sh.1

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Use file
.I word
as standard output.  If the file exists, output is appended (by seeking
to the end); otherwise the file is created.
.IP <<\fI\|word
The shell input is read up to a line the same as
.IR word ,
or end of file.  The resulting document becomes the standard input.  If
any character of
.I word
is quoted, no interpretation is placed upon the characters of the
document; otherwise, parameter and command substitution occurs,
.B
\enew-line
is ignored, and
.B
\e
is used to quote the characters
.B
\e $ \'
and the first character of
.I word.
.IP <\|&\fI\|digit
The standard input is duplicated from file descriptor
.I digit;
see
.MS dup 2 .
Similarly for the standard output using >\|.
.IP <\|&\|\-
The standard input is closed.  Similarly for the standard output using
>\|.
.PD
.PP
If one of the above is preceded by a digit, the file descriptor created
is that specified by the digit (instead of the default 0 or 1).  For
example,
.PP
	\&... 2>&1
.PP
creates file descriptor 2 to be 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
.PN /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.
.PP
.B Environment
.br
.NX R "sh command interpreter" "environment"
The environment is a list of name-value pairs that is passed to an
executed program in the same way as a normal argument list; see
.MS execve 2 
and
.MS environ 7 .
The shell interacts with the environment in several ways.
On invocation, the shell scans the environment and creates a
.I parameter
for each valid name found (except IFS),
giving it the corresponding value.
(IFS cannot be set by the environment;
it can only be set in the current shell session.)
Executed commands inherit the same environment.
If the user modifies the values of these
.I parameters
or creates new ones, none of these affects the environment unless the
.B export
command is used to bind the shell's
.I 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
.I parameters.
Thus these two lines are equivalent
.IP
TERM=450 cmd args
.br
(export TERM; TERM=450; cmd args)
.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 prints 'a=b c' and 'c':
.nf
echo a=b c
set \-k
echo a=b c
.fi
.PP
.B Signals
.br
.NX R "sh command interpreter" "signals"
The INTERRUPT and QUIT 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.
(But see also
.BR trap. )
.PP
.B Execution
.br
.NX R "sh command interpreter" "executing commands"
Each time a command is executed, the above substitutions are carried out.
Except for the special commands listed below, a new process is created and
an attempt is made to execute the command with an
.MS execve 2 .
.PP
The shell parameter
.B
$PATH
defines the search path for the directory containing the command.
Each alternative directory name is separated by a colon
.RB ( : ).
The default path is
.BR :/bin:/usr/bin .
If the command name contains a /, 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
.I a.out
file, it is assumed to be a file containing shell commands.
A subshell (that is, a separate process) is spawned to read it.
A parenthesized command is also executed in a subshell.
.PP
.B Special commands
.br
.NX R "sh command interpreter" "special commands"
The following commands are executed in the shell process
and, except where specified, 
no input output redirection is permitted for such commands.
.TP
.B :
No effect; the command does nothing.
.PD 0
.TP
.BI . \ file
Read and execute commands from
.I file
and return.  The search path
.B
$PATH
is used to find the directory containing
.IR file .
.TP
.B break
.RI
[\  n \ ]
.NX R "break command (sh)"
Exit from the enclosing
.B for
or
.B while
loop, if any.
If
.I n
is specified, break
.I n
levels.
.TP
.B continue
.RI
[\  n \ ]
.NX R "continue command (sh)"
Resume the next iteration of the enclosing
.B for
or
.B while
loop.  If
.I n
is specified, resume at the
.IR n th
enclosing loop.
.TP
\.B cd
.RI
[\  arg \ ]
.NX R "cd command (sh)"
Change the current directory to
.I arg.
The shell parameter
.B
$HOME
is the default
.IR arg .
.TP
.B eval
.RI
[\  arg ...\ ]
.NX R "eval command (sh)"
The arguments are read as input to the shell
and the resulting command(s) executed.
.TP
.B exec
.RI
[\  arg ...\ ]
.NX R "exec command (sh)"
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
.B exit
.RI
[\  n \ ]
.NX R "exit command (sh)"
Causes a noninteractive shell to exit with the exit status specified by
.I n.
If
.I n
is omitted, the exit status is that of the last command executed.  (An
end of file will also exit from the shell.)
.TP
.B export
.RI
[\  name ...\ ]
.NX R "export command (sh)"
The given names are marked for automatic export to the
.I environment
of subsequently executed commands.  If no arguments are given, a list
of exportable names is printed.
.TP
.B login
.RI
[\  arg ...\ ]
.NX R "login command (sh)"
Equivalent to 'exec login arg ...'.
.TP
.B read
.I name ...
