NAME
dash - command interpreter (shell)
SYNOPSIS
dash [-aCefnuvxIimqVEb] [+aCefnuvxIimqVEb] [-o option_name]
[+o option_name] [command_file [argument ...]]
dash -c [-aCefnuvxIimqVEb] [+aCefnuvxIimqVEb] [-o option_name]
[+o option_name] command_string [command_name [argument ...]]
dash -s [-aCefnuvxIimqVEb] [+aCefnuvxIimqVEb] [-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
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
terminal, 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
interpretative 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
directly by the shell.
Invocation
If no args are present and if the standard input of the shell is
connected 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
interactive shell generally prompts before each command and handles
programming 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
interactive 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
instead of from the standard input. Special
parameter 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
execute them. This is useful for checking the
syntax 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
debugging.
-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 login
shell.
-m monitor Turn on job control (set automatically when
interactive).
-s stdin Read commands from standard input (set
automatically 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)
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 arguments.
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 following
actions:
1. Leading words of the form “name=value” are stripped off and
assigned to the environment of the simple command.
Redirection 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
considered the command name and the command is located. The
remaining 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
existing 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 optional
number, 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 Duplicate standard input (or n1) from file descriptor
n2.
[n]<&- Close standard input (or n).
[n1]>&n2 Duplicate standard output (or n1) to 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 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 parameters
(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
described 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
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
performing 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
indication. 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
operators 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 control
operator |. The standard output of all but the last command is connected
to the standard input of the next command. The standard output 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
specified 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.
Otherwise, 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 status
is 1; if the last command returns greater than zero, the exit status 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 〈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
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 proceed 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
command, and then executes the second command iff the exit status of the
first command is zero. “||” is similar, but executes the second command
iff 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
repeatedly 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
continues with the next iteration of the innermost loop. These are
implemented 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.
Grouping Commands Together
Commands may be grouped by writing either
(list)
or
{ list; }
The first of these executes the commands in a subshell. Builtin commands
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
installs 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 exported
and readonly flags from the variable with the same name in the
surrounding scope, if there is one. Otherwise, the variable is initially
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
returns.
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
environment 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
alphabetics, numerics, and underscores - the first of which must not be
numeric. A parameter can also be denoted by a number or a special
character 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
special characters. The value of the parameter is listed next to its
character.
* 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
positional 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
option names concatenated into a string) as specified on
invocation, 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, arithmetic
expansions, and quote removals that occur within a single word expand to
a single field. It is only field splitting or pathname expansion 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,
Arithmetic 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
substitution, 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 characters 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
optional 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 expansion.
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; otherwise,
the value of parameter is substituted.
${parameter:=word} Assign Default Values. If parameter is unset or
null, the expansion of word is assigned to
parameter. 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 expansion
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 substring
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
effect.
${parameter%word} Remove Smallest Suffix Pattern. The word is
expanded to produce a pattern. The parameter
expansion 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
expanded to produce a pattern. The parameter
expansion 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
expanded to produce a pattern. The parameter
expansion 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
expanded to produce a pattern. The parameter
expansion 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
expansion 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 expansion
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
substitution 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
character 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
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(1), echo(1), test(1), etc).
:
true A null command that returns a 0 (true) 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 -
cd [-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 begin 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 specified
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.
\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
execute 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 variable
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
exported 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
previously 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
command 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
previous 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 option.
-number
A negative decimal number representing the command
that was executed number of commands previously.
For example, -1 is the immediately previous command.
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
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
arguments 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 initialized
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.
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 error. By
specifying a colon as the first character of optstring all errors
will be ignored.
A nonzero value is returned when the last option is reached. If
there are no remaining arguments, getopts will set var to the
special option, “--”, otherwise, it will set var to “?”.
The following code fragment shows how one might process the
arguments 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 ‘expr $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 -rv command ...
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. With the -v option, hash prints the locations of the
commands as it finds them. The -r option causes the hash command
to delete all the entries in the hash table except for functions.
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
standard 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 [arguments ...]
printf formats and prints its arguments, after the first, under
control of the format. The format is 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 argument.
The arguments after the first are treated as strings if the
corresponding format is either b, c or s; otherwise it is
evaluated as a C constant, with the following extensions:
· A leading plus or minus sign is allowed.
· If the leading character is a single or double quote,
the value is the ASCII code of the next character.
The format string is reused as often as necessary to satisfy the
arguments. Any extra format specifications are evaluated with
zero or the null string.
Character escape sequences are in backslash notation as defined in
ANSI X3.159-1989 (“ANSI C”). The characters and their meanings
are as follows:
\a Write a <bell> character.
\b Write a <backspace> 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
increased to force the first character of the
output 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 formats 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
always be a sign placed before the number when
using 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 characters 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
hexadecimal (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
precision 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
specification 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 characters
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 characters
indicated 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
truncation 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
human-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.
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
utility.
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 indicates
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
indicates 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 previous
versions of this program. Do not rely on its
existence; use -h instead.
-O file True if file exists and its owner matches the
effective user id of this process.
-G file True if file exists and its group matches the
effective 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.
file1 -ot file2
True if file1 and file2 exist and file1 is older
than file2.
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 algebraically
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
specified signals are received. The signals are specified by
signal number or as the name of the signal. If signal is 0, the
action is executed when the shell exits. action may be null,
which cause 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 | -tfdscmlpn [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 specifying
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
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
variable 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 the return an exit status of zero.
Command Line Editing
When dash is being used interactively from a terminal, the current
command 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 place sh into vi insert mode.
With vi-mode enabled, sh can be switched between insert mode and command
mode. The editor is not described in full here, but will be in a later
document. It’s similar to vi: typing 〈ESC〉 will throw you into command
VI command mode. Hitting 〈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 argument, 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
section 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 arrival
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
enabled, defaults to “+ ”.
IFS Input Field Separators. This is normally set to 〈space〉,
〈tab〉, and 〈newline〉. See the White Space Splitting section
for more details.
TERM The default terminal setting for the shell. This is inherited
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
significant security risk.
PS1, PS2, and PS4 should be subject to parameter expansion before being
displayed.