BASH Scripting

What is it

A set of commands that can be executed in terminal
Should begin with the shebang line (#!/bin/bash)
Bash scripts ends with .sh

Variables

Store data and configuration options
Declared with name followed by "=" and the value
Call: Put $ in front of the variable name, enclose the name in {} if directly followed by something else

Examples


#!/bin/bash
a=/home/web
echo "My home folder is $a"
echo "My data folder is ${a}/data"

Arrays

Arrays are variables containing multiple values
Declared with name followed by "=()". Values are space separated.
Values can be accessed by their index starting from 0

Example


#!/bin/bash
# declare a array variable color
color=(red blue green)
# print all elements of the array
echo ${color[*]}
# set 4th element of the array
color[3]=yellow
echo ${color[*]}
# print third element of the array
echo "The sunflower is ${color[2]}"
# print the number of elements in array
echo ${#color[*]}

Variable Expansion

Variable names are exapnded within double quotes, but not within single quotes.

Example


#!/bin/bash
string="abcdefg"
echo "$string"
echo "\$string"
echo '$string'

echo $'\U1f60e'

For Loop

Repetition


for((i=1; i<=5; ++i)); do
    echo ${i}
done

Functions


#!/bin/bash
function greeter() { echo "Hello ${USER}!"; }
greeter


#!/bin/bash
function greeter() { echo "Hello $1!"; }
greeter "My Friend"

Testing Conditions

[ ! <expression> ] # expression is false
[ <expr1> -a <expr2> ] # both are true
[ <expr1> -o <expr2> ] # either is true
[ -n <string> ] # string length is nonzero
[ -z <string> ] # string length is zero
[ <string1> = <string2> ] # strings are equal
[ <string1> != <string2> ] # strings are not equal

Testing Conditions

[ <int1> -eq <int2> ] # integers are equal
[ <int1> -ne <int2> ] # integers are not equal
[ <int1> -lt <int2> ] # int1 is less than int2
[ <int1> -le <int2> ] # int1 is less or equal int2
[ <int1> -gt <int2> ] # int1 is greater than int2
[ <int1> -ge <int2> ] # int1 is greater or equal int2

Testing Conditions

[ -e <file> ] # file exists
[ -f <file> ] # file exists and is a regular file
[ -L <file> ] # file exists and is a symbolic link
[ -d <file> ] # file exists and is a directory
[ -s <file> ] # file exists and has size greater than 0
[ -r <file> ] # file exists and is readable
[ -x <file> ] # file exists and is executable

Control Structures: if/elif/else/fi


#!/bin/bash
function f() {
    if [ $1 -lt 0 ]; then
        echo "negative"
    elif [ $1 -le 9 ]; then
        echo "one digit"
    elif [ $1 -le 99 ]; then
        echo "two digits"
    else
        echo "large!"
    fi
}

f 21345890

While Loop

Repetition


#!/bin/bash

count=10
i=0

while [ $i -lt $count ]; do
   echo "$i"
   i=$(($i+1))
done

This Presentation

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Reference Book

- The Linux Command Line

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