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Charles Steinkuehler's LEAF/LRP Website |
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NAME
sh - command interpreter (shell)
SYNOPSIS
sh [-/+aCefnuvxIimsVEb] [-/+o longname] [arg ...]
DESCRIPTION
Sh is the standard command interpreter for the system.
The current version of sh is in the process of being
changed to conform with the POSIX 1003.2 and 1003.2a spec
ifications 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 (run GNU's bash if
you want that). Only features designated by POSIX, plus a
few Berkeley extensions, are being incorporated into this
shell. We expect POSIX conformance by the time 4.4 BSD is
released. 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), 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/pro
file 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 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. 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 commands 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 posi
tional 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(1) builtin (described later).
-a allexport
Export all variables assigned to. (UNIMPLEMENTED
for 4.4alpha)
-C noclobber
Don't overwrite existing files with ``>''. (UNIM
PLEMENTED for 4.4alpha)
-e errexit
If not interactive, exit immediately if any
untested command fails. The exit status of a
command is considered to be explicitly tested if
the command is used to control an if, elif, while,
or until; or if the command is the left hand
operand of an ``&&'' or ``||'' operator.
-f noglob
Disable pathname expansion.
-n noexec
If not interactive, read commands but do not exe
cute them. This is useful for checking the 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. (UNIM
PLEMENTED for 4.4alpha)
-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.
-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(1)).
-V vi
Enable the built-in vi(1) command line editor (dis
ables -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 operator: < > >| << >> <& >& <<- <>
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 quot
ing: 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 lit
eral 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 sim
ple 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 per
forms 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 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 men
tioned 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 descrip
tor n2.
[n]<&- Close standard input (or n).
[n1]>&n2
Duplicate standard output (or n) from 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-docu
ment''.
[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 posi
tional 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 com
mand (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 defi
nition is executed. The positional parameters are
restored to their original values when the command com
pletes. 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 filesystem (as described in the next section).
When a normal program is executed, the shell runs the pro
gram, 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 loca
tion 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 proce
dure".
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 con
sists 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 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 com
mands return exit codes, as does an executed shell func
tion.
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 stan
dard input of command2. The standard input, standard out
put, or both of a command is considered to be assigned by
the pipeline before any redirection specified by redirec
tion 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 speci
fied 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 com
mand1 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 amper
sand (&), 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 termi
nated by one of these three characters. The commands in a
list are executed in the order they are written. If com
mand 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 sta
tus of the first list is zero. The until command is simi
lar, 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 continues with the next iteration of the inner
most 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 ``|'' char
acters.
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:
{ echo -n "hello"; echo " world" } > greeting
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 vari
able 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 vari
able named x.
The only special parameter than 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(1) 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 sep
arate argument. If there are no positional parame
ters, 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 argu
ments:
"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 com
mand, 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 back
ground 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 per
formed 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 Substi
tution, 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 expan
sions, 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 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.
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 pro
vide for substring processing. In each case, pattern
matching notation (see Shell Patterns), rather than regu
lar 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 fol
lows:
$(command)
or (``backquoted'' version):
`command`
The shell expands the command substitution by executing
command in a subshell environment and replacing the com
mand 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 split
ting, 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 expan
sions 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 substitution into fields.
Pathname Expansion (File Name Generation)
Unless the -f flag is set, file name generation is per
formed 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 sec
ond, 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(1) command.
Shell Patterns
A pattern consists of normal characters, which match them
selves, 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 intro
ducing 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 exclama
tion 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).
: 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 command arg...
Execute the specified builtin command. (This is
useful when you have a shell function with the same
name as a builtin command.)
cd [ directory ]
Switch to the specified directory (default $HOME).
If the an entry for CDPATH appears in the environ
ment 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 for
mat of CDPATH is the same as that of PATH. 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 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...
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.
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 com
mands. The editor string is a command name, sub
ject 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 deter
mined by the value of the HISTSIZE variable. The
value of first or last or both are one of the fol
lowing:
[+]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 com
mand.
string
A string indicating the most recently entered com
mand 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 embed
ded 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 access
able.
fg [ job ]
Move the specified job or the current job to the
foreground.
getopts optstring var
The POSIX getopts command. The getopts command
deprecates the older getopt command. The first
argument should be a series of letters, each possi
bly followed by a colon which indicates that the
option takes an argument. The specified variable
is set to the parsed option. The index of the next
argument is placed into the shell variable OPTIND.
If an option takes an argument, it is placed into
the shell variable OPTARG. If an invalid option is
encountered, var is set to '?'. It returns a false
value (1) when it encounters the end of the
options.
hash -rv command...
The shell maintains a hash table which remembers
the locations of commands. With no arguments what
soever, 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 speci
fied 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.
jobid [ job ]
Print the process id's of the processes in the job.
If the job argument is omitted, use the current
job.
jobs This command lists out all the background processes
which are children of the current shell process.
pwd Print the current directory. The builtin command
may differ from the program of the same name
because the builtin command remembers what the cur
rent directory is rather than recomputing it each
time. This makes it faster. However, if the cur
rent directory is renamed, the builtin version of
pwd will continue to print the old name for the
directory.
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 new
line 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.
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 back
slash and the newline will be deleted.
readonly name...
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.
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 val
ues of the shell's positional parameters to the
specified args. To change the positional parame
ters 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 $#''.)
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 positional
parameters, shifting doesn't do anything.
trap [ action ] signal...
Cause the shell to parse and execute action when
any of the specified signals are received. The sig
nals are specified by signal number. Action may be
null or omitted; the former causes the specified
signal to be ignored and the latter causes the
default action to be taken. 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.
type [name]...
Interpret each name as a command and print the res
olution 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 low
ered) and soft limit (which causes processes to be
signalled 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 pro
cess (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 (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 speci
fied, the limit is set to that number; otherwise the
current limit is displayed.
umask [ mask ]
Set the value of umask (see umask(2)) to the speci
fied 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 com
plete and the 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
Builtins) can be edited using vi-mode command-line edit
ing. This mode uses commands, described below, similar to
a subset of those described in the vi man page. The com
mand 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 com
mand VI command mode. Hitting <return> while in command
mode will pass the line to the shell.
HISTORY
A sh command appeared in Version 1 AT&T UNIX. It was,
however, unmaintainable so we wrote this one.
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