.NX R "read command (sh)"
One line is read from the standard input; successive words of the input
are assigned to the variables
.I name
in order, with leftover words to the last variable.  The return code is
0 unless the end-of-file is encountered.
.TP
.B readonly
.RI
[\  name ...\ ]
.NX R "readonly command (sh)"
The given names are marked readonly and the values of these names may
not be changed by subsequent assignment.  If no arguments are given, a
list of all readonly names is printed.
.TP
.B set
.RI
[\  \-eknptuvx [\  arg ...\ ]\ ]
.NX R "set command (sh)"
.RS
.PD 0
.TP 3m
.B \-e
If noninteractive, exit immediately if a command fails.
.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 \-
Turn off the
.B \-x
and
.B \-v
options.
.PD
.PP
These flags can also be used upon invocation of the shell.  The current
set of flags may be found in
.BR $\- .
.PP
Remaining arguments are positional parameters and are assigned, in
order, to
.BR $1 ,
.BR $2 ,
and so forth.  If no arguments are given, the values of all names are
printed.
.RE
.TP
.B shift
.NX R "shift command (sh)"
The positional parameters from
.BR $2 ...
are renamed
.BR $1 ...
.TP
.B times
.NX R "times command (sh)"
Print the accumulated user and system times for processes run from the
shell.
.TP
.B trap
.RI
[\  arg \ ]\ [\  n \ ] ...
.NX R "trap command (sh)"
The
.I arg
is a command to be read and executed when the shell receives signal(s)
.I n.
(Note that
.I arg
is scanned once when the trap is set and once when the trap is taken.)
Trap commands are executed in order of signal number.  If
.I arg
is absent, all trap(s)
.I n
are reset to their original values.
If
.I arg
is the null string, this signal is ignored by the shell and by invoked
commands.  If
.I n
is 0, the command
.I arg
is executed on exit from the shell, otherwise upon receipt of signal
.I n
as numbered in
.MS sigvec 2 .
The
.I trap
with no arguments prints a list of commands associated with each signal
number.
.TP
\fBumask \fR[\  \fInnn\fR \ ]
.NX R "umask command (sh)"
The user file creation mask is set to the octal value
.I nnn .
For further information, see 
.MS umask 2 .
If
.I nnn
is omitted, the current value of the mask is printed.
.TP
\fBwait\fP
.NX R "wait command (sh)"
Wait for all child background processes to terminate.  The return code
from this command is that returned upon termination of the last process
being waited for.
.PD
.PP
.PP
.B Invocation
.br
If the first character of argument zero is
.BR \- ,
commands are read from
.BR \s-2$HOME\s0/.\|profile ,
if such a file exists.
Commands are then read as described below.  The following flags are
interpreted by the shell when it is invoked.
.PD 0
.TP 11n
.BI \-c \ string
If the
.B \-c
flag is present, commands are read from
.I string\|.
.TP 11n
.B \-s
If the
.B \-s
flag is present or if no arguments remain, then commands are read from
the standard input.  Shell output is written to file descriptor 2.
.TP 11n
.B \-i
If the
.B \-i
flag is present or if the shell input and output are attached to a
terminal (as told by
.B gtty), 
then this shell is
.I interactive.
In this case the terminate signal SIGTERM is ignored (so that 'kill
0' does not kill an interactive shell) and the interrupt signal SIGINT
is caught and ignored (so that
.B wait
is interruptible).  For further information, see
.MS sigvec 2 .
In all cases SIGQUIT is ignored by the shell.
.PD
.PP
The remaining flags and arguments are described under the
.B set
command.
.SH Restrictions
.NX R "shell command interpreter" "restricted"
If
.B
<<
is used to provide standard input to an asynchronous process invoked by
.BR & ,
the shell becomes confused about naming the input document.  A garbage
file
.PN /tmp/sh*
is created, and the shell complains about not being able to find the
file by another name.
.PP
The 
.PN sh
command is not 8-bit clean. The 
.PN sh5
command is 8-bit clean.
.PP
.B VAX Only Restriction
.br
If 
.PN sh 
is run from another program (by the system or exec system calls) 
whose maximum descriptor in use is number 10, the prompt string 
is not printed. 
.SH Diagnostics
.NX R "shell command interpreter" "diagnostics"
Errors detected by the shell, such as syntax errors cause the shell to
return a nonzero exit status.  If the shell is being used
noninteractively, then execution of the shell file is abandoned.
Otherwise, the shell returns the exit status of the last command
executed (see also
.BR exit ).
.SH Files
.PN $HOME/.profile
.br
.PN /tmp/sh*
.br
.PN /dev/null
.SH See Also
csh(1), sh5(1), test(1), execve(2), environ(7)
.NX R "sh command interpreter"