SH(1) BSD General Commands Manual SH(1)NAMEsh — command interpreter (shell)
SYNOPSISsh [-aCefnuvxIimqVEb] [+aCefnuvxIimqVEb] [-o option_name]
[+o option_name] [command_file [argument ...]]
sh-c [-aCefnuvxIimqVEb] [+aCefnuvxIimqVEb] [-o option_name]
[+o option_name] command_string [command_name [argument ...]]
sh-s [-aCefnuvxIimqVEb] [+aCefnuvxIimqVEb] [-o option_name]
[+o option_name] [argument ...]
DESCRIPTIONsh is the standard command interpreter for the system. The current ver‐
sion of sh 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 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 imple‐
ments 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 interpre‐
tative 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 arguments 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 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 automati‐
cally 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 a 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 shell inside the ENV file. To set the ENV vari‐
able 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. Since the ENV file is
read for every invocation of the shell, including shell scripts and non-
interactive shells, the following paradigm is useful for restricting com‐
mands in the ENV file to interactive invocations. Place commands within
the “case” and “esac” below (these commands are described later):
case $- in *i*)
# commands for interactive use only
...
esac
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 have a corresponding name that 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 built-
in (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 op‐
erand 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 com‐
mand 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 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 debug‐
ging.
-q quietprofile If the -v or -x options have been set, do not
apply them when reading initialization files,
these being /etc/profile, .profile, and the file
specified by the ENV environment variable.
-I ignoreeof Ignore EOFs from input when interactive.
-i interactive Force the shell to behave interactively.
-m monitor Turn on job control (set automatically when inter‐
active).
-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). (See the
Command Line Editing section below.)
-E emacs Enable the built-in emacs style command line edi‐
tor (disables -V if it has been set). (See the
Command Line Editing section below.)
-b notify Enable asynchronous notification of background job
completion. (Not implemented.)
cdprint Make an interactive shell always print the new
directory name when changed by the cd command.
tabcomplete Enables filename completion in the command line
editor. Typing a tab character will extend the
current input word to match a filename. If more
than one filename matches it is only extended to
be the common prefix. Typing a second tab charac‐
ter will list all the matching names. One of the
editing modes, either -E or -V, must be enabled
for this to work.
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 char‐
acters 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 dollar sign ($), 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 built-in
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 dis‐
couraged.
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. Redirect‐
ion 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 remain‐
ing words are considered the arguments of the command. If no
command name resulted, then the “name=value” variable assign‐
ments 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 exist‐
ing 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, built-in 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 envi‐
ronment of the command (by placing assignments to them before the func‐
tion 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 built-ins are executed internally to the shell, without spawning a
new process.
Otherwise, if the command name doesn't match a function or built-in, 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 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 built-in command by that
name. If a built-in 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 behavior of other
shell commands. The paradigm is that a command exits with zero for nor‐
mal 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 built-in commands return exit codes,
as does an executed shell function.
If a command consists entirely of variable assignments then the exit sta‐
tus of the command is that of the last command substitution if any, oth‐
erwise 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 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 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. Other‐
wise, 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 built-in, 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 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 com‐
mand, and then executes the second command if and only if the exit status
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. Note that these operators are
left-associative, so “true || echo bar && echo baz” writes “baz” and
nothing else. This is not the way it works in C. Also, if you forget
the left-hand side (for example when continuing lines but forgetting to
use a backslash) it defaults to a true statement. This behavior is not
useful and should not be relied upon.
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 are expanded, and then the list is executed repeatedly with the
variable set to each word in turn. do and done may be replaced with “{”
and “}”.
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 implemented
as built-in 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. Built-in 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:
{ echo -n " hello " ; echo " world" ; } > 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 com‐
mand. This should appear as the first statement of a function, and the
syntax is
local [variable | -] ...
“Local” is implemented as a built-in command.
