Bash Scripting

From ben.goodacre.name/tech

BASH (Bourne Again Shell) is a command shell and interpretor for Linux.

Contents 1 Parameters 2 Get IP address 3 Loops 3.1 Iterate through words in a string 3.2 Looping through a series of numbers 4 If 4.1 Numeric Operators 4.1.1 Single Braces 4.1.2 Double Parentheses 4.2 String Operators 4.2.1 Single Braces 4.2.2 Double Braces 4.3 File Operators 4.3.1 Single Braces 4.4 Test if number 5 Force run as root 6 Favorite Resources

Parameters

$# Number of parameters

$1 First parameter

$0 Name of the script

$@ All the parameters in one string

Get IP address

The following command will print the IP address by itself allowing you to pipe it into a command or an environment variable:

ifconfig|grep "inet addr:"|grep -v "127.0.0.1"|cut -d: -f2|awk '{ print $1}'

Loops

Iterate through words in a string

users="tom jerry ben"
for character in $users
do
  echo $character is a good name
done

Looping through a series of numbers

for i in `seq 1 5`;
do
  echo $i
done

If

oldusers="tom jerry sam micky minnie"
if [ ! -d /var/spool/mail ]; then
        echo /var/spool/mail not found - check location
else
        echo Checking /var/spool/mail
        for user in $oldusers
        do
        if [ -e /var/spool/mail/$user ]; then
                echo /var/spool/mail/$user exists
        fi
fi

Numeric Operators

Single Braces

-eq	equal
-ne	not equal
-gt	greater than
-ge	greater than or equal to
-lt	less than
-le	less than or equal to

Double Parentheses

Standard mathematical operators apply

<=	less than or equal to
>	greater than
>=	greater than or equal to

String Operators

Single Braces

=	equal
==	equal, but has file globbing and word splitting when wildcarded without quotes, and literal matching with quotes
!=	not equal
-z	string is null
-n	string is not null - MUST be quoted string but string should always be quoted anyway...

Double Braces

<	less than in ASCII order
>	more than in ASCII order
==	equal, but has pattern matching when wildcarded, literal matching with quotes

File Operators

Single Braces

• Unary tests:

-d	Directory
-e OR -a	Exists
-f	Regular file
-h OR -L	Symbolic link
-p	Named pipe
-r	Readable by you
-s	Not empty
-S	Socket
-w	Writable by you
-N	Has been modified since last being read

• Compare two files:

-nt	file1 is newer than file 2. File modification time stamp is used.
-ot	file1 is older than file 2. File modification time stamp is used.
- ef	file1 is a hard link to file2.

Test if number

[[ "$var" =~ ^[0-9]+([.][0-9]+)?$ ]] && echo $var is a number

Force run as root

if [ $UID != "0" ];then echo Only root can run this command 1>&2;exit 1;fi

Favorite Resources

BASH for beginners book
BASH Scripting Techniques
Steve Parker's Shell Scrpting Book
If
Operators and constructs