Linux Commands

Here in this article, you will find the important Linux commands you should know in the Linux Operating System

What is Linux?

The first thing that we need to talk about in this article is the origins of Linux. In a nutshell, Linux is an operating system. For those of you who do not know what an operating system is, and for all of you who think you do but may have forgotten what an operating system is, it is actually the software layer that is between your hardware and the software that allows you to get something productive done on a computer.

The operating system is what allows the software to talk to the hardware. It is the one that lets you store information on hard drives, send out print jobs to printers, etc. If you are in a normal Windows environment. a Microsoft Windows Operating system. you have your hardware at the bottom, you have the Windows operating system on top of that, and then you have Microsoft Office, for example, that sits on top of the operating system.

Linux is an operating system that acts as an intermediary—i.e. a bridge—between the physical device and the instructional code of a program. The main thing that you just need to realize is that in the Linux world, the software that you will be running is of a completely different type, compared to the ones that you would run in the Windows operating system. Desktop applications like Microsoft Office and Adobe Photoshop are not usually run on a Linux environment. Linux normally
runs servers. Apache web servers, database servers, web virtualization servers, etc.

Linux is a Unix-like and mostly POSIX-compliant computer operating system assembled under the model of free and open-source software development and distribution.

More About Linux Operating System

Linux usually works as a server, because of its stability and security’s feature. Linux programs are extremely advantageous:

Linux programs are free, you’ll see, most Linux programs are.
They are frequently updated and for Zero charges!
Some of them are better than those in windows. And other doesn’t even exist in windows!
If Linux is free and almost all their softwares are free, it is for a reason; to understand we have to go back to 1984.

1984

So we are back in 1984, computer science was not very developed. Microsoft has just launched its first os: MS-DOS, but this one is far away from being done.

But, was MS-DOS the only one then? No! There was another operating system but less known by the public.
The one that was called the best was “Unix”. It was a lot powerful than MS-DOS but a lot complicated, which explains that only the professionals could use it.

Graphically UNIX looked a lot like MS-DOS they were both seen like a black screen with some white text in it. We must say that computers back then were not capable of doing better.

GNU Project

It is just in 1984, that Richard Stallman created GNU project.

The GNU Project is free software, mass collaboration project, announced on 27 September 1983, by Richard Stallman at MIT. Its aim is to give computer users freedom and control in their use of their computers and computing devices, by collaboratively developing and providing software that is based on the following freedom rights: users are free to run the software, share it (copy, distribute), study it and modify it.

GNU software guarantees these freedom-rights legally (via its license), and is therefore free software; the use of the word “free” always being taken to refer to freedom.

Richard Stallman was a researcher in Artificial intelligence in MIT. He wanted to create an operating system based on UNIX (the commands still the same).

But why would he create a copy of “UNIX”?

Because UNIX was not free and it was getting more expensive! Richard Stallman wanted to react by creating a free alternative: the project GNU was born.

GNU is an open operating system

GNU should not only be a free OS; it also had to be “open”

What is the difference?

A free program is a program where you can have the source code, that is to say, the “batch recipe.” In contrast, Windows is a proprietary OS whose source code is stored by Microsoft. Imagine it’s like Coca-Cola: nobody knows the recipe (there are many people who try to imitate it, but hey …). So we cannot change it or see how it works inside.

An open program is mostly a free program, it is also a program that has the right to copy, modify, redistribute. It’s a real ideology in computer science: people think it is better to give the source code of the programs that we create because it allows knowledge sharing and helps the computer to evolve faster. The slogan of the Free World might be: “Unity is strength.”

They say whenever the program is “open source” because its source code is open; everyone can see it. There are some slight differences between “open source” program and a “free” program, but we will not go into details here.

Linus Torvalds is doing his hobby

In 1991, Linus Torvalds, a student at the University of Helsinki (Finland), began creating his free own operating system. This system became known as Linux, referring to the name of its creator (Linux is a contraction of Linus and UNIX).

Linus Torvalds, creator of Linux

What relationship with GNU? Well it turns out that these two projects were complementary: while Richard Stallman created the basic programs (program file copy, delete, file, text editor), Linus had embarked on the creation of the “heart” an operating system kernel.

The GNU (free programs) and Linux (OS kernel) project merged to create GNU / Linux.

Theoretically, we should talk about GNU / Linux. But it is a bit difficult to write and pronounce, and by abuse of language, we often say just “Linux”. This is why I continue to speak of “Linux” in the rest of the book, even though the politically correct name is “GNU / Linux” because it is the merger of two complementary projects.

