SH(1)

NAME
sh, ., break, case, cd, continue, eval, exec, exit, export, for, if, read, readonly, set, shift, trap, umask, wait, while - shell

SYNOPSIS
sh [-eiknqstvxu] [-c str] [file]

OPTIONS

EXAMPLES

DESCRIPTION

Sh is the shell, which forms the user's main interface with the system. On startup, the shell reads /etc/profile and $HOME/.profile, if they exist, and executes any commands they contain. The Minix shell has most of the features of the V7 (Bourne) shell, including redirection of input and output, pipes, magic characters, background processes, and shell scripts. A brief summary follows, but whole books have been written on shell programming alone.

Some of the more common notations are:

date    #    Regular command
sort <file    #    Redirect stdin (standard input)
sort <file1  >file2    #    Redirect stdin and stdout
cc file.c  2>error    #    Redirect stderr
a.out >f  2>&1    #    Combine standard output and standard error
sort <file1  >>file2    #    Append output to file2
sort <file1  >file2 &    #    Background job
(ls -l; a.out) &    #    Run two background commands sequentially
sort <file | wc    #    Two-process pipeline
sort <f | uniq | wc    #    Three-process pipeline
ls -l *.c    #    List all files ending in .c
ls -l [a-c]*    #    List all files beginning with a, b, or c
ls -l ?    #    List all one-character file names
ls \?    #    List the file whose name is question mark
ls ′???′    #    List the file whose name is three question marks
v=/usr/ast    #    Set shell variable v
ls -l $v    #    Use shell variable v
PS1=′Hi! ′    #    Change the primary prompt to Hi!
PS2=′More: ′    #    Change the secondary prompt to More:
ls -l $HOME    #    List the home directory
echo $PATH    #    Echo the search path
echo $?    #    Echo exit status of previous command in decimal
echo $$    #    Echo shell's pid in decimal
echo $!    #    Echo PID of last background process
echo $#    #    Echo number of parameters (shell script)
echo $2    #    Echo second parameter (shell script)
echo "$2"    #    Echo second parameter without expanding spaces
echo $*    #    Echo all parameters (shell script)
echo $@    #    Echo all parameters (shell script)
echo "$@"    #    Echo all parameters without expanding spaces

The shell uses the following variables for specific purposes:

SHELL    the path of the current shell
HOME    the default value for the cd(1) command
PATH    the directories to be searched to find commands
IFS    the internal field separators for command strings
PS1    the primary shell prompt
PS2    the secondary shell prompt

There are various forms of substitution on the shell command line:

`...`    Command string between back-quotes is replaced by its output
"..."    Permits variable substitution between quotes
'...'    Inhibits variable substitution between quotes
$VAR    Replaced by contents of variable VAR
${VAR}    Delimits variable VAR from any following string

The expressions below depend on whether or not VAR has ever been set. If VAR has been set, they give:

${VAR-str}    Replace expression by VAR, else by str
${VAR=str}    Replace expression by VAR, else by str and set VAR to str
${VAR?str}    Replace expression by VAR, else print str and exit shell
${VAR+str}    Replace expression by str, else by null string

If a colon is placed after VAR, the expressions depend on whether or not VAR is currently set and non-null.

The shell has a number of built-in commands:

:    return true status
. fn    execute shell script fn on current path
break [n]    break from a for, until or while loop; exit n levels
continue [n]    continue a for, until or while loop; resume nth loop
cd [dir]    change current working directory; move to $HOME
eval cmd    rescan cmd, performing substitutions
eval    rescan the current command line
exec cmd    execute cmd without creating a new process
exec <|>    with no command name, modify shell I/O
exit [n]    exit a shell program, with exit value n
export [var]    export var to shell's children; list exported variables
pwd    print the name of the current working directory
read var    read a line from stdin and assign to var
readonly [var]    make var readonly; list readonly variables
set -f    set shell flag (+f unsets flag)
set str    set positional parameter to str
set    show the current shell variables
shift    reassign positional parameters (except ${0}) one left
times    print accumulated user and system times for processes
trap arg sigs    trap signals sigs and run arg on receipt
trap    list trapped signals
umask [n]    set the user file creation mask; show the current umask
wait [n]    wait for process pid n; wait for all processes

The shell also contains a programming language, which has the following operators and flow control statements:

#    Comment    The rest of the line is ignored
=    Assignment    Set a shell variable
&&    Logical AND    Execute second command only if first succeeds
||    Logical OR    Execute second command only if first fails
(...)    Group    Execute enclosed commands before continuing

for    For loop (for ... in ... do ... done)
case    Case statement ((case ... ) ... ;; ... esac)
esac    Case statement end
while    While loop (while ... do ... done)
do    Do/For/While loop start (do ... until ...)
done    For/While loop end
if    Conditional statement (if ... else ... elif ... fi)
in    For loop selection
then    Conditional statement start
else    Conditional statement alternative
elif    Conditional statement end
until    Do loop end
fi    Conditional statement end

SEE ALSO
echo(1), expr(1), pwd(1), true(1).