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

Testing

Infinite loop

Sometimes we need a simple test script, a simple infinite loop, for example to illustrate process commands (see Changing process priority).

#!/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

Backup and restore

Preliminaries

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

When doing:

$ echo {a,b}$PATH

the brace expansion does not expand the variable - this is done in a later step. Brace expansion just makes it being:

echo a$PATH b$PATH

Another common pitfall is to assume that a range like {1..200} can be expressed with variables using {$a..$b}. It simply is not possible, because it's the very first step in doing expansions. A possible way to achieve this, is using the eval 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})

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