Original operating systems

You should now have a better idea of the origin of the three major operating systems that exist today: Mac OS, Linux and Windows.

Thus, Mac OS and Linux are both based on UNIX, the ancestor of operating systems, while Windows, from MS-DOS is a separate branch. Overall, this is all you need to remember.

It is said that Mac OS and Linux are based on UNIX because they have “copied” its operation. It’s not pejorative, it’s quite the opposite: it’s been an honor to UNIX.

Linux programs do not use all the same source code as UNIX (it was also the owner, so private). They have been completely rewritten but work the same way.

If I told you all this is because I believe that knowing the origin of Linux is important. This will help you understand many things thereafter.

Also Read: What is API

Linux distributions

A Linux distribution (often called distro for short) is an operating system made as a collection of software-based around the Linux kernel and often around a package management system. The most well-known distributions are RedHat, SUSE, Debian, Mandriva, Slackware, and Ubuntu. You can find much different software and there are hundreds of different ways to install it.

To make life easier for users and allow them to make a choice, different Linux distributions were created. This is a concept that does not really exist in Windows. It’s like the difference between Windows 7 Home and Windows 7 Professional, but it goes much further than that.

Here’s what can differ from one distribution to another:

Installation: it can be greatly simplified as very complicated;

Installing management programs: If it is done well and centralized, it can make the installation of new software simpler than Windows, as discussed further!

The preinstalled programs on your computer (e.g. Windows is bundled with Internet Explorer and Windows Media Player).

In fact, distribution is somehow packing Linux. The heart itself remains the same for all distributions. Whichever distribution you install, you get a Linux compatible with others. Some distributions are just more or less easy to handle.

Various existing distributions

There are many different Linux distributions. Hard to choose, you will say: indeed, the first time it is unclear what to choose … especially since all are free! Do not worry; I’ll help you make your choice.

I will not list all existing distributions, but here at least the main ones:

Slackware: one of the oldest Linux distributions. It still exists today!

Mandriva: published by a French company, it is simple to use;

RedHat: published by an American company, this distribution is known and widespread, especially on servers;

SUSE: Novell published by the company;

Debian: Debian distribution alone which is managed by independent developers rather than a business. This is one of the most popular distributions.

As I have said, whatever the distro (short for distribution) you choose, you will have a Linux. Basically, “just” a screen on first boot and various software preinstalled (I’m simplifying a bit much, but the idea is there).

Summary

Windows, Mac OS and Linux are the most popular operating systems.

Linux usually works as a server, because of its stability and security’s feature.

Linux has the distinction of being free, that is to say that its source code (the manufacturing recipe) is open: anyone can view it. In contrast, the source code that was used to design Windows and Mac OS is closed; we say that these are proprietary operating systems.

There are many variants of Linux, called distributions.

Getting Started With Linux Commands

I assure you, we will start with simple things to become familiar with the console. We’ll really see the ABC, the basic survival guide of kits.

Also Read: What is Web Hosting?

Basic Linux Commands in 2021

a2p

translate awk to perl
e.g. a2p myfile.awk> myfile.pl
(translate myfile.awk into pl file)

alias

create another name for a command
e.g. alias p=”pwd”
(set p as alias for pwd)

apropos

view the searched term in man pages
e.g. apropos find
(list entries with “find” in man page)

apropos -e

view searched term in man pages
e.g. apropos –e nice
(-e: show exact word in man pages)

apt-get

install, remove or update a package
e.g. apt-get install libc6
(install libc6 package)

aspell

check and correct for misspellings
e.g. aspell -c test.txt
(-c:check spelling in test.txt file)

at

run a job at a schedule time
e.g. at 1 AM Fri
(run the job at 1am Friday)

awk

match text by regular expression
e.g. awk ‘length($0) > 88’ text.txt
(list only lines longer than 88 words)

basename

display the last part of a file path
e.g. basename /home/foo/usr/file.txt
(output: file.txt)

bc

perform a calculation by a calculator
e.g. bc 8+9
(output: 17)

bg

resume a stopped job in background
e.g. bg %3
(resume %3 job in background)

bunzip2

uncompress a file from zip format
e.g. bunzip2 myfile.tar.bz2
(uncompress myfile.tar.bz2)

bzip2

compress a file to zip format
e.g. bzip2 myfile.dat
(compress myfile.dat)

