Difference between revisions of "How to write a shell script"

Latest revision as of 20:28, 27 June 2007

Introduction
A Shell script is a program interpreted and executed by the shell, which is essentially a command line interpretor. So, think of a shell script as a list of commands that are run in sequence. This guide covers scripts created for the Borne shell and is based on Reg Quinton's Introduction to Shell Programing. 

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 
    % 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: 
Shell scripts are simple text files created with an editor. 
Shell scripts are marked as executeable 
    %chmod a+x sfind
Should be located in your search path and ~/bin should be in your search path. 
You likely need to rehash if you're a Csh (tcsh) user (but not again when you login). 
Arguments are passed from the command line and referenced. For example, as $1. 
#!/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 interpretors -- 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 specifica- tion: 
    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 sup- plied (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

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
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 Iteration

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 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
The value of the variable "p_01". 
${p}_01
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 envi- ronment. Ie. the user of the script can override variable assignments. Cf. programs like lpr(1) honor 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 vari- ables: 
$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. "$@". 
And, some commands: 
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 subsitution (ie. the dollar sign is interpreted) but no file name generation (ie. * 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 variable 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 procede?;  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 qui- etly 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 <
    rcpt to:
    data
    from: <$1@newshost.uwo.ca>
    to: 
    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
Pattern Matching

There are two kinds of pattern matching available, matching from the left and matching from the right. The operators, with their functions. Operator       Function        Example
${foo#t*is}    deletes the shortest possible match from the left       export $foo="this is a test"
echo ${foo#t*is}
is a test 
${foo##t*is}   deletes the longest possible match from the left        export $foo="this is a test"
echo ${foo#t*is}
a test 
${foo%t*st}    deletes the shortest possible match from the right      export $foo="this is a test" 
echo ${foo%t*st}
this is a 
${foo%%t*st}   deletes the longest possible match from the right       export $foo="this is a test"
echo ${foo#t*is}
  

Substitution

Another kind of variable mangling you might want to employ is substitution. There are four substitution operators in Bash. Operator        Function        Example
${foo:-bar}    If $foo exists and is not null, return $foo. If it doesn't exist, or is null, return bar.       export foo=""
echo ${foo:-one}
one
echo $foo
  
${foo:=bar}    If $foo exists and is not null, return $foo. If it doesn't exist, or is null, set $foo to bar and return barexport foo=""
echo ${foo:=one}
one
echo $foo
one 
${foo:+bar}    If $foo exists and is not null, return bar. If it doesn't exist, or is null, return a null.     export foo="this is a test" 
echo ${foo:+bar}
bar 
${foo:?"error message"}        If $foo exists and isn't null, return it's value. If it doesn't exist, or is null, print the error message. If no error message is given, it prints parameter null or not set.
Note: In a non-interactive shell, this will abort the current script. In an interactive shell, this will just print the error message.     export foo="one"
for i in foo bar baz; do
eval echo \${$foo:?}
one
bash: bar: parameter null or not set
bash: baz: parameter null or not set

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. IE. 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

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