Introduction to SH shell programming

Note: Like the /bin/csh shell page, here is a good introduction to /bin/sh programming.
Credit: I found this
here at North Carolina University...


Introduction

A shell is a command line interpreter. It takes commands and executes them. As such, it implements a programming language. The Bourne shell can be used to create shell scripts, i.e., programs that are interpreted/executed by the shell. You can write shell scripts with the C shell; however, this is not covered here.


Creating a Script

Suppose you often type the command

    find . -name file -print

and you'd rather type a simple command, say

    sfind file

Create a shell script like this:

    % cd ~/bin
% emacs sfind
% page sfind
find . -name $1 -print
% chmod a+x sfind
% rehash
% cd /usr/local/bin
% sfind tcsh
./shells/tcsh

Observations

This quick example is far from adequate but some observations:
  1. Shell scripts are simple text files created with an editor.
  2. Shell scripts are marked as executable
        %chmod a+x sfind
  3. Shell scripts should be located in your search path and ~/bin should be in your search path.
  4. You likely need to rehash if you're a Csh (tcsh) user (but not again when you login).
  5. Arguments are passed from the command line and referenced, for example, as $1.

Fundamentals

#!/bin/sh

All Bourne Shell scripts should begin with the sequence

    #!/bin/sh

From the man page for exec(2):

On the first line of an interpreter script, following the "#!", is the name of a program which should be used to interpret the contents of the file. For instance, if the first line contains "#!/bin/sh", then the con- tents of the file are executed as a shell script.

You can get away without this, but you shouldn't. All good scripts state the interpretor explicitly. Long ago there was just one (the Bourne Shell) but these days there are many interpreters -- csh, ksh, bash, and others.

Comments

Comments are any text beginning with the pound (#) sign. A comment can start anywhere on a line and continue until the end of the line.

Search Path

All shell scripts should include a search path specification:

    PATH=/usr/ucb:/usr/bin:/bin; export PATH

A PATH specification is recommended -- often times a script will fail for some people because they have a different or incomplete search path.

The Bourne Shell does not export environment variables to children unless explicitly instructed to do so by using the export command.

Argument Checking

A good shell script should verify that the arguments supplied (if any) are correct.

    if [ $# -ne 3 ]; then
echo 1>&2 Usage: $0 19 Oct 91
exit 127
fi

This script requires three arguments and gripes accordingly.

Exit status

All Unix utilities should return an exit status.

    # is the year out of range for me?

if [ $year -lt 1901 -o $year -gt 2099 ]; then
echo 1>&2 Year \"$year\" out of range
exit 127
fi

etc...

# All done, exit ok

exit 0

A non-zero exit status indicates an error condition of some sort while a zero exit status indicates things worked as expected.

On BSD systems there's been an attempt to categorize some of the more common exit status codes. See /usr/include/sysexits.h.

Using exit status

Exit codes are important for those who use your code. Many constructs test on the exit status of a command.

The conditional construct is:

    if command; then
command
fi

For example,

    if tty -s; then
echo Enter text end with \^D
fi

Your code should be written with the expectation that others will use it. Making sure you return a meaningful exit status will help.

Stdin, Stdout, Stderr

Standard input, output, and error are file descriptors 0, 1, and 2. Each has a particular role and should be used accordingly:

    # is the year out of range for me?

if [ $year -lt 1901 -o $year -gt 2099 ]; then
echo 1>&2 Year \"$year\" out of my range
exit 127
fi

etc...

# ok, you have the number of days since Jan 1, ...

case `expr $days % 7` in
0)
echo Mon;;
1)
echo Tue;;

etc...

Error messages should appear on stderr not on stdout! Output should appear on stdout. As for input/output dialogue:

    # give the fellow a chance to quit

if tty -s ; then
echo This will remove all files in $* since ...
echo $n Ok to procede? $c; read ans
case "$ans" in
n*|N*)
echo File purge abandoned;
exit 0 ;;
esac
RM="rm -rfi"
else
RM="rm -rf"
fi

Note: this code behaves differently if there's a user to communicate with (ie. if the standard input is a tty rather than a pipe, or file, or etc. See tty(1)).