cal

display a month or year calendar
e.g. cal 2014
(display 2014 calendar)

cal -3

display a month or year calendar
e.g. cal -3
(-3:display 3 months)

cat

display contents of one of more files
e.g. cat file1.txt file2.txt
(display contents of file1 and file2)

cat -n

display contents of one of more files
e.g. cat –n myfile.txt
(-n: specify number of output lines)

cd

change directory
e.g. cd /home/user/mydir
(change current directory to mydir)

chattr

set attributes for a file
e.g. chattr +i myfile.txt
(+i make the file as read-only)

chfn

change user’s finger information
e.g. chfn
(change all users information)

chfn -f

change user’s finger information
e.g. chfn –f Full-Name
(-f: change full name)

chgrp *

change the group ownership
e.g. chgrp groupname /usr/myfile.txt
(alter group ownership of myfile.txt)

chkconfig*

view and modify run level file
e.g. chkconfig –list
(-list: list services of run level)

chmod

change access permission
e.g. chmod 755 filename
(set file access permission as 755)

chown *

change ownership of file or directory
e.g. chown username myfile.txt
(alter file ownership of myfile.txt)

chpasswd*

change password for users.
e.g. chpasswd
(then enter username: password)

chsh *

change login shell for a user
e.g. chsh -s /bin/bash ray
(-s specify login shell)

cksum

produce a CRC checksum number
e.g. cksum file.txt
(output checksum number of file.txt)

clear

clear the screen.
e.g. clear
(clear the shell window)

cmp

compare two files text byte by byte
e.g. cmp first.txt second.txt
(compare first.txt and second.txt)

Also Read: What is a Domain Name

collectl

monitor the current system status
e.g. collectl
(list cpu, sys, inter .etc information)

comm

compare two files text line by line
e.g. comm first.txt second.txt
(compare first.txt second.txt)

cp

copy file(s) to another directory
e.g. cp myfile.txt /mydir
(copy myfile.txt to mydir)

cp -p

copy file(s) to another directory
e.g. cp –p myfile.txt /mydir
(-p: keep original permission)

cp -a

copy file(s) to another directory
e.g. cp –a myfile.txt /mydir
(-a: keep original attributes)

cpio -o

output archived cpio file
e.g. cpio -o > directory.cpio
(-o: backup to an archive cpio file)

cpio -i

input archived cpio file
e.g. cpio -i < directory.cpio
(-i: restore from an archive cpio file)

crontab

create a job to run at specified time
e.g. crontab
(set to run jobs at regular intervals. )

crontab -e

run a recurring job at a specified time
e.g. crontab -e
(-e allow edit the crontab file)

crontab -l

run a recurring job at a specified time
e.g. crontab -l
(-l: lists the crontab files)

crontab -r

run a recurring job at a specified time
e.g. crontab -r
(-r: remove the crontab file)

csplit

split a file into some separated files
e.g. csplit myfile.txt ”/part1/” “/part2/”
(separate files named xx00, xx01)

cut -d

show the specified field of a file
e.g. cut –d “:” myfile.txt
(-d: specify a field delimiter “:”)

cut -c

extract contents from a file
e.g. cut –c 6 myfile.txt
(-c6: the sixth character of each line)

cut -f

extract contents from a file
e.g. cut –f 3 myfile.txt
(-f3: specify a field number as 3)

date

show the date and time
e.g. date
(display the current date and time)

date -s

set the date and time
e.g. date -s “11/20/2014”
(-s set the date)

dc

open a command line desk calculator
e.g. dc
( “dc” means desk calculator)

dd

data dump to convert and copy a file
e.g. dd if=/dev/sda of=/dev/sdb
(copy data from sda to sdb)

dmesg

print out all kernel log messages
e.g. dmesg > kmsg.txt
(output kernel messages to kmsg.txt)

df

display free disk space
e.g. df
(display file system free space)

df -m

display free disk space
e.g. df -m
(-m: display sizes in Mb)

dhclient

configure network interfaces
e.g. dhclient eth0
(renew IP address of eth0)

diff

show difference between two files
e.g. diff firstfile.txt secondfile.txt
(display difference above two files)

diff3

show difference among three files
e.g. diff3 file1.txt file2.txt file3.txt
(display difference above three files)

dig

display the details of DNS servers
e.g. dig xvxaxx.com
(list information about xvxaxx.com)

dir

show directory contents
e.g. dir
(display current directory contents)

dircolors

show color settings for “ls” command.
e.g. dircolors
(display directory coloring of ls)

dirname

remove the last part of a file path.
e.g. dirname /foo/bar/baz/myfile.txt
(output: /foo/bar/baz)

du -s

display disk usage
e.g. du -s .
(-s list files size in current directory)

du -h

display disk usage
e.g. du -h myfile.txt
(-h: show human readable units)

dump -f

makes backup of filesystem
e.g. dump -f0 filebk /mydir
(-f:backup-0:dump-level filebk: dump-file)

echo

display input on standard output
e.g. echo “Hello World!”
(show “Hello World!”)

