SysIII/usr/src/man/man1/sh.1

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.TH SH 1
.SH NAME
sh \- shell, the standard command programming language
.SH SYNOPSIS
.B sh
[
.B \-ceiknrstuvx
] [ args ]
.SH DESCRIPTION
.I Sh\^
is a command programming language
that executes commands read from a terminal
or a file.
See
.I Invocation\^
below
for the meaning of arguments to the shell.
.SS Commands.
A
.I simple-command\^
is a sequence of non-blank
.I words\^
separated by
.I blanks\^
(a
.I blank\^
is a
tab
or a
space).
The first word specifies the name of the command to
be executed.
Except as specified below,
the remaining words are passed as arguments
to the invoked command.
The command name is passed as argument 0
(see
.IR exec (2)).
The
.I value\^
of a simple-command is its exit status
if it terminates normally, or (octal) 200+\f2status\^\fP if
it terminates abnormally (see
.IR signal (2)
for a list of
status values).
.PP
A
.I pipeline\^
is a sequence of one or more
.I commands\^
separated by
.BR \(bv .
The standard output of each command but the last
is connected by a
.IR pipe (2)
to the standard input of the next command.
Each command is run as a separate process;
the shell waits for the last command to terminate.
.PP
A
.I list\^
is a sequence of one or more
pipelines
separated by
.BR ; ,
.BR & ,
.BR && ,
or
.BR \(bv\|\(bv ,
and optionally terminated by
.B ;
or
.BR & .
Of these four symbols,
.B ;
and
.B &
have equal precedence,
which is lower than that of
.B &&
and
.BR \(bv\|\(bv .
The symbols
.B &&
and
.B \(bv\|\(bv
also have equal precedence.
A semicolon
.RB ( ; )
causes sequential execution of the preceding pipeline; an ampersand
.RB ( & )
causes asynchronous execution of the preceding pipeline (i.e., the shell does
.I not\^
wait for that pipeline to finish).
The symbol
.B &&
.RB (\| \(bv\|\(bv \^)
causes the
.I list\^
following it to be executed only if the preceding
pipeline
returns a zero (non-zero) exit status.
An arbitrary number of new-lines may appear in a
.IR list ,
instead of semicolons,
to delimit commands.
.PP
A
.I command\^
is either a simple-command
or one of the following.
Unless otherwise stated,
the value returned by a command is that of the
last simple-command executed in the command.
.PP
.PD 0
.TP
\f3for\fP \f2name\^\fP \*(OK \f3in\fP \f2word\^\fP .\|.\|. \*(CK \f3do\fP \f2list\^\fP \f3done\fP
Each time a
.B for
command is executed,
.I name\^
is set to the next
.I word\^
taken from the
.B in
.I word\^
list.
If
.BI in " word\^"
\&.\|.\|.
is omitted, then
the
.B for
command executes the \f3do\fP \f2list\^\fP once for each positional parameter
that is set
(see
.I "Parameter Substitution\^"
below).
Execution ends when there are no more words in the list.
.TP
\f3case\fP \f2word\^\fP \f3in\fP \*(OK \f2pattern\^\fP \*(OK \(bv \
\f2pattern\^\fP \*(CK .\|.\|. \f3)\fP \f2list\^\fP \f3;;\fP \*(CK .\|.\|. \f3esac\fP
A
.B case
command executes the
.I list\^
associated with the first
.I pattern\^
that matches
.IR word .
The form of the patterns is
the same as that used for
file-name generation (see 
.I "File Name Generation\^"
below).
.TP
\f3if\fP \f2list\^\fP \f3then\fP \f2list\^\fP \*(OK \
\f3elif\fP \f2list\^\fP \f3then\fP \f2list\^\fP \*(CK .\|.\|. \
\*(OK \f3else\fP \f2list\^\fP \*(CK \f3f\&i\fP
The
.I list\^
following \f3if\fP is executed and,
if it
returns a zero exit status, the
.I list\^
following
the first
.B then
is executed.
Otherwise, the
.I list\^
following \f3elif\fP
is executed and, if its value is zero,
the
.I list\^
following
the next
.B then
is executed.
Failing that, the
.B else
.I list\^
is executed.
If no
.B else
.I list\^
or
.B then
.I list\^
is executed, then the
.B if
command returns a zero exit status.
