Tag: bash

Basic If Statements

#!/bin/bash
# Basic if statement
if [ $1 -gt 100 ]
then
echo Hey that\'s a large number.
pwd
fi
date

If Else

#!/bin/bash
# else example
if [ $# -eq 1 ]
then
nl $1
else
nl /dev/stdin
fi

If Elif Else

#!/bin/bash
# elif statements
if [ $1 -ge 18 ]
then
echo You may go to the party.
elif [ $2 == 'yes' ]
then
echo You may go to the party but be back before midnight.
else
echo You may not go to the party.
fi

Boolean Operations

#!/bin/bash
# and example
if [ -r $1 ] && [ -s $1 ]
then
echo This file is useful.
fi

#!/bin/bash
# or example
if [ $USER == 'bob' ] || [ $USER == 'andy' ]
then
ls -alh
else
ls
fi

Multiple logical operators, ((A || B) && C)

use double brackets

if [[ ($A -eq 0 || $B -ne 0) && $C -eq 0 ]]; then …

References
https://ryanstutorials.net/bash-scripting-tutorial/bash-if-statements.php
https://unix.stackexchange.com/questions/290146/multiple-logical-operators-a-b-c-and-syntax-error-near-unexpected-t

Ask the User for Input

#!/bin/bash
# Ask the user for their name
echo Hello, who am I talking to?
read varname
echo It\'s nice to meet you $varname

More variables

#!/bin/bash
# Demonstrate how read actually works
echo What cars do you like?
read car1 car2 car3
echo Your first car was: $car1
echo Your second car was: $car2
echo Your third car was: $car3

More with Read

#!/bin/bash
# Ask the user for login details
read -p 'Username: ' uservar
read -sp 'Password: ' passvar
echo
echo Thankyou $uservar we now have your login details

-p allows you to specify a prompt and -s makes the input silent

References
https://ryanstutorials.net/bash-scripting-tutorial/bash-input.php

source is a bash shell built-in command that executes the content of the file passed as argument, in the current shell

Syntax

. filename [arguments]
source filename [arguments]

References
https://superuser.com/questions/46139/what-does-source-do

mkdir -p foo

mkdir -p foo/bar/baz

References
https://stackoverflow.com/questions/793858/how-to-mkdir-only-if-a-dir-does-not-already-exist

[ "$UID" -eq 0 ] || exec sudo "$0" "$@"

References
https://askubuntu.com/questions/746350/request-root-privilege-from-within-a-script/748362
https://unix.stackexchange.com/questions/28791/prompt-for-sudo-password-and-programmatically-elevate-privilege-in-bash-script
https://stackoverflow.com/questions/3976362/bash-scripts-requiring-sudo-password

You can redirect the output of a command to a file:

cat file > copy_file

or append to it

cat file >> copy_file

If you want to write directly the command is echo ‘text’

echo 'Hello World' > file

Or

# possibility 1:
echo "line 1" >> greetings.txt
echo "line 2" >> greetings.txt

# possibility 2:
echo "line 1
line 2" >> greetings.txt

# possibility 3:
cat <<EOT >> greetings.txt
line 1
line 2
EOT

 

References
https://stackoverflow.com/questions/11162406/open-and-write-data-to-text-file-using-bash-shell-scripting
https://unix.stackexchange.com/questions/77277/how-to-append-multiple-lines-to-a-file

#!/bin/bash

References :

http://askubuntu.com/questions/223691/how-do-i-create-a-script-file-for-terminal-commands