echo -e

display text using escape sequence
e.g. echo –e “\n Hello World!”
(-e: allow use \n to show text)

ed

open a command-line text editor
e.g. ed myfile.txt
(open myfile.txt with text editor)

egrep

search file(s) for a specified regex
e.g. egrep “new | string” myfile.txt
(search myfile.txt for “new | string”)

eject

eject the cd or dvd tray
e.g. eject cdrom
(eject cdrom tray)

eject -t

eject or close the cd or dvd tray
e.g. eject –t cdrom
(-t: close an open cdrom tray)

emacs

powerful, extensible file editor
e.g. emacs file.txt
(launch emacs and open file.txt )

env

show, set the environment variables
e.g. env
(list current environment variables)

eval

make a command from its arguments
e.g. UPLS=”eval cd.. ; ls ”
(create a command named UPLS)

exit

exit the shell
e.g. exit
(terminate the program and log out)

expand

convert tabs into spaces
e.g. expand myfile.txt
(convert tabs to spaces for myfile.txt)

expand -t

convert tabs into spaces
e.g. expand -t 3 myfile.txt
(-t: set tabs 3 characters apart)

export

set an environment variable & value
e.g. export newvar=8 echo $newvar
(output: 8)

expr

evaluate an expression
e.g. expr 10+8
(output: 18)

factor

show the prime factors of a number
e.g. factor 1001
(output: 7 11 13)

fc

list, edit, re-execute last commands
e.g. fc -l
(-l:list the history of commands)

fdisk*

manipulate the hard disk partitions
e.g. fdisk /dev/hdb
(list hard disk partitions information)

fg

resume a stopped job in foreground
e.g. fg %3
(resume the job 3 in foreground)

fgrep

search file(s) for a specified string
e.g. fgrep “good” myfile.txt
(search myfile.txt for “good”)

file

detect the file type.
e.g. file myfile.tar
(determine file type of myfile.tar)

find –print

find file(s) in a directory named dir
e.g. find dir -print -name ‘abc.txt’
(-print:print, -name:specify file name)

finger

show user’s information
e.g. finger username
( list the user’s login name, time. etc.)

fmt

format text files
e.g. fmt myfile1.txt > myfile2.txt
(format myfile1 & output to myfile2)

fmt -u

format text files
e.g. fmt –u myfile.txt
(-u: provide uniform word spacing)

fold

wrap each line to fit a specified width
e.g. fold -w 15 myfile.txt
(-w specify how many words)

for in

set conditional parameter for loop
e.g. for n in 3 6 9 do echo $n done
(output: 3 6 9 )

free

displays free memory information
e.g. free
(list free, used, total memory…)

free -m

displays free memory information
e.g. free -m
(-m: show sizes in Mb.)

fsck *

file system check
e.g. fsck
(check or fix Linux file system)

ftp

transfer files by File Transfer Protocol
e.g. ftp ServerURL
(transfer files using ftp)

gawk

find or replace text in a file
e.g. gawk ‘length($0) > 88’
(list lines longer than 88 characters)

grep

match a specified string or regex.
e.g. grep onestring myfile.txt
(search myfile.txt for onestring)

groups

list groups to which the user belongs
e.g. groups
(print the groups of user)

groupadd *

add a new group
e.g. groupadd newgroup
(create a new group)

groupadd * -f

add a new group
e.g. groupadd –f newgroup
(-f: check group doesn’t exist)

groupdel *

delete an existing group.
e.g. groupdel existinggroup
(remove an existing group)

groupmod *-n

modify an existing group
e.g. groupmod -n newgrp oldgrp
(-n change group name)

gunzip

uncompress a file from gzip format
e.g. gunzip myfile.txt.gz
(uncompress myfile.txt.gz)