.TP
\f3while\fP \f2list\^\fP \f3do\fP \f2list\^\fP \f3done\fP
A
.B while
command repeatedly executes the
.B while
.I list\^
and, if the exit status of the last command in the list is zero, executes
the
.B do
.IR list ;
otherwise the loop terminates.
If no commands in the
.B do
.I list\^
are executed, then the
.B while
command returns a zero exit status;
.B until
may be used in place of
.B while
to negate
the loop termination test.
.TP
\f3(\fP\f2list\^\fP\f3)\fP
.br
Execute
.I list\^
in a sub-shell.
.TP
\f3{\fP\f2list\^\fP\f3;}\fP
.br
.I list\^
is simply executed.
.PD
.PP
The following words
are only recognized as the first word of a command
and when not quoted:
.if t .RS
.PP
.B
.if n if then else elif fi case esac for while until do done { }
.if t if  then  else  elif  f\&i  case  esac  for  while  until  do  done  {  }
.if t .RE
.SS Comments.
A word beginning with
.B #
causes that word and all the following characters up to a new-line
to be ignored.
.SS Command Substitution.
The standard output from a command enclosed in
a pair of grave accents (\^\f3\*`\^\*`\fP\^) may be used as part or all
of a word;
trailing new-lines are removed.
.SS Parameter Substitution.
The character
.B $
is used to introduce substitutable
.IR parameters .
Positional parameters may be assigned values by
.BR set .
Variables may be set by writing:
.RS
.PP
.IB name = value\^
\*(OK
.IB name = value\^
\*(CK .\|.\|.
.RE
.PP
Pattern-matching is not performed on
.IR value .
.PP
.PD 0
.TP
\f3${\fP\f2parameter\^\fP\f3}\fP
A
.I parameter\^
is a sequence of letters, digits, or underscores (a
.IR name ),
a digit,
or any of the characters
.BR \(** ,
.BR @ ,
.BR # ,
.BR ? ,
.BR \- ,
.BR $ ,
and
.BR !\\^ .
The value, if any, of the parameter is substituted.
The braces are required only when
.I parameter\^
is followed by a letter, digit, or underscore
that is not to be interpreted as part of its name.
A
.I name\^
must begin with a letter or underscore.
If
.I parameter\^
is a digit then it is a positional parameter.
If
.I parameter\^
is
.B \(**
or
.BR @ ,
then all the positional
parameters, starting with
.BR $1 ,
are substituted
(separated by spaces).
Parameter
.B $0
is set from argument zero when the shell
is invoked.
.TP
\f3${\fP\f2parameter\^\fP\f3:\-\fP\f2word\^\fP\f3}\fP
If
.I parameter\^
is set and is non-null then substitute its value;
otherwise substitute
.IR word .
.TP
\f3${\fP\f2parameter\^\fP\f3:=\fP\f2word\^\fP\f3}\fP
If
.I parameter\^
is not set or is null
then set it to
.IR word ;
the value of the parameter is then substituted.
Positional parameters may not be assigned to
in this way.
.TP
\f3${\fP\f2parameter\^\fP\f3:?\fP\f2word\^\fP\f3}\fP
If
.I parameter\^
is set and is non-null then substitute its value;
otherwise, print
.I word\^
and exit from the shell.
If
.I word\^
is omitted, then the message
``parameter null or not set''
is printed.
.TP
\f3${\fP\f2parameter\^\fP\f3:+\fP\f2word\^\fP\f3}\fP
If
.I parameter\^
is set and is non-null then substitute
.IR word ;
otherwise substitute nothing.
.PD
.PP
In the above,
.I word\^
is not evaluated unless it is
to be used as the substituted string,
so that, in the following example,
.B pwd
is executed only if
.B d
is not set or is null:
.RS
.PP
echo \|${d:\-\^\*`\^pwd\^\*`\^}
.RE
.PP
If the colon
.RB ( : )
is omitted from the above expressions, then the
shell only checks whether 
.I parameter\^
is set or not.
.PP
The following
parameters
are automatically set by the shell:
.RS
.PD 0
.TP
.B #
The number of positional parameters in decimal.
.TP
.B \-
Flags supplied to the shell on invocation or by
the
.B set
command.