<!-- -------------------------------------------------------------- -->

Language Constructs

for - loop iteration

Substitute values for variable and perform task:

    for variable in word ...
do
command
done

For example:

    for i in `cat $LOGS`
do
mv $i $i.$TODAY
cp /dev/null $i
chmod 664 $i
done

Alternatively you may see:

    for variable in word ...; do command; done

case - multiway branch

Switch to statements depending on pattern match:

    case word in
[ pattern [ | pattern ... ] )
command ;; ] ...
esac

For example:

    case "$year" in

[0-9][0-9])
year=19${year}
years=`expr $year - 1901`
;;
[0-9][0-9][0-9][0-9])
years=`expr $year - 1901`
;;
*)
echo 1>&2 Year \"$year\" out of range ...
exit 127
;;
esac

if - conditional execution

Test exit status of command and branch:

    if command
then
command
[ else
command ]
fi

For example:

    if [ $# -ne 3 ]; then
echo 1>&2 Usage: $0 19 Oct 91
exit 127
fi

Alternatively you may see:

    if command; then command; [ else command; ] fi

while/until - tested iterations

Repeat task while command returns good exit status.

    {while | until} command
do
command
done

For example:

    # for each argument mentioned, purge that directory

while [ $# -ge 1 ]; do
_purge $1
shift
done

Alternatively you may see:

    while command; do command; done

Variables

Variables are sequences of letters, digits, or underscores beginning with a letter or underscore. To get the contents of a variable you must prepend the name with a $.

Numeric variables (eg. like $1, etc.) are positional vari- ables for argument communication.

Variable Assignment

Assign a value to a variable by variable=value. For example:

    PATH=/usr/ucb:/usr/bin:/bin; export PATH

or

    TODAY=`(set \`date\`; echo $1)`

Exporting Variables

Variables are not exported to children unless explicitly marked.

    # We MUST have a DISPLAY environment variable

if [ "$DISPLAY" = "" ]; then
if tty -s ; then
echo "DISPLAY (`hostname`:0.0)? \c";
read DISPLAY
fi
if [ "$DISPLAY" = "" ]; then
DISPLAY=`hostname`:0.0
fi
export DISPLAY
fi

Likewise, for variables like the PRINTER which you want hon- ored by lpr(1). From a user's .profile:

    PRINTER=PostScript; export PRINTER

Note that the Cshell automatically exports all environment variables.

Referencing Variables

Use $variable (or, if necessary, ${variable}) to reference the value.

    # Most user's have a /bin of their own

if [ "$USER" != "root" ]; then
PATH=$HOME/bin:$PATH
else
PATH=/etc:/usr/etc:$PATH
fi

The braces are required for concatenation constructs.

$p_01

is the value of the variable "p_01".

${p}_01

is the value of the variable "p" with "_01" pasted onto the end.

Conditional Reference

${variable-word}

If the variable has been set, use it's value, else use word.

POSTSCRIPT=${POSTSCRIPT-PostScript};
export POSTSCRIPT

${variable:-word}

If the variable has been set and is not null, use it's value, else use word.

These are useful constructions for honoring the user environment, i.e., the user of the script can override variable assignments. Compare to programs like lpr(1), which honors the PRINTER environment variable. You can do the same trick with your shell scripts.

${variable:?word}

If variable is set use it's value, else print out word and exit. Useful for bailing out.

Arguments

Command line arguments to shell scripts are positional variables:

$0, $1, ...

The command and arguments, with $0 the command and the rest the arguments.

$#

The number of arguments.

$*, $@

All the arguments as a blank separated string. Watch out for "$*" vs. "$@".

