1. What is bash and why would you use it?

• bash is a Unix shell and commmand language, which makes performing any task you want on Unix/Linux system programatically

• anything you’ve ever done using OS GUI can be done with bash. The main benefit is the possibility to automate any task and keep track of its changes with git

2. Why am I writing this tutorial?

• bash is the most basic Linux

disclaimer: all the GNU/Linux/Unix systems I call Linux. I think it became a standard now. I know it’s incorrect (e.g. OSX), but shorter and everybody seems to understand ;)

skill but it is quite oldschool and sometimes daunting, especially because it probably is not you main programming language, as a developer. Even the simplest commands (if statements, assigning variables, executing commands, for loops) have rather non-intuitive, quirky syntax. This tutorial will hopefully serve me as a reference during future development.

3. Examples

shebang

Two possible ways:

#!/bin/bash
#!/usr/bin/env bash

The second one is more popular.

running code

There are at least three possible ways to execute a bash script:

• bash script.sh - creates a new bash session and runs the code in it. The variables in your curretn session will be available, however if you exit your code, the current (~host) session will not exit. If you assign a variable using export, it will not be available further on.

• source script.sh and .script.sh - so far I haven’t noticed any differences. If there are differences, sorry for my ignorance.

variables assignment

• simple interior assignment - these variables are available only in the script

x=10
var=hi

• exterior assignment - these variables will be available after the script was run, unless it was run with bash script.sh command:

export var=hello

• assigment of the result of executed statement - if you want to execute a statement in shell and assign its result to the variable, use the following sytnax:

val=$(date)
var=$(ls | wc -l)

if statement

Keep in mind that you have to leave spaces between a condition inside square brackets and the brackets themselves:

if [ hi == hi ]
then
    echo yes
elif [ hi == hello ]
then
    echo no
else
    echo what
fi

A common practice is to write if [ hi == hi ]; then in one line.

Logical operators:

• not [ ! 1 == 1 ]

• and [ cond1 && cond2 ]

• or [ cond1 || cond2 ]

• is equal - for strings it’s simply ==, while for integers it’s [[ $var -eq 1 ]]. String comparison should be sufficient for most cases, unless you explicitly declare integers.

• combining [[ cond1 ]] && [[ cond2 || cond3 ]]

Check out this link for more detailed information.

for loop

There are many objects you can iterate over. You will find a pretty exhaustive list here with examples of break and continue. And here’s a trivial example:

for i in 1 2 3 4 5
do
   echo $i
done

4. Links

• excellent bash cheatsheet