.TP
.B ?
The decimal value returned by the last synchronously executed command.
.TP
.B $
The process number of this shell.
.TP
.B !
The process number of the last background command invoked.
.PD
.RE
.PP
The following
parameters
are used by the shell:
.RS
.PD 0
.TP
.B
.SM HOME
The default argument (home directory) for the
.I cd\^
command.
.TP
.B
.SM PATH
The search path for commands (see
.I Execution\^
below).
.TP
.B
.SM MAIL
If this variable is set to the name of
a mail file, then the shell informs the user of
the arrival of mail in the specified file.
.TP
.SM
.B PS1
Primary prompt string, by default
.RB `` "$ \|" ''.
.TP
.SM
.B PS2
Secondary prompt string, by default
.RB `` "> \|" ''.
.TP
.SM
.B IFS
Internal field separators,
normally
.BR space ,
.BR tab ,
and
.BR new-line .
.PD
.RE
.PP
The shell gives default values to
\f3\s-1PATH\s+1\fP, \f3\s-1PS1\s+1\fP, \f3\s-1PS2\s+1\fP, and \f3\s-1IFS\s+1\fP,
while
.SM
.B HOME
and
.SM
.B MAIL
are
not set at all by the shell (although
.SM
.B HOME
.I is\^
set by
.IR login (1)).
.SS Blank Interpretation.
After parameter and command substitution,
the results of substitution are scanned for internal field separator
characters (those found in
.BR \s-1IFS\s+1 )
and split into distinct arguments where such characters are found.
Explicit null arguments (\^\f3"\^"\fP or \f3\*'\^\*'\fP\^) are retained.
Implicit null arguments
(those resulting from
.I parameters\^
that have no values) are removed.
.SS File Name Generation.
Following substitution, each command
.I word\^
is scanned for
the characters
.BR \(** ,
.BR ? ,
and
.BR \*(OK .
If one of these characters appears
then the word is regarded as a
.IR pattern .
The word is replaced with alphabetically sorted file names that match the pattern.
If no file name is found that matches the pattern, then
the word is left unchanged.
The character
.B .
at the start of a file name
or immediately following a
.BR / ,
as well as the character
.B /
itself,
must be matched explicitly.
.PP
.PD 0
.RS
.TP
.B \(**
Matches any string, including the null string.
.TP
.B ?
Matches any single character.
.TP
.BR \*(OK .\|.\|.\^ \*(CK
Matches any one of the enclosed characters.
A pair of characters separated by
.B \-
matches any
character lexically between the pair, inclusive.
.PD
.RE
.SS Quoting.
The following characters have a special meaning to the shell
and cause termination of a word unless quoted:
.RS
.PP
\f3;  &  (  )  \(bv  <  >  new-line  space  tab\fP
.RE
.PP
A character may be
.I quoted\^
(i.e., made to stand for itself)
by preceding
it with a
.BR \e .
The pair
.B \enew-line
is ignored.
All characters enclosed between a pair of single quote marks (\^\f3\*'\^\*'\fP\^),
except a single quote,
are quoted.
Inside double quote marks
(\f3"\^"\fP),
parameter and command substitution occurs and
.B \e
quotes the characters
.BR \e ,
.BR \*` ,
\f3"\fP,
and
.BR $ .
.B
"$\(**"
is equivalent to
\f3"$1 \|$2\fP \|.\|.\|.\f3"\fP,
whereas
.B
"$@"
is equivalent to
.B
"$1"\|
.B
"$2"\|
\&.\|.\|.\|.
.SS Prompting.
When used interactively,
the shell prompts with the value of
.SM
.B PS1
before reading a command.
If at any time a new-line is typed and further input is needed
to complete a command, then the secondary prompt
(i.e., the value of
.BR \s-1PS2\s+1 )
is issued.
.SS Input/Output.
Before a command is executed, its input and output
may be redirected using a special notation interpreted by the shell.
The following may appear anywhere in a simple-command
or may precede or follow a
.I command\^
and are
.I not\^
passed on to the invoked command;
substitution occurs before
.I word\^
or
.I digit\^
is used:
.PP
.PD 0
.TP 14
.B <word
Use file
.I word\^
as standard input (file descriptor 0).
.TP
.B >word
Use file
.I word\^
as standard output (file descriptor 1).