Commands for Variables

shift

Shift the postional variables down one and decrement number of arguments.

set arg arg ...

Set the positional variables to the argument list.

Command line parsing uses shift:

    # parse argument list

while [ $# -ge 1 ]; do
case $1 in
process arguments...
esac
shift
done

A use of the set command:

    # figure out what day it is

TODAY=`(set \`date\`; echo $1)`

cd $SPOOL

for i in `cat $LOGS`
do
mv $i $i.$TODAY
cp /dev/null $i
chmod 664 $i
done

Special Variables

$$

Current process id. This is very useful for constructing temporary files.

         tmp=/tmp/cal0$$
trap "rm -f $tmp /tmp/cal1$$ /tmp/cal2$$"
trap exit 1 2 13 15
/usr/lib/calprog >$tmp

$?

The exit status of the last command.

         $command
# Run target file if no errors and ...

if [ $? -eq 0 ]
then
etc...
fi


Quotes/Special Characters

Special characters to terminate words:

      ; & ( ) | ^ < > new-line space tab

These are for command sequences, background jobs, etc. To quote any of these use a backslash (\) or bracket with quote marks ("" or '').

Single Quotes

Within single quotes all characters are quoted, including the backslash. The result is one word.

         grep :${gid}: /etc/group | awk -F: '{print $1}'

Double Quotes

Within double quotes you have variable substitution (i.e., the dollar sign is interpreted) but no file name generation (i.e., * and ? are quoted). The result is one word.

         if [ ! "${parent}" ]; then
parent=${people}/${group}/${user}
fi

Back Quotes

Back quotes mean run the command and substitute the output.

         if [ "`echo -n`" = "-n" ]; then
n=""
c="\c"
else
n="-n"
c=""
fi

and

         TODAY=`(set \`date\`; echo $1)`

Functions

Functions are a powerful feature that aren't used often enough. Syntax is

    name ()
{
commands
}

For example:

    # Purge a directory

_purge()
{
# there had better be a directory

if [ ! -d $1 ]; then
echo $1: No such directory 1>&2
return
fi

etc...
}

Within a function the positional parmeters $0, $1, etc. are the arguments to the function (not the arguments to the script).

Within a function use return instead of exit.

Functions are good for encapsulations. You can pipe, redi- rect input, etc. to functions. For example:

    # deal with a file, add people one at a time

do_file()
{
while parse_one

etc...
}

etc...

# take standard input (or a specified file) and do it.

if [ "$1" != "" ]; then
cat $1 | do_file
else
do_file
fi

Sourcing Commands

You can execute shell scripts from within shell scripts. A couple of choices:

sh command

This runs the shell script as a separate shell. For example, on Sun machines in /etc/rc:

         sh /etc/rc.local

. command

This runs the shell script from within the current shell script. For example:

         # Read in configuration information
. /etc/hostconfig

What are the virtues of each? What's the difference? The second form is useful for configuration files where environment variables are set for the script. For example:

    for HOST in $HOSTS; do

# is there a config file for this host?

if [ -r ${BACKUPHOME}/${HOST} ]; then
. ${BACKUPHOME}/${HOST}
fi
etc...

Using configuration files in this manner makes it possible to write scripts that are automatically tailored for differ- ent situations.


Some Tricks

Test

The most powerful command is test(1).

    if test expression; then

etc...

and (note the matching bracket argument)

    if [ expression ]; then

etc...

On System V machines this is a builtin (check out the com- mand /bin/test).

On BSD systems (like the Suns) compare the command /usr/bin/test with /usr/bin/[.

Useful expressions are:

test { -w, -r, -x, -s, ... } filename

is file writeable, readable, executeable, empty, etc?

test n1 { -eq, -ne, -gt, ... } n2

are numbers equal, not equal, greater than, etc.?

test s1 { =, != } s2

Are strings the same or different?

test cond1 { -o, -a } cond2

Binary or; binary and; use ! for unary negation.

