Basic Linux Commands

Basic BASH Commands:

ls: Displays the content of a directory

If no external option is given, it will display the contents of present working directory.

Eg: ls ./abc

ls

ls ~/user

- a : Option to print all files(including hidden files)

- l : Option to give a long of file details.

Eg: ls -a

ls -al

ls -l /

NB: ~ : Denotes the home directory of current user

/ : Denotes the root directory of the Linux System.

Eg: ls /

ls -a ~/

pwd: Prints the current(present) working directory.

cd <directory> : Changes the working directory to the directory specified.

Eg: cd ~

cd /

NB: cd .. will make the terminal to move to the parent directory of present working directory.

cd . Will make the terminal to move to the current directory itself.

passwd : Changes the password of current user account.

touch <filename> : Creates a new file

Eg: touch abc.txt

touch ~/abc.txt

nano : A simple text editor

cp <file_src> <file_dest> : Copies a source file to a destination directory with or without renaming (File will be renamed if file_dest is not a directory).

-r or -R or –recursive : Recursive copy or copy all the subdirectories as well.

Eg: cp ./abc.txt ./dir2/efg.txt

cp -r ./abc ./dir2

mkdir <dir_name> : Creates a new directory.

-p, --parents : Creates intervening parent directories if they do not exist.

mv <source> <dest> : Cut and paste source file to the destination and rename if dest is not a directory. mv is used also for renaming a file.

Eg: mv ./abc/sample.txt ./efg

mv ./efg/sample.txt ./efg/sample2.txt

rmdir <dir_name> : Deletes an empty directory.

rm <file_name>: Removes a file.

-r option can be used to remove directories as well recursively.

-i option can be used to warn before deleting.

command --help : Displays a simple help page for a command.

man <command> : To get the manual page for a command. Press 'q' to exit.

info <command> : To get the info page for a command.

whatis <command> : gives a short description about the command.

Eg: whatis ls

apropos <keyword> : search the manual page names and descriptions for commands

Eg: apropos editor

time : Displays the current system time.

cal : Displays the calender for the current month.

Eg: cal #Calender for the current month

cal 2 2013 #Calender for February 2103

cal 2012 #Calender for the year 2012

ps : Displays the processes created by the current user.

who : Displays all the users currently logged into the system.

w : Displays all the users currently logged into the system and their resours utilization.

cat : Display the content of a file on to the screen.

wc : Word Count – Counts the number of lines, words and characters in a text file.

Eg: wc sample.txt

which <command> : Displays the actual location of the command / application.

exit or logout : Terminates the current session.

Redirecting output of a command to file:

Eg: ls -al > new.txt

Typing two or more commands in a single line:

Eg: ls -al ; mkdir abc

Comments given after commands:

Eg: ls -al #Long listing of pwd contents.

echo “<TEXT>” : Prints a text into console.

Eg: echo “Hello World”

su : Used to change user account, either to super user or any other user account.

NB: In Ubuntu based linux systems, we use sudo (SUper User DO) to exicute a command with root privilage.

Eg: sudo mkdir /abc

sudo apt-get update

script

---------------------------

--------------------------- : Records the BASH Session to the file typescript

---------------------------

exit

sort <filename> : Sorts a text file

uname -a : Prints the information about the linux installed in the system.

lsb_release -a : Prints the version information about the linux installed.

Keyboard Shortcuts:

Key Combination	Use
Ctrl + D	Same as exit , exits or terminates current session
Ctrl + C	Closes the currently running program in foreground
Ctrl + Z	Suspends the currently running program
Ctrl + H	Same as backspace
Ctrl + L	Clears the terminal Same as clear command
TAB	Predicts and completes a command
TAB TAB	Lists all command completion possibilities.

Files & Directories in Linux Systems:

• In Linux all the things excepts processes are generally considered as files(devices, directories, etc).
• Directories can be treated as special file that contains the list of files contained in it.
• In the tree structure for directory all the directories contains two pointers . and .. which points to the current directory and parent directory respectively.
• All the files also have an inode number which along with . And .. pointers identifies a file in the directory tree structure. Give -i option along ith ls to get the inode number as well.
• Hidden files have names preceding with a '.'

Important Directiories in / (Root Directory):

bin : Common files shared by system, administrator and users.(Eg: commands)

boot : vmlinuz(Kernel Files), GRUB

dev : References to hardware peripherals.

etc : Configuration files(Similar to the control panel in windows)

home : Directories and home folders for users.

initrd : Information needed during booting.

lib : Library Files

lost + found : Files saved during failures.

var :Variable files like log details, server pages, temporary variables etc.

usr : Programs, libraries, documentation for all user related programs.

media : Mount points for media devices.

df -h : Gives information about the partitions.

-h : Human readable, ie. Prints the details in a way easily unstaandable by human beings.

df -h . : Give the details of the disk partition that contains the present working directory.

du -h : Prints the disk usage details for the current directory.

which <command>: Give the absolute path for the command / program.

Eg: which ls

/bin/ls

$PATH is an environment variable that contains the list of directories which has executable files in it. So if we give a command it will search all the folders in the $PATH variable and if it is there then it will get executed, else it will print and error “Command not found..”.

echo $PATH : Prints the content of PATH variable.