gzip

compress a file to gzip format
e.g. gzip myfile.txt
(compress myfile.txt)

halt

shutdown the system
e.g. halt
(power off the system)

hash

access the hash table
e.g. hash
(list commands from hash table)

head

display some front lines in a file
e.g. head myfile.txt
(output the first ten lines of myfile.txt)

head -n

display some front lines in a file
e.g. head –n 4 myfile.txt
(-n: specify a number of lines)

help

show help information of commands
e.g. help echo
(show information about echo)

history

show the commands history
e.g. history
(list commands in this shell session)

host

find the ip address of a domain name
e.g. host websprogram.com
(show ip of websprogram.com)

hostid

display id of the current host in hex.
e.g. hostid
(print the current host id)

hostname

show or set the host name
e.g. hostname
(display the name of current host )

id

show the user or group id number
e.g. id
(display the root user uid, gid. etc.)

ifconfig

show, configure the network interface
e.g. ifconfig
(display the network settings)

init *

set the system run level
e.g. init 5
(change to run level 5)

info

show help information of a command
e.g. info man
(show help page for man)

install

copy files, set permission, ownership
e.g. install myfiles /home/user
(copy myfiles to user directory)

install -o

copy files, set permission, ownership
e.g. install –o myfiles ray /home
(-o: specify ownership)

jobs

show all jobs’ status
e.g. jobs
(list all running jobs’ information)

join

join lines of files having common field
e.g. join myfile1.txt myfile2.txt
(join lines of two files by same field)

join

join lines of two files
e.g. join –i myfile1.txt myfile2.txt
(-i: ignore the differences)

kill

stop a job by number.
e.g. kill %3
(terminate job %3)

kill

stop a process by pid.
e.g. kill 3956
(terminate process 3956)

killall

stop a process by name
e.g. killall no respond
(terminate process no respond)

last

show most recently logged-in users
e.g. last
(list recent users’ date, time…)

lastb

show bad login attempts
e.g. lastb
(display bad login attempts)

lastlog

display the last login information
e.g. lastlog -u username
(-u: specify a user)

less

show contents page by page
e.g. less myfile.txt
(display myfile.txt page by page)

let

perform arithmetic on shell variables
e.g. let a=12; let a=a+8; echo $a
(output: 20)

link

create a link to a file.
e.g. link file1.txt file2.txt
(create a link from file1 to file2)

ln

create a hard link to a file
e.g. ~/myfile.txt
(create a hard link to myfile.txt)

ln -s

create a link between two files
e.g. ln -s file1.txt file2.txt
(create a symbolic link to file1/file2)

locate

find the location of a file or a directory
e.g. locate myfile.txt
(locate myfile.txt on local machine)

logname

show the current user’s login name
e.g. logname
(display the login name of user)

look

show words matching a given prefix
e.g. look ab
(output: aba, abb, abc, abd…)

lpc

run the line printer control program
e.g. lpc status
(show status of current print queue)

lpq

show the printer queue status
e.g. lpq
(list the print queue)

lpr

send a print request to printer
e.g. lpr myfile.txt
(send myfile.txt to printer)

lprm

cancel the printing job in print queue
e.g. lprm 2
(remove printing job 2)

ls

list the contents of current directory
e.g. ls
(list files and sub-directories)

ls -l

long list contents of current directory
e.g. ls -l
(-l:long lists including permissions)

ls -a

lists all entries including hidden files
e.g. ls -a
(-a: show all files)

ls -t

lists all entries by time stamps
e.g. ls -t
(-t: show by time stamps)

ls -lh

lists contents in current directory
e.g. ls -lh
(-lh: list files with size in mb and gb. )

lsattr

list the attribute of a file or a directory
e.g. lsattr myfile.txt
(show myfile.txt attribute)

lsof

list opened files
e.g. lsof
(list all opened files)

man

get command help from manual
e.g. man cat
(show manual page for cat)

man -k

search manual pages for keyword
e.g. man –k printf
(-k: specify a keyword)

md5sum

create a md5 checksum number
e.g. md5sum -c file.txt
(-c validate file against a checksum)

mesg

enable or disable messaging
e.g. mesg
(show the current write status)

mesg y/n

enable or disable messaging
e.g. mesg y/n
(y or n: permit or deny messaging)

mail

send and receive mails
e.g. mail ray@websprogram.com
(email to ray@websprogram.com)

mkdir

make a new directory
e.g. mkdir mydir
(create a directory named mydir)

mknod

make a device file
e.g. mknod /dev/dk b 45 0
(dk:device; b:block; 45:major no.; 0:minor no.)