If the file does not exist then it is created;
otherwise, it is truncated to zero length.
.TP
.B >\h@-.3m@>word
Use file
.I word\^
as standard output.
If the file exists then output is appended to it (by first seeking to the end-of-file);
otherwise, the file is created.
.TP
\f3<\h@-.3m@<\fP\*(OK\f3\-\fP\*(CK\f3word\fP
The shell input is read up to a line that is the same as
.IR word ,
or to an end-of-file.
The resulting document becomes
the standard input.
If any character of
.I word\^
is quoted, then no interpretation
is placed upon the characters of the document;
otherwise, parameter and command substitution occurs,
(unescaped)
.B \enew-line
is ignored,
and
.B \e
must be used to quote the characters
.BR \e ,
.BR $ ,
.BR \*` ,
and the first character of
.IR word .
If
.B \-
is appended to
.BR <\h@-.3m@< ,
then all leading tabs are stripped from
.I word\^
and from the document.
.TP
.B <&digit
The standard input is duplicated from file descriptor
.I digit\^
(see
.IR dup (2)).
Similarly for the standard output using
.BR > .
.TP
.B <&\-
The standard input is closed.
Similarly for the standard output using
.BR > .
.PD
.PP
If one of the above is preceded by a digit,
then the
file descriptor created is that specified
by the digit
(instead of the default 0 or 1).
For example:
.RS
.PP
\&.\|.\|. \|2>&1
.RE
.PP
creates file descriptor 2 that is a duplicate
of file descriptor 1.
.PP
If a command is followed by
.B &
then the default standard input
for the command
is the empty file
.BR /dev/null .
Otherwise, the environment for the execution of a command contains the
file descriptors of the invoking shell as modified by
input/output specifications.
.SS Environment.
The
.I environment\^
(see
.IR environ (7))
is a list of name-value pairs that is passed to
an executed program in the same way as a normal argument list.
The shell interacts with the environment in several ways.
On invocation, the shell scans the environment
and creates a
parameter
for each name found,
giving it the corresponding value.
Executed commands inherit the same environment.
If the user modifies the values of these
parameters
or creates new ones,
none of these affects the environment
unless the
.B export
command is used to bind the shell's
parameter
to the environment.
The environment seen by any executed command is thus composed
of any unmodified name-value pairs originally inherited by the shell,
plus any modifications or additions,
all of which must be noted in
.B export
commands.
.PP
The environment for any
.I simple-command\^
may be augmented by prefixing it with one or more assignments to
parameters.
Thus:
.RS
.PP
\s-1TERM\s+1=450 \|cmd \|args				and
.br
(export \|\s-1TERM\s+1; \|\s-1TERM\s+1=450; \|cmd \|args)
.RE
.PP
are equivalent (as far as the above execution of
.I cmd\^
is concerned).
.PP
If the
.B \-k
flag is set,
.I all\^
keyword arguments are placed in the environment,
even if they occur after the command name.
The following
first prints
.B "a=b c"
and then
.BR c:
.PP
.RS
.nf
echo \|a=b \|c
set \|\-k
echo \|a=b \|c
.fi
.RE
.SS Signals.
The \s-1INTERRUPT\s+1 and \s-1QUIT\s+1 signals for an invoked
command are ignored if the command is followed by
.BR & ;
otherwise signals have the values
inherited by the shell from its parent,
with the exception of signal 11
(but see also
the
.B trap
command below).
.SS Execution.
Each time a command is executed, the above substitutions
are carried out.
Except for the
.I "Special Commands\^"
listed below, a new
process is created and
an attempt is made to execute the command via
.IR exec (2).
.PP
The shell parameter
.B
.SM PATH
defines the search path for
the directory containing the command.
Alternative directory names are separated by
a colon
.RB ( : ).
The default path is
.B :/bin:/usr/bin
(specifying the current directory,
.BR /bin ,
and
.BR /usr/bin ,
in that order).
Note that the current directory is specified by a null path name,
which can appear immediately after the equal sign
or between the colon delimiters anywhere else in the path list.
If the command name contains a \f3/\fP then the search path
is not used.
Otherwise, each directory in the path is
searched for an executable file.
If the file has execute permission but is not an
.B a.out
file,
it is assumed to be a file containing shell commands.
