopts will then unset OPTARG and write output to standard er-
ror. By specifying a colon as the first character of optstring
all errors will be ignored.
After the last option getopts will return a non-zero value and
set var to “?”.
The following code fragment shows how one might process the argu-
ments for a command that can take the options [a] and [b], and
the option [c], which requires an argument.
while getopts abc: f
do
case $f in
a | b) flag=$f;;
c) carg=$OPTARG;;
\?) echo $USAGE; exit 1;;
esac
done
shift $((OPTIND - 1))
This code will accept any of the following as equivalent:
cmd -acarg file file
cmd -a -c arg file file
cmd -carg -a file file
cmd -a -carg -- file file
hash [command ...]
hash -r
The shell maintains a hash table which remembers the locations of
commands. With no arguments whatsoever, the hash command prints
out the contents of this table. Entries which have not been
looked at since the last cd command are marked with an asterisk;
it is possible for these entries to be invalid.
With arguments, the hash command removes the specified commands
from the hash table (unless they are functions) and then locates
them. The -r option causes the hash command to delete all the
entries in the hash table except for functions.
jobs [-lp] [job ...]
Display the status of all, or just the specified, jobs:
By default display the job number, currency (+-) status, if
any, the job state, and its shell command.
-l also output the PID of the group leader, and
just the PID and shell commands of other members
of the job.
-p Display only leader PIDs, one per line.
kill [-s sigspec | -signum | -sigspec] [pid | job ...]
Equivalent to kill(1), but a job spec may also be specified.
Signals can be either case-insensitive names without SIG prefixes
or decimal numbers; the default is TERM.
kill -l [signum | exitstatus]
List available signal names without the SIG prefix (sigspecs).
If signum specified, display just the sigspec for that signal.
If exitstatus specified (> 128), display just the sigspec that
caused it.
pwd [-LP]
builtin command remembers what the current directory is rather
than recomputing it each time. This makes it faster. However,
if the current directory is renamed, the builtin version of pwd
will continue to print the old name for the directory. The -P
option causes the physical value of the current working directory
to be shown, that is, all symbolic links are resolved to their
respective values. The -L option turns off the effect of any
preceding -P options.
read [-p prompt] [-r] variable [...]
The prompt is printed if the -p option is specified and the stan-
dard input is a terminal. Then a line is read from the standard
input. The trailing newline is deleted from the line and the
line is split as described in the section on word splitting
above, and the pieces are assigned to the variables in order. At
least one variable must be specified. If there are more pieces
than variables, the remaining pieces (along with the characters
in IFS that separated them) are assigned to the last variable.
If there are more variables than pieces, the remaining variables
are assigned the null string. The read builtin will indicate
success unless EOF is encountered on input, in which case failure
is returned.
By default, unless the -r option is specified, the backslash “\”
acts as an escape character, causing the following character to
be treated literally. If a backslash is followed by a newline,
the backslash and the newline will be deleted.
readonly name ...
readonly -p
The specified names are marked as read only, so that they cannot
be subsequently modified or unset. The shell allows the value of
a variable to be set at the same time it is marked read only by
writing
readonly name=value
With no arguments the readonly command lists the names of all
read only variables. With the -p option specified the output
will be formatted suitably for non-interactive use.
printf format [value]...
printf formats and prints its arguments according to format, a
character string which contains three types of objects: plain
characters, which are simply copied to standard output, character
escape sequences which are converted and copied to the standard
output, and format specifications, each of which causes printing
of the next successive value.
Each value is treated as a string if the corresponding format
specification is either b, c, or s; otherwise it is evaluated as
a C constant, with the following additions:
• A leading plus or minus sign is allowed.
• If the leading character is a single or double quote,
the value of the next byte.
The format string is reused as often as necessary until all
values are consumed. Any extra format specifications are evalu-
ated with zero or the null string.
Character escape sequences are in backslash notation as defined
in ANSI X3.159-1989 (“ANSI C89”). The characters and their mean-
ings are as follows:
\a Write a <bell> character.
\b Write a <backspace> character.
\e Write an <escape> (ESC) character.
\f Write a <form-feed> character.
\n Write a <new-line> character.
\r Write a <carriage return> character.
\t Write a <tab> character.
\v Write a <vertical tab> character.
\\ Write a backslash character.
\num Write an 8-bit character whose ASCII value is the
1-, 2-, or 3-digit octal number num.