When a variable is made local, it inherits the initial value and exported
and read-only flags from the variable with the same name in the surround‐
ing 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 causes 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 built-in 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 environ‐
ment 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 alphabet‐
ics, 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 built-in 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
option names concatenated into a string) as specified on
invocation, by the set built-in 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 com‐
mand 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, 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 substi‐
tution, 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 the special rules for @.
In addition, a parameter expansion can be modified by using one of the
following formats. If the “:” is omitted in the following modifiers,
then the expansion is applied only to unset parameters, not null ones.
${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 parame‐
ter. In all cases, the final value of parameter is
substituted. Only variables, not positional param‐
eters 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 writ‐
ten 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.
${#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 pat‐
tern 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 pat‐
tern 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 pat‐
tern 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 pat‐
tern 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 sub‐
shell environment and replacing the command substitution with the stan‐
dard 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:
$((expression))
The expression is treated as if it were in double-quotes, except that a
double-quote inside the expression is not treated specially. The shell
expands all tokens in the expression for parameter expansion, command
substitution, and quote removal.
Next, the shell treats this as an arithmetic expression and substitutes
the value of the expression.
Arithmetic expressions use a syntax similar to that of the C language,
and are evaluated using the ‘intmax_t’ data type (this is an extension to
POSIX, which requires only ‘long’ arithmetic). Shell variables may be
referenced by name inside an arithmetic expression, without needing a “$”
sign.
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 use the
delimiters to split the results of parameter expansion and command sub‐
stitution into fields.
Non-whitespace characters in IFS are treated strictly as parameter termi‐
nators. So adjacent non-whitespace IFS characters will produce empty
parameters.
If IFS is unset it is assumed to contain space, tab, and newline.
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, sep‐
arated 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 com‐
mand.
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 between
the square brackets. A range of characters may be specified using a
minus sign. The character class may be complemented by making an excla‐
mation 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.
Built-ins
This section lists the built-in commands which are built-in 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 built in for efficiency (e.g. printf(1), echo(1), test(1), etc).
: 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 built-in 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 built-in 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 built-in.
-v Do not execute the command but search for the command and
print the absolute pathname of utilities, the name for
built-ins or the expansion of aliases.
cd [-P] [directory [replace]]
Switch to the specified directory (default $HOME). If replace is
specified, then the new directory name is generated by replacing
the first occurrence of directory in the current directory name
with replace. If directory is ‘-’, then the current working
directory is changed to the previous current working directory as
set in OLDPWD. Otherwise 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, or its first
(or only) component isn't dot or dot dot, then the directories
listed in CDPATH will be searched for the specified directory.
The format of CDPATH is the same as that of PATH.
The -P option instructs the shell to update PWD with the specified
physical directory path and change to that directory. This is the
default.
When the directory changes, the variable OLDPWD is set to the
working directory before the change.
Some shells also support a -L option, which instructs the shell to
update PWD with the logical path and to change the current direc‐
tory accordingly. This is not supported.
In an interactive shell, 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 a symbolic link
was crossed.
eval string ...
Concatenate all the arguments with spaces. Then re-parse and exe‐
cute 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
built-in 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 built-in 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 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 oper‐
and 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
-derivedgetopt(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 built-in 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 vari‐
able OPTIND. When the shell is invoked, OPTIND is initialized to
1. For each option that requires an argument, the getopts built-
in 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 whitespace. 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 specify‐
ing 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 spe‐
cial option, “--”, otherwise, it will 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 $(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 com‐
mands as it finds them. The -r option causes the hash command to
delete all the entries in the hash table except for functions.
inputrc file
Read the file to set keybindings as defined by editrc(5).
jobid [job]
Print the process id's of the processes in the job. If the job
argument is omitted, the current job is used.
jobs This command lists out all the background processes which are
children of the current shell process.
pwd [-LP]
Print the current directory. If -L is specified the cached value
(initially set from PWD) is checked to see if it refers to the
current directory; if it does the value is printed. Otherwise the
current directory name is found using getcwd(3). The environment
variable PWD is set to the printed value.
