An Introduction to Unix Shell Scripting

UNIX Shells

/bin/sh –Borne Shell/bin/csh – C Shell/bin/ksh – Korn Shell/bin/bash Borne Again Shell

Unix Shell Scripting

The Bourne shell is what we will use for scripting

It's portableVirtually every Unix system has sh installed

It has a rich set of programming constructsIt is arguable that it is the most popular UNIX shell.

BASH Comments

# is the comment symbol in the shell# can occur anywhere on a line

The shell ignores anything following a # until the end-of-line

One special exception#! on the first line is used to tell the shell what program to use to interpret the script fileExamples:

#!/bin/sh -tells shell to use the Bourne shell to execute the script#!/usr/bin/perl -tells shell to use Perl to execute the script

Shell Outputecho is the primary way to perform output from the shellSyntax: echo [-n] arguments

-n - do not append a NEWLINE (suppress the enter)arguments - may be variables or explicit strings

Examplesecho "Hi there, I'm a Unix whiz kid!"echo "My home directory is $HOME"echo $PATHecho

To suppress output from shell commands in a script, redirect the output to /dev/null

Assigning Shell VariablesTo store values in a shell variable, write the name of the variable followed by an = followed by the valuecount=1my_dir=/home/bobo

Note that spaces are NOT allowed on either side of the =Also, the shell has no concept of data typesNo matter what assignment you make, the shell considers the value as a string of charactersVariables don't need to be declared, they're simply assigned values when you want to use then

Referring to VariablesIn order to refer to the value of a variable, preface the variable name with a $

echo $count displays 1echo count displays count

echo $my_dir displays /home/boboecho my_dir displays my_dir

If you want to ensure a variable is not accidentally modified, specify it as readonly. Further attempts to modify it will result in an error from the shell.

my_dir=/home/boboreadonly my_dir

File Name Substitution & Variables

If you define a variable as x=* ls $x will produce a directory list of all filesDid the shell store the * in x or the list of files in your current directory?

File Name Substitution & Variables

The shell stored the * in x, the shell does not perform filename substitution when assigning values to variablesWhat actually happens is:

The shell scans ls $x, substituting * for $xIt then rescans the line and substitutes all the files in the current directory for the *

Environment

Create a file called firstscript that contains:

#!/bin/bashx=123echo the variable x equals $x

Save it and make it executable (chmod 744 firstscript)

Then execute firstscript - what happens?

Variables in the Environment

When variables are assigned, they are local to the current shellSince scripts are executed in a sub-shell, these local variables aren't visible to the scriptTo make them visible (inherited by) subsequent sub-shells, they must be exported

export my_dir

The env command lists all currently defined exported variables

Advanced echoThe echo command allows for special support of control characters. These special control features include such functions as new line, tab and bell

To Enable the interpretation of special control(escape) characters. One must use the –e switch echo –e “This will \n be on two lines”

echo escaped characters \a alert (bell)\b backspace\c suppress trailing newline\f form feed\n new line\r carriage return\t horizontal tab\v vertical tab\\ backslash\nnn The character whose ASCII code is nnn(octal)

Debugging Scripts with -x

When developing scripts, it is often difficult to debug them

In order to get a trace of what is happening, you can invoke your script by using the shell's -x option sh -x case.scr This will trace(show the commands) the

statements in the script as they are being executed

(similar to leaving echo on in batch files)

Command Line Arguments

Supposing you ran the scripted called big, using the following command.

big red sea

You would be providing your program with two variables. (red and sea)The program can use these variables.

Command Line Arguments

When the shell invokes your script, it assigns the command line arguments to a set of variables

$0, $1, $2,…$9$0 is the script name$1 is the first argument, $2 the second, up through $9You can then refer to these variables in your script.

What if you have more then 9 command line arguments?

If more than 9 arguments are used, you must access them using the shift command

shift simply does a left shift on all the arguments, discarding $1 and making $1 = $2, $2 = $3, etcNote, this means that if you still need $1 you must save it in another variable BEFORE performing the shift

Other Pre-defined Variables

$# - number of arguments on command line$* - collectively references all of the positional

parameters ($1, $2, $3)$0 - name of program being executed$$ - PID of current process$? - exit status of last command not executed in

background

Interactive Input

Aside from command line arguments input can also be provided by the user as the program in running.to accomplish this simply use the read command

The read Command

Syntax: read var1 var2 var3 …Example:

read text

This would wait for the user to input a string then the variable text would be set to it.

read gets input from STDIN so it can be redirected from a file

read text < data

QuotingQuotes in shell scripting have special

meaning.There are four types of quoting;

Single Quote ‘ (next to the enter key) removes the special meaning of all enclosed characters

Double Quote" removes the special meaning of all enclosed characters EXCEPT $, `, and \

Quoting

\<character> removes the special meaning of the character that follows the \; inside double quotes it removes the special meaning of $ ` " NEWLINE and \ but is otherwise not interpreted

Back Quote ` (matilda on the ~ key) executes command inside the quotes and inserts standard output at that point.

Examples of Back Quoting

echo Your current directory is `pwd` Outputs "Your current directory is

/home/bobo"

echo There are `who | wc -l` users logged on Outputs "There are 13 users logged on"

Single Quotes

Since single quotes protect everything, the following output should make sense:echo ‘`who | wc –l` tells how many users are logged on’

Outputs `who -| wc –l` tells how many users are logged on

But back quotes are interpreted inside “echo "You have `ls | wc -l` files in your directory"Outputs You have 24 files in your directory

Shell Arithmetic

The Bourne shell has no idea how to do arithmetic

For examplenumber=2number=$number + 4echo $number2 + 4

That makes shell scripting pretty useless as a programming language doesn't it?

expr

Fortunately, there is a Unix command that will allow us to perform arithmetic within a script

The expr command "evaluates" it's arguments and writes it's output on STDOUT

Example:expr 1 + 23expr 6 / 2 + 58

expr

Note, since it is evaluating arguments, they must be separated by spacesAlso, expr only works with integer arithmetic expressionsa=1, b=2, c=3expr $a / $b + $c = 3