Each format specification is introduced by the percent character
(``%''). The remainder of the format specification includes, in
the following order:
Zero or more of the following flags:
# A `#' character specifying that the value should
be printed in an ``alternative form''. For b, c,
d, and s formats, this option has no effect. For
the o format the precision of the number is in-
creased to force the first character of the out-
put string to a zero. For the x (X) format, a
non-zero result has the string 0x (0X) prepended
to it. For e, E, f, g, and G formats, the result
will always contain a decimal point, even if no
digits follow the point (normally, a decimal
point only appears in the results of those for-
mats if a digit follows the decimal point). For
g and G formats, trailing zeros are not removed
from the result as they would otherwise be.
- A minus sign `-' which specifies left adjustment
of the output in the indicated field;
+ A `+' character specifying that there should al-
ways be a sign placed before the number when us-
ing signed formats.
‘ ’ A space specifying that a blank should be left
before a positive number for a signed format. A
`+' overrides a space if both are used;
0 A zero `0' character indicating that zero-padding
should be used rather than blank-padding. A `-'
overrides a `0' if both are used;
Field Width:
An optional digit string specifying a field width; if the
output string has fewer characters than the field width
it will be blank-padded on the left (or right, if the
left-adjustment indicator has been given) to make up the
field width (note that a leading zero is a flag, but an
embedded zero is part of a field width);
Precision:
An optional period, ‘.’, followed by an optional digit
string giving a precision which specifies the number of
digits to appear after the decimal point, for e and f
formats, or the maximum number of bytes to be printed
from a string (b and s formats); if the digit string is
missing, the precision is treated as zero;
Format:
A character which indicates the type of format to use
(one of diouxXfwEgGbcs).
A field width or precision may be ‘*’ instead of a digit string.
In this case an argument supplies the field width or precision.
The format characters and their meanings are:
diouXx The argument is printed as a signed decimal (d or i),
unsigned octal, unsigned decimal, or unsigned hexa-
decimal (X or x), respectively.
f The argument is printed in the style [-]ddd.ddd where
the number of d's after the decimal point is equal to
the precision specification for the argument. If the
precision is missing, 6 digits are given; if the pre-
cision is explicitly 0, no digits and no decimal
point are printed.
eE The argument is printed in the style [-]d.ddde±dd
where there is one digit before the decimal point and
the number after is equal to the precision specifica-
tion for the argument; when the precision is missing,
6 digits are produced. An upper-case E is used for
an `E' format.
gG The argument is printed in style f or in style e (E)
whichever gives full precision in minimum space.
b Characters from the string argument are printed with
backslash-escape sequences expanded.
The following additional backslash-escape sequences
are supported:
\c Causes dash to ignore any remaining charac-
ters in the string operand containing it, any
remaining string operands, and any additional
characters in the format operand.
\0num Write an 8-bit character whose ASCII value is
the 1-, 2-, or 3-digit octal number num.
c The first character of argument is printed.
s Characters from the string argument are printed until
the end is reached or until the number of bytes indi-
cated by the precision specification is reached; if
the precision is omitted, all characters in the
string are printed.
% Print a `%'; no argument is used.
In no case does a non-existent or small field width cause trunca-
tion of a field; padding takes place only if the specified field
width exceeds the actual width.
set [{ -options | +options | -- }] arg ...
The set command performs three different functions.
With no arguments, it lists the values of all shell variables.
If options are given, it sets the specified option flags, or
clears them as described in the section called “Argument List
Processing”. As a special case, if the option is -o or +o and no
argument is supplied, the shell prints the settings of all its
options. If the option is -o, the settings are printed in a hu-
man-readable format; if the option is +o, the settings are
printed in a format suitable for reinput to the shell to affect
the same option settings.
In addition to the option names listed in the “Argument List
Processing” section, the following options may be specified as
arguments to -o or +o:
pipefail Derive the exit status of a pipeline from the exit sta-
tuses of all of the commands in the pipeline, not just
the last command, as described in the “Pipelines” sec-
tion.
The third use of the set command is to set the values of the
shell's positional parameters to the specified args. To change
the positional parameters without changing any options, use “--”
as the first argument to set. If no args are present, the set
command will clear all the positional parameters (equivalent to
executing “shift $#”.)
shift [n]
Shift the positional parameters n times. A shift sets the value
of $1 to the value of $2, the value of $2 to the value of $3, and
so on, decreasing the value of $# by one. If n is greater than
the number of positional parameters, shift will issue an error
message, and exit with return status 2.
test expression
[ expression ]
The test utility evaluates the expression and, if it evaluates to
true, returns a zero (true) exit status; otherwise it returns 1
(false). If there is no expression, test also returns 1 (false).
All operators and flags are separate arguments to the test util-
ity.
The following primaries are used to construct expression:
-b file True if file exists and is a block special file.
-c file True if file exists and is a character special
file.
