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
-c Execute the commands in str
-e Quit on error
-i Interactive mode; ignore QUIT, TERMINATE, INTERRUPT
-k Look for name=value everywhere on command line
-n Do not execute commands
-q Change qflag from sig_ign to sig_del
-s Read commands from standard input
-t Exit after reading and executing one command
-v Echo input lines as they are read
-x Trace
-u Unset variables
EXAMPLES
sh script # Run a shell script
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).