more

show content one screen at one time
e.g. more +2 myfile.txt
(+2: beginning at line 2)

mount

mount a storage device
e.g. mount /dev/cd
(mount a device cd)

mount -l

mount or show devices
e.g. mount -l
(-l: list all mounted devices)

mt

magnetic tape drive control
e.g. mt -f /dev/tape eod
(-f select eod; move to end of data)

mv

move a file to another directory
e.g. mv myfile.txt /home/user/mydir
(move myfile.txt to mydir directory)

mv

rename a file
e.g. mv myfile1.txt myfile2.txt
(rename mayfile1 to myfile2)

netstat

display network status
e.g. netstat
( print network connections, routing tables.etc )

nice *

set the priority level of a job
e.g. nice -19 ftp
(set priority level as 19 for ftp)

nl

add text with number lines
e.g. nl mylist.txt
(make number lines for mylist.txt)

nohup

ignore hangup signals
e.g. nohup find ftp
(run ftp ignoring hangup signals)

nslookup

query internet name servers for IP
e.g. nslookup myxxexample.com
(return IP like 75.126.162.XXX)

passwd

modify a user password
e.g. passwd username
(change password for username)

paste

merge lines of multiple files
e.g. paste file1.txt file2.txt
(merge contents for file1 and file2)

pidof

show process ID of running program
e.g. pidof console
(display console’s process id)

ping

send data to a host, await response
e.g. ping xvxmjfz.com
(test if remote host can be reached)

ping -c

test if remote host can be reached
e.g. ping –c5 xvxmjfz.com
(-c5: specify the number of pings)

pkill

kill a running process
e.g. pkill firefox
(stop web browser firefox)

pr

prepare text files for printing
e.g. pr myfile.txt
(prepare myfile.txt for printing)

pr -n

prepare text files for printing
e.g. pr –n myfile.txt
(-n: specify number in each line)

pr -h

prepare text files for printing
e.g. pr –h “Good” myfile.txt
(-h: specify a header)

printenv

show the environment variables
e.g. printenv
(list values of environment variables)

printf

format and print data
e.g. printf “start\b”
(\b: backspace output: star)

printf

format and print data
e.g. printf ‘hello \n world \n !’
(\n: prints by newlines. output 3 lines)

ps -f

show the process status
e.g. ps -f
(-f full information of current process)

ps -u

show the process status
e.g. ps -u ray
(-u specify a user’s current process)

pstree

displays process in tree structure
e.g. pstree
(show all process as a tree)

pwd

print working directory
e.g. pwd
(display current directory)

rcp

remotely copy file between two hosts
e.g. rcp file.txt host2:/dir2/file.txt
(remotely copy file.txt to host2)

read

read a line from standard input
e.g. read name (input ray)
echo “$name” (output ray)

reboot*

restart the system
e.g. reboot
(cause the computer to restart)

renice *

change the priority level of a job
e.g. renice 3 23001
(set priority level as 3 for job 23001)

restore

restores data from the backup file
e.g. restore -f databackup
(-f:specify a backup file)

rlogin

remotely login to a system
e.g. rlogin -l username domain.com
(-l: specify a username)

rm

remove one or more files
e.g. rm myfile.txt
(remove myfile.txt)

rm -r

remove non-empty directory
e.g. rm -r /NonEmptyDir
(-r: remove directory and its content)

rm -i

remove non-empty directory or a file
e.g. rm -i myfile.txt
(-i: ask before removing)

rmdir

remove empty directory
e.g. rmdir /emptydir
(delete directory without contents)

route

show or modify the IP routing table
e.g. route -n
(-n: show in numerical format)

rsync

remotely synchronize files
e.g. rsync myfile host2:/dir2/myfile
(sync. myfile with remote host2)

scp

securely copy files amid two hosts
e.g. scp file.txt host2:/dir2/file.txt
(securely copy file to remote host2)

screen

open the terminal window manager
e.g. screen
(start a new screen)

sdiff

show two files’ difference side by side
e.g. sdiff myfile1.txt myfile2.txt
(compare two files side by side)

sed

filter and transform input text
e.g. sed “{print $3}” myfile.txt
(display the third word of each line)

seq

list sequent numbers in given range
e.g. seq 1 8
(output: 1 2 3 4 5 6 7 8)