The default is pwd -L, but note that the built-in cd command
doesn't currently support the -L option and will cache (almost)
the absolute path. If cd is changed, pwd may be changed to
default to pwd -P.
If the current directory is renamed and replaced by a symlink to
the same directory, or the initial PWD value followed a symbolic
link, then the cached value may not be the absolute path.
The built-in command may differ from the program of the same name
because the program will use PWD and the built-in uses a sepa‐
rately cached value.
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. 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 built-in 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.
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.
The third use of the set command is to set the values of the
shell's positional parameters to the specified arguments. To
change the positional parameters without changing any options, use
“--” as the first argument to set. If no arguments are present,
the set command will clear all the positional parameters (equiva‐
lent to executing “shift $#”.)
setvar variable value
Assigns value to variable. (In general it is better to write
variable=value rather than using setvar. setvar is intended to be
used in functions that assign values to variables whose names are
passed as parameters.)
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 there are zero posi‐
tional parameters, shift does nothing.
trap [-l]
trap [action] signal ...
Cause the shell to parse and execute action when any of the speci‐
fied signals are received. The signals are specified by signal
number or as the name of the signal. If signal is 0 or its equiv‐
alent, EXIT, 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.
On non-interactive shells, the trap command has no effect on sig‐
nals that were ignored on entry to the shell. On interactive
shells, the trap command will catch or reset signals ignored on
entry. Issuing trap with option -l will print a list of valid
signal names. 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 -l
Print a list of valid signals
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 built-in, 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 | -tfdscmlpnv [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
-b show or set the limit on the socket buffer size of a
process (in bytes)
-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 of files a process
can have open at once
-v show or set the limit on how large a process address
space can be
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 name ...
The specified variables and functions are unset and unexported.
If a given name corresponds to both a variable and a function,
both the variable and the function are 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 then return an exit status of zero.
Command Line Editing
When sh is being used interactively from a terminal, the current command
and the command history (see fc in Built-ins) can be edited using emacs-
mode or vi-mode command-line editing. The command ‘set -o emacs’ enables
emacs-mode editing. The command ‘set -o vi’ enables vi-mode editing and
places the current shell process into vi insert mode. (See the Argument
List Processing section above.)
The vi mode uses commands similar to a subset of those described in the
vi(1) man page. With vi-mode enabled, sh can be switched between insert
mode and command mode. It's similar to vi(1): pressing the ⟨ESC⟩ key
will throw you into command VI command mode. Pressing the ⟨return⟩ key
while in command mode will pass the line to the shell.
The emacs mode uses commands similar to a subset available in the
emacs(1) editor. With emacs-mode enabled, special keys can be used to
modify the text in the buffer using the control key.
sh uses the editline(3) library.
ENVIRONMENT
HOME Set automatically by login(1) from the user's login directory
in the password file (passwd(5)). This environment variable
also functions as the default argument for the cd built-in.
PATH The default search path for executables. See the above sec‐
tion Path Search.
CDPATH The search path used with the cd built-in.
LANG The string used to specify localization information that
allows users to work with different culture-specific and lan‐
guage conventions. See nls(7).
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.
FILES
$HOME/.profile
/etc/profile
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. Oth‐
erwise the shell will return the exit status of the last command exe‐
cuted, or if the exit built-in is used with a numeric argument, it will
return the argument.
SEE ALSOcsh(1), echo(1), getopt(1), ksh(1), login(1), printf(1), test(1),
editline(3), getopt(3), editrc(5), passwd(5), environ(7), nls(7),
sysctl(8)HISTORY
A sh command appeared in Version 1 AT&T UNIX. It was, however, unmain‐
tainable so we wrote this one.
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.
The characters generated by filename completion should probably be quoted
to ensure that the filename is still valid after the input line has been
processed.
BSD October 4, 2011 BSD
