Shell scripting

From Linux 101, The beginner's guide to all things Linux.

This is the fifth article on bash.

The Bash Articles Series
The shells and specifically bash | The bash environment | Processes and signals | Simple shell scripts | Shell scripting

Text below contributed by Robert Long of (Easy-Linux.com)

This falls into the bash series, but is written in a generic manner so as to apply to shell scripting across UNIX platforms. Many UNIX systems still do not default to bash as a system shell.

Defining shell to execute in: The first line of a shell script determines the shell used to run the script, also known as the interpreter.

The syntax is typically #!<shell path>

Examples:

Use system default shell (/bin/sh):

#!/bin/sh

Use bash shell (/bin/bash):

#!/bin/bash

Use perl interpreter (/usr/bin/perl):

#!/usr/bin/perl

Other common shells to find: ksh ash csh tcsh awk

Commonalities in most shell languages:

In most shell languages you have access to certain facilities, for example: Variable creation and storage, control flow (loops, conditionals), file status, system paths and accessible executables, standard functions for formatting data, file handles (such as STDIN/STDOUT/STDERR, etc), return/exit codes.

Creating a variable:

VARIABLE = "value";

Accessing a variable:

echo $VARIABLE;

conditionals with file status:

if( -e /path/to/file ) {

  do something; 

}

(depending on the shell you may need to end the if statement with

fi endif

or a similar terminator

Flow control:

while true do printf "number of sendmail processes on %s\t%d\n" $HOSTNAME `ps ax | grep sendmail | wc -l`; sleep 1; done

This will show you how many sendmail processes are running, and it will update once per second.

How about running the same command across a list of systems?

for i in `seq 1 9` do ssh host$i finger username done

this is equivalent to

for i in host1 host2 .... host 9 do ssh $i finger username done

where ... is host3 host4... etc

notice we have access to system available commands and tools.

These are very useful when monitoring systems as an administrator / systems engineer.