seq -w

list sequent numbers in given range
e.g. seq –w 1 3
(-w: with zeros output: 01 02 03 )

seq

list sequent numbers in given range
e.g. seq –s| 1 3
(-s: with separators output: 1| 2 |3 )

set

set shell variable or function
e.g. set n=who am i; echo $n
(output: who am i)

sftp

securely transfer files by ftp
e.g. sftp SeverURL
(securely transfer files to a Server)

shopt

show the shell option settings
e.g. shopt
(show the shell behavior settings)

shutdown *

close system
e.g. shutdown 22:00
(shut down at 22:00 o’clock)

shutdown-h

close system
e.g. shutdown –h +5
(-h+5: halt after 5 minutes)

shutdown -r

shutdown and restart
e.g. shutdown -r now
(-r:shut down and instantly restart)

sleep

pause for a specified amount of time
e.g. sleep 10
(pause for 10 seconds)

sort

show sorted contents alphabetically
e.g. sort -r myfile.txt
(-r sort file in reverse order)

split -b

split a file to some files in given size
e.g. split -b 11 file.txt
(split file to some 11 byte files named xaa,xab,xac, etc.)

split -l

split a file to some files in given size
e.g. split -l 8 file.txt
(split file to some 8 line files named xaa,xab,xac, etc.)

ssh

login to remote secure shell
e.g. ssh ray@myexample.com
(securely connect to a remote host)

ssh -l

login to remote secure shell
e.g. ssh -l username hostname
(-l specify your remote username)

stat

list status about file size, access, etc.
e.g. stat myfile.txt
(show myfile.txt statistics)

su

switch user
e.g. su user2
(switch user named user2)

su -l

login as a root super user
e.g. su –l
(-l:enter password, login root account )

sudo -u

execute a command as another user
e.g. sudo -u user2 ls /home/mydir
(-u: specify user2 to execute ls cmd)

sudo -v

refresh the authentication timeout
e.g. sudo -v
(next sudo will not require password.)

sudo -k

expire the authentication timeout
e.g. sudo -k
(next sudo will require password.)

sum

summarize a file with a checksum
e.g. sum myfile.txt
(create a checksum for myfile.txt)

suspend

suspend the working shell
e.g. suspend
(pause system during execution)

sync

synchronize disk data with memory
e.g. sync
(flush all file system buffers to disk)

tac

display lines of a file in reverse order
e.g. tac myfile.txt
(print file from last line to first line)

tail

show the final part of a file
e.g. tail -n 20 file.txt
(-n20:output last 20 lines of file.txt)

talk

communicate with another user
e.g. talk ray
(talk to user ray)

tar -xf

extract an archived tar file
e.g. tar -xf archive.tar
(-xf: extract an archive tar file)

tar -cf

create an archived tar file
e.g. tar -cf archive.tar
(-cf: create an archive tar file)

tee

print standard output, write to a file
e.g. sort file1.txt | tee file2.txt
(sort file1.txt and write to file2.txt)

tee -a

print standard output, write to a file
e.g. sort file1.txt | tee –a file2.txt
(-a: append instead of overwrite)

test

calculate a boolean expression
e.g. [ 8 -gt 6 ]; echo $?
(output: 0 0:true; 1:false)

test

calculate a boolean expression
e.g. [ 5 -eq 6 ]; echo $?
(output: 1 0:true; 1:false)

time

show the time taken to run a program
e.g. time ftp
(display the time taken to execute ftp)

times

show the uptime of sell
e.g. times
(display the system uptime)

tload

show a graphic report of system load
e.g. tload
(show the current system load average to a specified process)

top

list the top active or specified process
e.g. top -p pid
(-p:display specific process by pid)

touch

update timestamp for an existing file
e.g. touch myfile.txt
(modify myfile.txt to the current time)

touch -t

update timestamp for an existing file
e.g. touch –t myfile.txt
(-t: specify a time)

tr

translates sets of characters
e.g. echo apple | tr “apple” “banana”
(output: banana)

traceroute*

trace the route to a host
e.g. traceroute xvauhdhxv.com
(trace packets route to another host)

trap

run a command on receiving a signal
e.g. trap
(display the current signal traps)

tree -p

list directory contents in tree format
e.g. tree –p
(-p: also show the file permissions)

tty

show the name of the terminal device
e.g. tty
(show the terminal filename)

type

detect the type of a command
e.g. type wait
(output: wait is a shell builtin)

