If you want to get a little into bash, this article is for you. We will cover very basic commands in bash, so that after this, you can google everything else

Tested Configuration:
OS: Ubuntu 16.04 LTS

Basics

# declare a variable
# /!\ no SPACE: if you type var = "white rabbit", you'll have an error
var="white rabbit"

# read a variable, access its value (2 ways)
$var
${var}

# print a variable value
echo $var

# length
${#var}

# export through shell
export var
declare -x var

# print every variable defined in the shell
set

# removes a variable
unset var

Array

# store the variable inside the array "arr"
arr[0]=val

# read the first element of "arr"
${arr[0]}
$arr

# every element from "arr"
${arr[*]}

# 11th element from "arr"
${#arr[11]}

# number of element inside "arr"
${#arr[*]}

Cursor Position

# script name
$0

# positon parameters
$1 $2 ... ${10}   paramètres positionnels (1, 2 et 10)

# number of positon parameters
$#

# every positon parameters = $1 $2 ... ${n}      (2 ways)
$*
$@

# "$1 $2 ... ${n}"
"$*"

# "$1" "$2" ... "${n}"
"$@"

Specials


# reapeat last command
!!

# current shell PID
$$

# last background task PID
$!

# last command
$?

# home path
$HOME

# last dir path
$OLDPWD

# current shell PID
$PATH   liste des chemins de recherche des commandes exécutables

# Parent PID = PID of the mother of the shell
$PPID

#  Default interaction prompt
$PS1

# Continuation interactive prompt
$PS2

#  Prompt used by “select” inside shell script
$PS3

# Used by “set -x” to prefix tracing output -> xtrace DEBUG option
$PS4

# current path
$PWD

# random number
$RANDOM

# seconds this shell has lived
$SECONDS

Conditions

There are two identical ways for testing an expression:
[ XXXX ]
test XXXX
They both return 0 for true, and 1 for false

Well, there is a third way for testing, more bulletproof:
[[ XXXX ]] recommended version read more about it here

Note: in bash, we don’t use if with =. use eq, lt etc… instead

Note: the spaces inside the brackets are important => this is a wrong way [[XXXXXXX]]

[[ 2 = 2 ]]
echo $?

returns 0

[[ 2 = 3 ]]
echo $?

returns 1

Possible operators:

  1. Strings
    XXXX = YYYY
    XXXX != c2
    -z XXXX (true if empty)
    -n XXXX (true if not empty)

  2. Numbers XXXX -eq YYYY (equal)
    XXXX -ne YYYY (non equal)
    XXXX -lt YYYY (strictly lower than)
    XXXX -le YYYY (lower or equal)
    XXXX -gt YYYY (strictly greater than)
    XXXX -ge YYYY (greater or equal)

  3. Expressions
    ! XXXX (true if e is false)
    XXXX -a YYYY (true if XXXX and YYYY are true)
    XXXX -o YYYY (true if at least XXXX or YYYY is true)

Use conditions with if / else

if [[ condition ]]
then instruction(s)
else instruction(s)
fi

Example

read -p "Si vous etes d'accord entrez o ou oui : " reponse
if [ ! "$reponse" = "o" -a ! "$reponse" = "oui" ]; then
    echo "Non, je ne suis pas d'accord !"
else
    echo "Oui, je suis d'accord"
fi

Simple oneliner condition &&

if you want to trigger an action only if the previous one succeded, you can do

ls && echo "previous command succeded"

Note: || is the opposite, and ; will trigger both whatever the output of the first command

Create / append to a file

# creates or overwrites a file called read.txt
echo "hello tom" > read.txt

# read this file
cat read.txt

# append a new line to the same file
echo "hello siva" >> read.txt

# read this file again
cat read.txt
Advanced tip:

You can specify what will be outputted with these codes:
1 No error (stdout)
2 error (stderr)

# Nothing will happen (no errors)
echo "hello tom" 2>> error-log.txt

# will create succeded-log.txt and append "hello tom"
echo "hello tom" 1>> succeded-log.txt

# will create error-log.txt and append "-bash: notacommand: command not found"
notacommand 2>> error-log.txt

# will do both
mycommand 1>> succeded-log.txt 2>> error-log.txt

# shortcut when using the same log file
mycommand >> log.txt 2>&1

Check for an existing file

if [[ -f "README.md" ]]
then
    echo 'yes the README file exists'
else
    echo 'no it does not'
fi

Note: to check the existence of a directory, change the option from f to d like so : if [[ -d "myfolder" ]]

Check hardware configuration of your machine

lshw -short

Put a process in the background

This applies to the current shell environment. Let’s say you are editing a file with vi or nano. Temporarily, you want to check something in your machine. Then type :

bg

or you can also type Ctrl + z (Note: some editor can block this shortcut sometimes to avoid confusion)

And you editor will disappear. It’s because it was sent as a background process.

List all the tasks in the background
jobs
Get back your process
fg
# or if you know its number after typing 'jobs' (here the second one)
fg 2

More info here

Tip

& is a shortcut for running vi in the forground and ls -la in the background

vi log.txt & ls -la

Difference between && and |

command1 | command2 will take the output of command1 and provide it to command2
While
command1 && command2 will execute command2 only if command1 succeded

echo "test1" && echo "test2"
# outputs
> test1
> test2

echo "test1" | echo "test2"
# outputs
test2

What is /dev/null

it’s the the null device in a UNIX system : whatever you write to /dev/null will be discarded. more info here

Known use : commande >/dev/null 2>&1 to delete every output (stdout & stderr) of command

Template from HackerNews

#!/usr/bin/env bash

set -o errexit
set -o nounset
set -o pipefail
if [[ "${TRACE-0}" == "1" ]]; then
    set -o xtrace
fi

if [[ "${1-}" =~ ^-*h(elp)?$ ]]; then
    echo 'Usage: ./script.sh arg-one arg-two

This is an awesome bash script to make your life better.

'
    exit
fi

cd "$(dirname "$0")"

main() {
    echo do awesome stuff
}

main "$@"

Ressources

More about ps1, ps2 … thegeek
More about testing
More about checking if a file exist: linuxize
Difference between bash & sh
lshw manual and tips
bg & fg manual and tips
bg & fg [shorter]((https://til.secretgeek.net/linux/bg_fg_background_and_foreground.html)
very 4 to understand french documentation
explanation & how to use dev/null
from HackerNews 10/2022: best practises