A sub-shell (i.e., a separate process) is spawned to read it.
A parenthesized command is also executed in
a sub-shell.
.SS Special Commands.
The following commands are executed in the shell process
and, except as specified,
no input/output redirection is permitted for such commands:
.PP
.PD 0
.TP
.B :
No effect; the command does nothing.
A zero exit code is returned.
.br
.TP
.BI ".\| " file\^
Read and execute commands from
.I file\^
and return.
The search path
specified by
.B
.SM PATH
is used to find the directory containing
.IR file .
.TP
\f3break\fP \*(OK \f2n\^\fP \*(CK
Exit from the enclosing \f3for\fP or
.B while
loop, if any.
If
.I n\^
is specified then break
.I n\^
levels.
.TP
\f3continue\fP \*(OK \f2n\^\fP \*(CK
Resume the next iteration of the enclosing
\f3for\fP or
.B while
loop.
If
.I n\^
is specified then resume at the
.IR n -th
enclosing loop.
.TP
\f3cd\fP \*(OK \f2arg\^\fP \*(CK
Change the current directory to
.IR arg .
The shell
parameter
.B
.SM HOME
is the default
.IR arg .
.br
.ne 2.1v
.TP
\f3eval\fP \*(OK \f2arg\^\fP .\|.\|. \*(CK
The arguments are read as input
to the shell
and the resulting command(s) executed.
.TP
\f3exec\fP \*(OK \f2arg\^\fP .\|.\|. \*(CK
The command specified by
the arguments is executed in place of this shell
without creating a new process.
Input/output arguments may appear and, if no other
arguments are given, cause the shell
input/output to be modified.
.TP
\f3exit\fP \*(OK \f2n\^\fP \*(CK
Causes a shell to exit
with the exit status specified by
.IR n .
If
.I n\^
is omitted then the exit status is that of the last command executed
(an end-of-file will also cause the shell to exit.)
.TP
\f3export\fP \*(OK \f2name\^\fP .\|.\|. \*(CK
The given
.IR name s
are marked
for automatic export to the
.I environment\^
of subsequently-executed commands.
If no arguments are given, then a list of all
names that are exported in this shell is printed.
.TP
\f3newgrp\fP \*(OK \f2arg\^\fP .\|.\|. \*(CK
Equivalent to
.BI "exec newgrp" " arg\^"
\&.\|.\|.\|.
.TP
\f3read\fP \*(OK \f2name\^\fP .\|.\|. \*(CK
One line is read from the standard input and
the first
word is assigned to the first
.IR name ,
the second word
to the second
.IR name ,
etc., with leftover words assigned to the last
.IR name .
The return code is 0 unless an end-of-file is encountered.
.TP
\f3readonly\fP \*(OK \f2name\^\fP .\|.\|. \*(CK
The given
.IR name s
are marked
.I readonly\^
and
the values of the these
.IR name s
may not be changed
by subsequent assignment.
If no arguments are given, then a list
of all
.I readonly\^
names is printed.
.TP
\f3set\fP \*(OK \f3\-ekntuvx\fP \*(OK \f2arg\^\fP .\|.\|. \*(CK \*(CK
.RS
.TP
.B \-e
If the shell is non-interactive then exit immediately if a command
exits with a non-zero exit status.
.TP
.B \-k
All keyword arguments are placed in the environment for a command,
not just those that precede the command name.
.TP
.B \-n
Read commands but do not execute them.
.TP
.B \-t
Exit after reading and executing one command.
.TP
.B \-u
Treat unset variables as an error when substituting.
.TP
.B \-v
Print shell input lines as they are read.
.TP
.B \-x
Print commands and their arguments as they are executed.
.TP
.B \-\-
Do not change any of the flags; useful in setting $1 to -.
.PP
Using
.B \+
rather than
.B \-
causes these flags to be turned off.
These flags can also be used upon invocation of the shell.
The current set of flags may be found in
.BR $\- .
The remaining arguments are positional
parameters and are assigned, in order, to
.BR $1 ,
.BR $2 ,
\&.\|.\|.\|.
If no arguments are given then the values
of all names are printed.
.RE
.TP
\f3shift\fP
.br
The positional parameters from
.B $2
\&.\|.\|.
are renamed
.B $1
\&.\|.\|.\|.