For example

    if [ $year -lt 1901  -o  $year -gt 2099 ]; then
echo 1>&2 Year \"$year\" out of range
exit 127
fi

Learn this command inside out! It does a lot for you.

String matching

The test command provides limited string matching tests. A more powerful trick is to match strings with the case switch.

    # parse argument list

while [ $# -ge 1 ]; do
case $1 in
-c*) rate=`echo $1 | cut -c3-`;;
-c) shift; rate=$1 ;;
-p*) prefix=`echo $1 | cut -c3-`;;
-p) shift; prefix=$1 ;;
-*) echo $Usage; exit 1 ;;
*) disks=$*; break ;;
esac

shift

done

Of course getopt would work much better.

SysV vs BSD echo

On BSD systems to get a prompt you'd say:

    echo -n Ok to proceed?;  read ans

On SysV systems you'd say:

    echo Ok to procede? \c; read ans

In an effort to produce portable code we've been using:

    # figure out what kind of echo to use

if [ "`echo -n`" = "-n" ]; then
n=""; c="\c"
else
n="-n"; c=""
fi

etc...

echo $n Ok to procede? $c; read ans

Is there a person?

The Unix tradition is that programs should execute as quietly as possible, especially for pipelines, cron jobs, etc.

User prompts aren't required if there's no user.

    # If there's a person out there, prod him a bit.

if tty -s; then
echo Enter text end with \^D
fi

The tradition also extends to output.

    # If the output is to a terminal, be verbose

if tty -s <&1; then
verbose=true
else
verbose=false
fi

Beware: just because stdin is a tty that doesn't mean that stdout is too. User prompts should be directed to the user terminal.

    # If there's a person out there, prod him a bit.

if tty -s; then
echo Enter text end with \^D >&0
fi

Have you ever had a program stop waiting for keyboard input when the output is directed elsewhere?

Creating Input

We're familiar with redirecting input. For example:

    # take standard input (or a specified file) and do it.

if [ "$1" != "" ]; then
cat $1 | do_file
else
do_file
fi

alternatively, redirection from a file:

    # take standard input (or a specified file) and do it.

if [ "$1" != "" ]; then
do_file < $1
else
do_file
fi

You can also construct files on the fly.

    rmail bsmtp << EOF
helo news
mail from:<$1@newshost.uwo.ca>
rcpt to:<listserv@$3>
data
from: <$1@newshost.uwo.ca>
to: <listserv@$3>
Subject: Signon $2

subscribe $2 Usenet Feeder at UWO
.
quit
EOF

Note that variables are expanded in the input.

String Manipulations

One of the more common things you'll need to do is parse strings. Some tricks:

    TIME=`date | cut -c12-19`

TIME=`date | sed 's/.* .* .* \(.*\) .* .*/\1/'`

TIME=`date | awk '{print $4}'`

TIME=`set \`date\`; echo $4`

TIME=`date | (read u v w x y z; echo $x)`

With some care, redefining the input field separators can help.

    #!/bin/sh
# convert IP number to in-addr.arpa name

name()
{ set `IFS=".";echo $1`
echo $4.$3.$2.$1.in-addr.arpa
}

if [ $# -ne 1 ]; then
echo 1>&2 Usage: bynum IP-address
exit 127
fi

add=`name $1`

nslookup << EOF | grep "$add" | sed 's/.*= //'
set type=any
$add
EOF

Debugging

The shell has a number of flags that make debugging easier:

sh -n command

Read the shell script but don't execute the commands, i.e., check syntax.

sh -x command

Display commands and arguments as they're executed. In a lot of my shell scripts you'll see

    # Uncomment the next line for testing
# set -x

Based on An Introduction to Shell Programing by:

Reg Quinton <reggers@julian.uwo.ca>
Computing and Communications Services
The University of Western Ontario
London, Ontario N6A 5B7
Canada

The original version of this document is here.

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