View source for Shell scripting

Shell scripts are good for automating repetitive shell tasks. Bash and other shells include the “usual” constructs found in programming languages, such as for loops, tests, if and case statements, but there is only one type of variable: strings.

== Shells ==
Unix has variety of Shells. Bourne shell (sh), Bourne again shell (bash), C shell (csh), Korn shell (ksh), Tenex C shell (tcsh). Use the which or whereis unix commands to find out where a specific shell is located:
 # which bash
 /bin/bash
or:
 # whereis bash
 bash: /bin/bash /etc/bash.bashrc /usr/share/man/man1/bash.1.gz

== Hello World ==

== Print system related information ==

== Environment ==

== Testing ==

Sometimes we need a simple test script, a simple infinite loop (see [[Kinky_linux_command-line#Changing_process_priority|Changing process priority]] to illustrate process commands.

<code>#!/bin/bash
# set an infinite loop
while :
do
	clear
        # display menu
        echo "Server Name - $(hostname)"
	echo "-------------------------------"
	echo "     M A I N - M E N U"
	echo "-------------------------------"
	echo "1. Display date and time."
	echo "2. Display all processes owned by user (with CPU/MEM)."
	echo "3. Display network connections."
	echo "4. Exit"
        # get input from the user 
	read -p "Enter your choice [ 1 -4 ] " choice
        # make decision using case..in..esac 
	case $choice in
		1)
			echo "Today is $(date)"
			read -p "Press [Enter] to continue..." readEnterKey
			;;
		2) 
		# Show all running processes (with CPU/MEM) of user
			ps -u user u	
			read -p "Press [Enter] to continue..." readEnterKey
			;;
		3) 
		# show all tcp connections with no dns resolution (no reverse dns lookups are done)
			netstat -nat
			read -p "Press [Enter] to continue..." readEnterKey
			;;
		4)
			echo "Goodbye!"
			exit 0
			;;
		*)
			echo "Error: Out of my range ..."	
			read -p "Press [Enter] to continue..." readEnterKey
			;;
	esac		
 
done
</code>

== Backup and restore ==

=== Preliminaries ===

The brace expansion is present in two basic forms, <strong>string lists</strong> and <strong>ranges</strong>. It can be switched on and off under runtime by using the <code>set</code> builtin and the option <code>-B</code> and <code>+B</code> or the long option <code>braceexpand</code>. If brace expansion is enabled, the stringlist in <code>SHELLOPTIONS</code> contains <code>braceexpand</code>.

When doing:
 $ echo {a,b}$PATH
the brace expansion <strong>does not expand the variable</strong> - this is done in a <strong>later step</strong>. Brace expansion just makes it being:
 echo a$PATH b$PATH

Another common pitfall is to assume that a range like <code>{1..200}</code> can be expressed with variables using <code>{$a..$b}</code>. It <strong>simply is not possible</strong>, because it's the very first step in doing expansions. A possible way to achieve this, is using the <code>eval</code> command, which basically evaluates a commandline twice: 
 eval echo {$a..$b}

For instance, when embedded inside a for loop in a shell script: 
 for i in $(eval echo {$a..$b})

=== Backup script ===

=== Restore script ===

== Arp sweeps ==
If you regularly do arp sweeps of your network, you can use arp-scan or develop your own from this basic script:
 #!/bin/bash
 PREFIX=$1
 INTERFACE=$2
 for SUBNET in {1..255}
 do
    for HOST in {1..255}
    do
       echo "[*] IP : "$PREFIX"."$SUBNET"."$HOST
       arping –c 3 –i $INTERFACE $PREFIX"."$SUBNET"."$HOST 2> /dev/null
    done
 done
If named arpsweep (or arpsweep.sh) call with (for example):
 $ arpsweep 192.168 eth0