Linux Scripting - Paiet/Tech-Journal-for-Everything GitHub Wiki

Managing the Shell Environment

• Login scripts:
  • /etc/profile
  • /etc/profile.d/*
  • ~/.profile
  • ~/.bash_profile
• Bash configuration:
  • /etc/bashrc
  • ~/.bashrc
• Logout script
  • ~/.bash_logout
• Search path for executables defined:
  • Globally in /etc/profile
  • Per user in ~/.profile or ~/.bash_profile
• When setting a variable, they are typically local to that specific shell instance.
  • Use export to convert them to global
    • export MESG
  • Add the following command to your .profile to make this automatic:
    • set -o allexport
• Aliases
  • Add these to the .bashrc to automate commands
    • alias ls='ls --color=auto'
    • alias ls='ls --color=auto -la'
    • unalias ls
• Functions
  • Add to .bashrc to create custom command sets
  • Can combine multiple commands in to a single command
  • Example system status function:

To Begin

function sysinfo()
	{
	echo -e "\nKernel Information:" ; uname -a
	echo -e "\nOS Version:" ; cat /etc/centos-release
	echo -e "\nSystem Uptime:" ; uptime
	echo -e "\nMemory Utilization:" ; free -m
	echo -e "\nFilesystem Utilization:"; df -h
	}

• Update /etc/skel/.bashrc to populate values for new users

Writing Scripts

Example script

#!/bin/bash
MESG="This is my message"
echo $MESG

• Set execute permissions chmod +x ./sample.sh

• Add folder to PATH if you wish to call it without entering the directory

• Basic Commands:

  • echo - Displays text on screen
  • echo -e - Displays text on screen and then awaits user input
  • read - Copies user input in to a variable
  • if/then/else - Performs a calculation and then acts on the results

Sample script to create a user account

#!/bin/bash
echo -e "Username:"
read username
if id -u $username > -1; then
	echo "That username already exists"
else
	echo -e "Full Name:"
	read fullname
	echo -e "Password:"
	read password
	useradd -c "$fullname" $username -p $password
	if id -u $username > -1; then
		echo "User created successfully"
	else
		echo "An error occurred while creating the user"
	fi
fi

• More Commands:
  • for - Perform an action for each entry in a list
  • do - Execute an action
  • seq - Generate a list of numbers
  • while - Perform an action if certain conditions are met and loop
  • test - Perform a comparison and return true or false

Solution #1: Sample script to create ten files called video#.mp4

#!/bin/bash
for filenum in `seq 10`
do
    touch video$filenum.mp4
done

Solution #2: Sample script to create ten files called video#.mp4

#!/bin/bash
filenum=10
while [ $filenum -gt 0 ]
do
    touch video$filenum.mp4
    filenum=$(($filenum - 1))
done

Test if a user already has a home directory:

#!/bin/bash
echo -e "Username:"
read username
test -d /home/$username && echo "Home Directory Exists" || echo "Home Directory Does Not Exist"