-d file True if file exists and is a directory.
-e file True if file exists (regardless of type).
-f file True if file exists and is a regular file.
-g file True if file exists and its set group ID flag is
set.
-h file True if file exists and is a symbolic link.
-k file True if file exists and its sticky bit is set.
-n string True if the length of string is nonzero.
-p file True if file is a named pipe (FIFO).
-r file True if file exists and is readable.
-s file True if file exists and has a size greater than
zero.
-t file_descriptor
True if the file whose file descriptor number is
file_descriptor is open and is associated with a
terminal.
-u file True if file exists and its set user ID flag is
set.
-w file True if file exists and is writable. True indi-
cates only that the write flag is on. The file is
not writable on a read-only file system even if
this test indicates true.
-x file True if file exists and is executable. True indi-
cates only that the execute flag is on. If file is
a directory, true indicates that file can be
searched.
-z string True if the length of string is zero.
-L file True if file exists and is a symbolic link. This
operator is retained for compatibility with previ-
ous versions of this program. Do not rely on its
existence; use -h instead.
-O file True if file exists and its owner matches the ef-
fective user id of this process.
-G file True if file exists and its group matches the ef-
fective group id of this process.
-S file True if file exists and is a socket.
file1 -nt file2
True if file1 and file2 exist and file1 is newer
than file2, or if file1 exists but file2 doesn't.
file1 -ot file2
True if file1 and file2 exist and file1 is older
than file2, or if file2 exists but file1 doesn't.
file1 -ef file2
True if file1 and file2 exist and refer to the same
file.
string True if string is not the null string.
s1 = s2 True if the strings s1 and s2 are identical.
s1 != s2 True if the strings s1 and s2 are not identical.
s1 < s2 True if string s1 comes before s2 based on the
ASCII value of their characters.
s1 > s2 True if string s1 comes after s2 based on the ASCII
value of their characters.
n1 -eq n2 True if the integers n1 and n2 are algebraically
equal.
n1 -ne n2 True if the integers n1 and n2 are not alge-
braically equal.
n1 -gt n2 True if the integer n1 is algebraically greater
than the integer n2.
n1 -ge n2 True if the integer n1 is algebraically greater
than or equal to the integer n2.
n1 -lt n2 True if the integer n1 is algebraically less than
the integer n2.
n1 -le n2 True if the integer n1 is algebraically less than
or equal to the integer n2.
These primaries can be combined with the following operators:
! expression True if expression is false.
expression1 -a expression2
True if both expression1 and expression2 are true.
expression1 -o expression2
True if either expression1 or expression2 are true.
(expression) True if expression is true.
The -a operator has higher precedence than the -o operator.
times Print the accumulated user and system times for the shell and for
processes run from the shell. The return status is 0.
trap [action signal ...]
Cause the shell to parse and execute action when any of the spec-
ified signals are received. The signals are specified by signal
number or as the name of the signal. If signal is 0 or EXIT, the
action is executed when the shell exits. action may be empty
(''), which causes the specified signals to be ignored. With
action omitted or set to `-' the specified signals are set to
their default action. When the shell forks off a subshell, it
resets trapped (but not ignored) signals to the default action.
The trap command has no effect on signals that were ignored on
entry to the shell. trap without any arguments cause it to write
a list of signals and their associated action to the standard
output in a format that is suitable as an input to the shell that
achieves the same trapping results.
Examples:
trap
List trapped signals and their corresponding action
trap '' INT QUIT tstp 30
Ignore signals INT QUIT TSTP USR1
trap date INT
Print date upon receiving signal INT
type [name ...]
Interpret each name as a command and print the resolution of the
command search. Possible resolutions are: shell keyword, alias,
shell builtin, command, tracked alias and not found. For aliases
the alias expansion is printed; for commands and tracked aliases
the complete pathname of the command is printed.
ulimit [-H | -S] [-a | -tfdscmlpnvwr [value]]
Inquire about or set the hard or soft limits on processes or set
new limits. The choice between hard limit (which no process is
allowed to violate, and which may not be raised once it has been
lowered) and soft limit (which causes processes to be signaled
but not necessarily killed, and which may be raised) is made with
these flags:
-H set or inquire about hard limits
-S set or inquire about soft limits. If neither -H nor
-S is specified, the soft limit is displayed or both
limits are set. If both are specified, the last one
wins.