ulimit -a

limit user resources
e.g. ulimit -a
(-a:display all limits for the system)

umask

show or set the file permission value
e.g. umask 0022
(allow user read, write privileges and all others to read)

umask

show or set the file permission value
e.g. umask 0002
(allow group read, write privileges and all others to read)

umask

show or set the file permission value
e.g. umask 0077
(allow user read, write privileges and no for others)

umount

unmount a device or filesystem
e.g. umount /dev/dvd
( unmount a device DVD )

unalias

remove an alias
e.g. unalias aliasname
( delete a specified alias)

uname -a

show the current system information
e.g. uname -a
( -a: display all information)

uname -n

show the current system information
e.g. uname -n
( -n: display the host name)

unexpand

convert spaces into tabs
e.g. unexpand myfile.txt
(convert spaces to tabs for myfile.txt)

uniq

filter out repeated lines in a file
e.g. uniq myfile.txt
(show unique line in myfile.txt)

unset

remove shell variable or function
e.g. unset var
(delete a variable)

unzip

uncompress files from zip format
e.g. unzip archive.zip
(uncompress file from archive.zip)

uptime

show system uptime
e.g. uptime
(display system uptime)

useradd *

add a new user account
e.g. useradd username
(create a user account)

useradd* -d

display default value for new users
e.g. useradd -d
(show default data for a new user)

userdel *

delete an existing user account
e.g. userdel username
(remove a user account)

usermod* -d

modify home directory
e.g. usermod -d /home/mydir andy
(-d: specify home directory for andy.)

usermod * -l

modify an existing user account
e.g. usermod -l oldname newname
(-l: login name change)

usermod * -p

modify an existing user account
e.g. usermod -p password username
(-p:modify password of a user)

users

display current logged-in users
e.g. users
(list users currently logged in)

vdir

verbosely show directory contents
e.g. vdir
(vdir just like ls, but more verbose)

vi

open the vi text editor
e.g. vi filename
(open a file with vi text editor)

vmstat

report virtual memory statistics…
e.g. vmstat
(also report swap, disk i/o devices…)

w

list current processes for each users
e.g. w username
(show the user’s process)

w -s

list current processes by summary
e.g. w -s
(-s: show a summary of shell process)

wait

wait for a process to change state
e.g. wait 10788
(wait for 10788 to change state)

watch

execute a command periodically
e.g. watch –n 5 date
(-n5:update date every 5 seconds)

wc

show word count, line count, etc
e.g. wc myfile.txt
(list word, line count… for myfile.txt)

wc -c

show word count, line count, etc
e.g. wc –c myfile.txt
(-c: show the byte counts)

wget

download a web page from a website
e.g. wget
(download webpage from above url)

wget -c

download a web page from a website
e.g. wget –c http://www.examp.com
(-c: continue download previous web)

whatis

show manual page of a command
e.g. whatis ping
(show manual page of ping)

whereis

locate source, man for a command
e.g. whereis ls
(show source, man locations of ls)

which

show path of a executable command
e.g. which ftp
(show the full path of ftp command)

who

show who currently logged in
e.g. who
(list all logged-in users, date, time…)

who -a

list all users currently logged in
e.g. who -a
(-a: all information)

whoami

show the current user’s login name
e.g. whoami
(show your own user name)

whois

show domain owner’s information
e.g. whois xvqizx.com
(list xvqizx.com owner’s information)

write

write a message to another user
e.g. write username
(then write your message…)

xargs

execute a command with arguments
e.g. find -name ” a. ” | xargs rm
(find files named a., remove them)

xcalc

launch a graphical calculator
e.g. xcalc
(open a scientific calculator)

xclock

launch graphical clock
e.g. xclock -digital
(-digital: specify a digital clock)

yes

output a string repeatedly until killed
e.g. yes “hello”
(output hello repeatedly until killed)

yum

rpm-based package manager
e.g. yum install update
(install a package named “update”)

zcat

output compressed text
e.g. zcat myfiles.txt.gz | less
(uncompress file and show contents)

zless

show un/compressed file contents
e.g. zless myfile.txt.gz
(zless: show contents by page)

zmore

show un/compressed file contents
e.g. zmore myfile.txt.gz
(zmore: show contents by screen)

zip

compress files to zip format
e.g. zip documents *
(create documents.zip for all files)

unzip

uncompress files from zip format
e.g. unzip myfile.zip
(uncompress myfile.zip.)