We can temporarily change the $PATH variable using export command.

Eg:

export PATH=/usr/local/bin:/bin:/usr/sbin

or

export PATH=$PATH:/usr/sbin

$HOME – Contains the home folder for current user.

Eg: echo $HOME

file <file_name> : Displays the type of the file.

cmp <file_1> <file_2> : Compares two files

Searching in Linux:

Wild char symbols:

* : Matches any number of characters

? : Matches exactly one character.

[a-z] : Matches any character within the specified range.

Eg: ls *.txt

mv ./abc/*.txt ./abc/*.c

find <path> -name <search_string>

find <path> -size <size>

Eg: find /temp -name “abc.txt”

find . -size +50k

locate <search_string> : Similar to find but more friendly and less efficient.

~/.bash_history : is a file that contains the bash usage history.

grep: Command used to filter input lines and return only some patterns.

Eg: grep find ~/.bash_history

grep apple ./fruits.txt

NB: /usr/share/dict/words is a dictionary containing all the English words

Eg: grep man /usr/share/dict/words

cat : Concatenates and displays the output of one or more files to the screen. Output displayed in an uncontrolled way.

more : Displays the output with scrolling downward option.

nano : A simple text editor.

head : Display first few lines of a text file.

tail : Display last few lines of a text file.

File and Directory Permissions:

For each file any users can be classified into any of the three categories:

• u – User / Creator of the file
• g – Group User
• o – Any other user

ls -l gives a detailed listing along with the user and privilege details. There are separate privileges for read(r), write(w) and execute(x) for user, group user and other users.

chmod : Used to change the user privileges of any file.

Use -r or –recursive to apply privileges excursively to all subfiles in a directory.

Eg: chmod u+x abc.txt #Add user the privilege to execute.

chmod ug+rwx abc.txt #Add user and group user the privilege to read,write andexecute.

chmod o-x abc.txt #Deducing the power of other user to execute the file.

chmod 777 abc.txt #Give all privileges to all categories of users.

Processes:

A program executing in memory is called a process.

Using a terminal we can execute a process in:

• Foreground
  OR
• Background

<regular_command> : Runs the command in background.

<regular_command> & : Runs the command in background.

jobs : Command to display all the jobs running in background.

Ctrl + Z : Suspends a program running in foreground.

Ctrl + C : Terminate and quit a process running in foreground.

%n : Every process running in background has got a unique number, we can get that using job command.

bg <%n> : Reactivate a suspended program in background.

fg <%n> : Puts the job back in background.

kill <%n> : Terminates a background process.

• All the process has got a unique identification number called PID or Process ID.
• A process is being created by another by the process of forking in which the address space of one process is being cloned.
• Every process also have a Parent Process Id or PPID.

ps : Displays the processes created by the current user.

-f : Option to display PPID as well.

-e or -A : Display all the system process as well.

-u <username> : diplay all the processes created by any particular user.

Eg:

ps -e | grep tty

ps -e

kill <PID> :Terminates a process.

top : Displays the currently running tasks and their CPU utilization.

nohup <command> : Executes the command even if we log out of the system and writes the output to the file nohup.out.

Eg: nohup wget -c www.athena.nitc.ac.in/file.txt &

NB: wget is used to download a file.

pstree : Displays all currently running processes along with their parent child relationships.

time <command>: Executes the command and displays how much time it takes.

netstat : Displays the current network usage and traffic.

vmstat : Displays the current virtual memory usage.

sleep <seconds> : Do nothing for sometime.

Eg: sleep 5

(sleep 1800 ; echo “Lunch Time”) &

I/O Redirection:

> : Used to direct the output to a file.

>> : Used to append the output to a file.

| : Used to take the output of one command to another.

Eg: ls -al > ls.list

top > current.txt

Package Managers:

Redhat Package Manager : RPM

installing an rpm:

rpm -Uvh <path>

-U : Upgrade

-v : Generate more verbose output

-h : Gives a progress bar of installation

To check whether a program is being installed:

rpm -qa | grep gimp

Uninstalling:

rpm -e application

Debian Package Manager : dpkg

To install an application:

sudo dpkg -i package.deb

To check version and details of an installed application:

dpkg -l application

OR

dpkg -l *application*

APT – Advanced Package Manager

sudo apt-get update : Will update the database with list of available packages.

sudo apt-get install package : Installs a new package.

sudo apt-get upgrade : Performs and upgrade.

ssh user@host : Secure Shell Login to a remote server

scp sort.cpp abijith_bcs10@athena.nitc.ac.in: : SSH File Copy

write user [tty] : Used to send messages to other users.

Using gcc:

gcc <filename.c> -o <output_filename>

./output_filename

Different Shells:

sh : Traditional Unix Shell or Bourne Shell

bash : Bourne Again Shell, the default linux shell

ksh : Korn Shell

csh : C-Shell

tcsh : Turbo C Shell

$SHELL : Environment variable that contains the current shell

/etc/shells - File containing the list of available shells.

Shell scripts starts with

#! /bin/bash

so that when they are executed, it will be using BASH by default.

Eg:

#! /bin/bash

echo “Hello World!..”

Save it as sample.sh and give chmod +x sample.txt

Execute it as: ./sample.sh