The limit to be interrogated or set, then, is chosen by specify-
ing any one of these flags:
-a show all the current limits
-t show or set the limit on CPU time (in seconds)
-f show or set the limit on the largest file that can be
created (in 512-byte blocks)
-d show or set the limit on the data segment size of a
process (in kilobytes)
-s show or set the limit on the stack size of a process
(in kilobytes)
-c show or set the limit on the largest core dump size
that can be produced (in 512-byte blocks)
-m show or set the limit on the total physical memory
that can be in use by a process (in kilobytes)
-l show or set the limit on how much memory a process
can lock with mlock(2) (in kilobytes)
-p show or set the limit on the number of processes this
user can have at one time
-n show or set the limit on the number files a process
can have open at once
-v show or set the limit on the total virtual memory
that can be in use by a process (in kilobytes)
-w show or set the limit on the total number of locks
held by a process
-r show or set the limit on the real-time scheduling
priority of a process
If none of these is specified, it is the limit on file size that
is shown or set. If value is specified, the limit is set to that
number; otherwise the current limit is displayed.
Limits of an arbitrary process can be displayed or set using the
sysctl(8) utility.
umask [mask]
Set the value of umask (see umask(2)) to the specified octal
value. If the argument is omitted, the umask value is printed.
unalias [-a] [name]
If name is specified, the shell removes that alias. If -a is
specified, all aliases are removed.
unset [-fv] name ...
The specified variables and functions are unset and unexported.
If -f or -v is specified, the corresponding function or variable
is unset, respectively. If a given name corresponds to both a
variable and a function, and no options are given, only the vari-
able is unset.
wait [job]
Wait for the specified job to complete and return the exit status
of the last process in the job. If the argument is omitted, wait
for all jobs to complete and return an exit status of zero.
Command Line Editing
When dash is being used interactively from a terminal, the current com-
mand and the command history (see fc in “Builtins”) can be edited using
vi-mode command-line editing. This mode uses commands, described below,
similar to a subset of those described in the vi man page. The command
‘set -o vi’ enables vi-mode editing and places sh into vi insert mode.
With vi-mode enabled, sh can be switched between insert mode and command
mode. It is similar to vi: typing ⟨ESC⟩ enters vi command mode. Hit-
ting ⟨return⟩ while in command mode will pass the line to the shell.
EXIT STATUS
Errors that are detected by the shell, such as a syntax error, will
cause the shell to exit with a non-zero exit status. If the shell is
not an interactive shell, the execution of the shell file will be
aborted. Otherwise the shell will return the exit status of the last
command executed, or if the exit builtin is used with a numeric argu-
ment, it will return the argument.
ENVIRONMENT
HOME Set automatically by login(1) from the user's login directory
in the password file (passwd(4)). This environment variable
also functions as the default argument for the cd builtin.
PATH The default search path for executables. See the above sec-
tion “Path Search”.
CDPATH The search path used with the cd builtin.
MAIL The name of a mail file, that will be checked for the arrival
of new mail. Overridden by MAILPATH.
MAILCHECK The frequency in seconds that the shell checks for the ar-
rival of mail in the files specified by the MAILPATH or the
MAIL file. If set to 0, the check will occur at each prompt.
MAILPATH A colon “:” separated list of file names, for the shell to
check for incoming mail. This environment setting overrides
the MAIL setting. There is a maximum of 10 mailboxes that
can be monitored at once.
PS1 The primary prompt string, which defaults to “$ ”, unless you
are the superuser, in which case it defaults to “# ”.
PS2 The secondary prompt string, which defaults to “> ”.
PS4 Output before each line when execution trace (set -x) is en-
abled, defaults to “+ ”.
IFS Input Field Separators. This is normally set to ⟨space⟩,
⟨tab⟩, and ⟨newline⟩. See the “White Space Splitting” sec-
tion for more details.
TERM The default terminal setting for the shell. This is inher-
ited by children of the shell, and is used in the history
editing modes.
HISTSIZE The number of lines in the history buffer for the shell.
PWD The logical value of the current working directory. This is
set by the cd command.
OLDPWD The previous logical value of the current working directory.
This is set by the cd command.
PPID The process ID of the parent process of the shell.
FILES
$HOME/.profile
/etc/profile
SEE ALSO
csh(1), echo(1), getopt(1), ksh(1), login(1), printf(1), test(1),
getopt(3), passwd(5), environ(7), sysctl(8)
HISTORY
dash is a POSIX-compliant implementation of /bin/sh that aims to be as
small as possible. dash is a direct descendant of the NetBSD version of
ash (the Almquist SHell), ported to Linux in early 1997. It was renamed
to dash in 2002.
BUGS
Setuid shell scripts should be avoided at all costs, as they are a sig-
nificant security risk.
PS1, PS2, and PS4 should be subject to parameter expansion before being
displayed.
Debian January 19, 2003 DASH(1)
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