bash

A repository of valuable information around the bash scripting

Main command lines :

pwd :

Indicates in which directory the shell is running

mkdir :

Creates a directory. It has many options, the main ones are :

• v for verbose to say that the script created the directory and
• p to has idempotent command, which means that if the directory already exists it won't return an error

less, head and tail :

• Less : To read a text file, you can add the -N to have the line number
• Tail, head : returns by default the 10 last/recent lines in a file. You can specify the number of lines with -n

ls :

Returns the list of files in a directory :

• -l : using this option allows you to have some details on the files such as the : date, size, permission and the type of file
• -lh : -lh is equivalent to -l -h, when adding the -h the size of the files are human readable
• -a : to show the hidden files
• ../. : indicates the parent directory, while "." indicates the current directory

file :

This command tells the type of a file, it can be a symbolic file, an emty file, an executable file, an ASCII file ...

rm :

To remove a file or a directory. by default you can't remove a directory, you have to add the option -r

mv :

to move a file/ directory to a target directory.

ln :

Enables the creation of a symbolic link, file that likes to another file. It is necessary to use an absolute path for the source.

cp :

To copy a file, you can use the tag : --report-identical-files to make sure that the files are identical

echo :

to print a text

• \n : to go to the next line
• \t : to mark a tab space
• • : to print all the files in a directory
• {} : to set variables and iterate over them : example echo hello{rida,adam,lamy,safa} or echo hello{1..5}
• $(()) : to do basic arithmetic operations : +,-,*,%,**

grep :

to search for something in a file, text or command response :

• -i: to ignore the case
• -c: to count the number of instances

Nested command lines in bash

It is possible to chain commands such as

echo here is the list of the files in this directory $(ls -l)

Commands to monitor applications :

df -h

returns stats about the disc. example :

df -h /data 

Users control with bash

adduser, addgroup, deluser and delgroup

adduser creates a new user, and creates a file in the home directory addgroup creates a new groupe, to add a user to the group we use the command : usermod -g [groupName] [userName] delgroupe to delete a group deluser to delete a user, you need to add "--remove-home" to remove the directory related to the user

file access parameters

When executing the command ls -l, we get the following line

on the left of every file are the permissions to access it. there are 3 groups of them :

Ignore the first "d", it just indicates that this is a directory 1- The first group indicates the permissions for the user :

• w : write permission
• r : read permission
• x : execute permission 2- The second group indicates the permissions for the group where the user belongs 3- The last group indicates the permissions for other users

chown and chgrp :

these command are here to transfer the permissions from a user to another and from a group to another

chmod :

This is the main command that changes the permissions of a file. It has the following syntax : sudo chmod [entity] [operation] [permission]

• entity : it can have 3 values (u for user, g for group, o for other)
• operation : it can have 2 values (+ to add permissions, - to remove permissions)
• permission : it can have 3 values and it is cumulative (w to write, r to read and x to execute)

Useful commands to writes scripts :

You will find within the repository multiple file with classic examples of scripts

read :

to get an input value

if statement :

read price
echo $price

if [[ $price >30 ]]; 
	then
	echo "this is too expensive !"
else
	echo "not so expensive :)"
fi

loops :

while loop :

read price
while [[ $price != 10 ]]; do
	echo "not enough try again"
	read price
done
echo bravo you have got it ! 

for loop :

for var in {rida,adam,brahim,fatima}; do
	echo user $var
done

functions :

sayHello(){
	echo "hello et bienvenue $nom ";
}

clear;
read nom;
sayHello;

arrays :

utilisateurs=("rida" "adam" "safa" "fatima")
for var in ${utilisateurs[@]}; do
	echo the user is $var
done

Be careful the white space can cause errors when declaring your arrays, there shouldn't be any white space between the "=" and "("

the pipe operation | :

This operation is used to send the result of a command as an input to the next one : for example

ls -1 |while read filename; do echo "here is a file "; done

&& and ; operator :

To run a command after the other, we can either use ';' or '&&'. The difference is that if a command fails with ';' the following one will still bne executed, while with '&&' all the following command will not be executed