.TP
\f3test\fP
.br
Evaluate conditional expressions. See
.IR test (1)
for usage and description.
.TP
\f3times\fP
.br
Print the accumulated user and system times for processes
run from the shell.
.TP
\f3trap\fP \*(OK \f2arg\^\fP \*(CK \*(OK \f2n\^\fP \*(CK .\|.\|.
.I arg\^
is a command to be read and executed when the shell
receives signal(s)
.IR n .
(Note that
.I arg\^
is scanned once when
the trap is set and once when the trap
is taken.)
Trap commands are executed in order of signal number.
Any attempt to set a trap on a signal that
was ignored on entry to the current shell
is ineffective.
An attempt to trap on signal 11 (memory fault) produces an error.
If
.I arg\^
is absent then all trap(s)
.I n\^
are reset
to their original values.
If
.I arg\^
is the null
string then this signal is ignored by the shell and by the commands
it invokes.
If
.I n\^
is 0 then the command
.I arg\^
is executed
on exit from the shell.
The
.B trap
command
with no arguments prints a list
of commands associated with each signal number.
.TP
\f3umask\fP \*(OK \f2nnn\^\fP \*(CK
The user file-creation mask is set to
.I nnn\^
(see
.IR umask (2)).
If
.I nnn\^
is omitted, the current value of the mask is printed.
.TP
\f3wait\fP
Wait for all child processes to terminate
report the termination status.
If
.I n\^
is not given then all currently active child processes are waited for.
The return code from this command is always zero.
.PD
.PP
.SS Invocation.
If the shell is invoked through
.IR exec (2)
and the first character of argument zero
is
.BR \- ,
commands are initially read from
.B /etc/profile
and then from
.BR \s-1$HOME\s+1/.profile ,
if such files exist.
Thereafter, commands are read as described below, which
is also the case when the shell is invoked as
.BR /bin/sh .
The flags below are interpreted by the shell on invocation only; Note
that unless the 
.B \-c
or
.B \-s
flag is specified, the first argument is assumed to be the
name of a file containing commands, and the remaining
arguments are passed as positional parameters
to that command file:
.PP
.PD 0
.TP 10
.BI \-c "\| string\^"
If the
.B \-c
flag is present then
commands are read from
.IR string .
.TP
.B \-s
If the
.B \-s
flag is present or if no
arguments remain
then commands are read from the standard input.
Any remaining arguments specify the positional parameters.
Shell output is written to
file descriptor 2.
.TP
.B \-i
If the
.B \-i
flag is present or
if the shell input and output are attached to a terminal,
then this shell is
.IR interactive .
In this case \s-1TERMINATE\s+1 is ignored (so that \f3kill 0\fP
does not kill an interactive shell) and \s-1INTERRUPT\s+1 is caught and ignored
(so that
.B wait
is interruptible).
In all cases, \s-1QUIT\s+1 is ignored by the shell.
.TP
.B \-r
If the
.B \-r
flag is present the shell is a restricted shell
(see
.IR rsh (1)).
.PD
.PP
The remaining flags and arguments are described under the
.B set
command above.
.PP
.SH EXIT STATUS
Errors detected by the shell, such as syntax errors,
cause the shell
to return a non-zero exit status.
If the shell is being used non-interactively
then execution of the shell file is abandoned.
Otherwise, the shell returns the exit status of
the last command executed (see also the
.B exit
command above).
.SH FILES
/etc/profile
.br
\s-1$HOME\s+1/\f3.\fPprofile
.br
/tmp/sh\(**
.br
/dev/null
.SH SEE ALSO
cd(1),
env(1),
login(1),
newgrp(1),
rsh(1),
test(1),
umask(1),
dup(2),
exec(2),
fork(2),
pipe(2),
signal(2),
umask(2),
wait(2),
a.out(5),
profile(5),
environ(7).
.SH BUGS
The command
.B readonly
(without arguments) produces the same output as the command
.BR export .
.br
If
.B <\h@-.3m@<
is used to provide standard input to an
asynchronous process invoked by
.BR & ,
the shell gets mixed up about naming
the input document;
a garbage file
.B /tmp/sh\(**
is created and the shell complains about not being able
to find that file by another name.