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). Only fea-
tures designated by POSIX, plus a few Berkeley extensions, are being
incorporated 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 ter-
minal, 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 command). The
shell implements a language that has flow control constructs, a macro
facility that provides a variety of features in addition to data stor-
age, along with built in history and line editing capabilities. It in-
corporates many features to aid interactive use and has the advantage
that the interpretative language is common to both interactive and non-
interactive use (shell scripts). That is, commands can be typed di-
rectly to the running shell or can be put into a file and the file can
be executed directly 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 program-
ming and command errors differently (as described below). When first
starting, the shell inspects argument 0, and if it begins with a dash
`-', the shell is also considered a login shell. This is normally done
automatically 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 op-
tion. 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 noclobber Don't overwrite existing files with ">".
-e errexit If not interactive, exit immediately if any
-f noglob Disable pathname expansion.
-n noexec If not interactive, read commands but do not ex-
-u nounset Write a message to standard error when attempt-
-v verbose The shell writes its input to standard error as
-x xtrace Write each command to standard error (preceded
-I ignoreeof Ignore EOF's from input when interactive.
-i interactive Force the shell to behave interactively.
-m monitor Turn on job control (set automatically when in-
-s stdin Read commands from standard input (set automati-
-V vi Enable the built-in vi command line editor
-E emacs Enable the built-in emacs command line editor
-b notify Enable asynchronous notification of background
-p priviliged Do not attempt to reset effective uid if it does
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:
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 . A backslash preceding a 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:
$ ` " \ .
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
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
would become
ls -F foobar
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
2. The remaining words are expanded as described in the section
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 existing reference to a file. The overall format used for redirec-
tion is:
[n] redir-op file
where redir-op is one of the redirection operators mentioned previ-
ously. Following is a list of the possible redirections. The [n] is
an optional 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
The following redirection is often called a "here-document".
[n]<< delimiter
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
arithmetic 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 in-
voked to handle the ad-hoc shell script, except that the location of
hashed commands 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 sepa-
rated by colons. Each entry consists of a directory name. The
current directory may be indicated implicitly by an empty direc-
tory 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 in-
dication. The man page for each command should indicate the various
exit codes and what they mean. Additionally, the builtin commands re-
turn 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 oper-
ators or reserved words, together creating a larger complex command.
More generally, a command is one of the following:
o simple command
o pipeline
o list or compound-list
o compound command
o 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 reserved word ! does not precede the pipeline, the exit status
is the exit status of the last command specified in the pipeline. Oth-
erwise, the exit status is the logical NOT of the exit status of the
last command. That is, if the last command returns zero, the exit sta-
tus is 1; if the last command returns greater than zero, the exit sta-
tus is zero.
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 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
environment 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; oth-
erwise 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 un-
til 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 con-
tinues 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 be-
fore 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
second 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 char-
acter 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
character.
* Expands to the positional parameters, starting from one.
@ Expands to the positional parameters, starting from one.
# 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-
$ Expands to the process ID of the invoked shell. A sub-
! Expands to the process ID of the most recent background
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
expand to a single field. It is only field splitting or pathname ex-
pansion that can create multiple fields from a single word. The single
exception 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 un-
less 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 directory. If the username is missing (as in ~/foobar), the tilde
is replaced with the value of the HOME variable (the current user's
home directory).
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 charac-
ters in embedded arithmetic expansions, command substitutions, and
variable expansions, 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
${parameter:=word} Assign Default Values. If parameter is unset or
${parameter:?[word]} Indicate Error if Null or Unset. If parameter is
${parameter:+word} Use Alternative Value. If parameter is unset or
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; omis-
sion of the colon results in a test for a parameter that is only unset.
${#parameter} String Length. The length in characters of the
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 evalu-
ate the patterns. If parameter is * or @, the result of the expansion
is unspecified. Enclosing the full parameter expansion string in dou-
ble-quotes does not cause the following four varieties of pattern char-
acters to be quoted, whereas quoting characters within the braces has
this effect.
${parameter%word} Remove Smallest Suffix Pattern. The word is ex-
${parameter%%word} Remove Largest Suffix Pattern. The word is ex-
${parameter#word} Remove Smallest Prefix Pattern. The word is ex-
${parameter##word} Remove Largest Prefix Pattern. The word is ex-
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
s at the end of the substitution. (Embedded s before
the end of the output are not removed; however, during field splitting,
they may be translated into 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 ex-
pansion 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 start-
ing with a period unless the first character of the pattern is a pe-
riod. 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 char-
acter class. The end of the character class is indicated by a ("]");
if the "]" is missing then the "[" matches a "[" rather than introduc-
ing a character class. A character class matches any of the characters
between the square brackets. A range of characters may be specified
using a minus sign. The character class may be complemented by making
an exclamation 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, echo, test, etc).
:
true A null command that returns a 0 (true) exit value.
false A null command that returns a 1 (false) exit value.
. file
alias [name[=string ...]]
bg [job] ...
command [-p] [-v] [-V] command [arg ...]
cd|chdir -
cd|chdir [-LP] [directory]
echo [-n] args...
eval string ...
exec [command arg ...]
exit [exitstatus]
export name ...
export -p
fc [-e editor] [first [last]]
fc -l [-nr] [first [last]]
fc -s [old=new] [first]
fg [job]
getopts optstring var [arg ...]
hash [command ...]
hash -r
jobs [-lp] [job ...]
kill [-s sigspec | -signum | -sigspec] [pid | job ...]
kill -l [signum | exitstatus]
pwd [-LP]
read [-p prompt] [-r] variable [...]
readonly name ...
readonly -p
printf format [value]...
set [{ -options | +options | -- }] arg ...
shift [n]
test expression
[ expression ]
times Print the accumulated user and system times for the shell and
trap [action signal ...]
type [name ...]
ulimit [-H | -S] [-a | -tfdscmlpnvwr [value]]
umask [mask]
unalias [-a] [name]
unset [-fv] name ...
wait [job]
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 be-
low, 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 in-
sert mode. With vi-mode enabled, sh can be switched between insert
mode and command mode. It is similar to vi: typing enters vi
command mode. Hitting 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 from the user's login direc-
PATH The default search path for executables. See the above sec-
CDPATH The search path used with the cd builtin.
MAIL The name of a mail file, that will be checked for the ar-
MAILCHECK The frequency in seconds that the shell checks for the ar-
MAILPATH A colon ":" separated list of file names, for the shell to
PS1 The primary prompt string, which defaults to "$ ", unless
PS2 The secondary prompt string, which defaults to "> ".
PS4 Output before each line when execution trace (set -x) is en-
IFS Input Field Separators. This is normally set to ,
TERM The default terminal setting for the shell. This is inher-
HISTSIZE The number of lines in the history buffer for the shell.
PWD The logical value of the current working directory. This is
OLDPWD The previous logical value of the current working directory.
PPID The process ID of the parent process of the shell.
FILES
$HOME/.profile
/etc/profile
SEE ALSO
csh, echo, getopt, ksh, login, printf, test,
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 re-
named 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.