Linux Commands

1.  Everything in Linux is a file including the hardware and even the directories.

2. # : Denotes the super(root) user

3.  $ : Denotes the normal user

4.  /root: Denotes the super user’s directory

/home: Denotes the normal user’s directory.

5.  Switching between Terminals

§  Ctrl + Alt + F1-F6: Console login

§  Ctrl + Alt + F7: GUI login

6.  The Magic Tab: Instead of typing the whole filename if the unique pattern for a particular file is given then the remaining characters need not be typed and can be obtained automatically using the Tab button.

7.   ~(Tilde): Denotes the current user’s home directory

8.   Ctrl + Z: To stop a command that is working interactively without terminating it.

9.  Ctrl + C: To stop a command that is not responding. (Cancellation).

10.  Ctrl + D: To send the EOF( End of File) signal to a command normally when you see ‘>’.

11.  Ctrl + W: To erase the text you have entered a word at a time.

12.  Up arrow key: To redisplay the last executed command. The Down arrow key can be used to print the next command used after using the Up arrow key previously.

13.  The history command can be cleared using a simple option –c (clear).

14.  cd :   The cd command can be used trickily in the following ways:

cd : To switch to the home user

cd * : To change directory to the first file in the directory (only if the first file is a directory)

cd .. : To move back a folder

cd - : To return to the last directory you were in

15.  Files starting with a dot (.) are a hidden file.

16.   To view hidden files: ls -a

17.   ls: The ls command can be use trickily in the following ways:

ls -lR : To view a long list of all the files (which includes directories) and their subdirectories recursively .

ls *.* : To view a list of all the files with extensions only.

18.   ls -ll: Gives a long list in the following format

drwxr-xr-x 2 root root 4096 2010-04-29 05:17 bin where

drwxr-xr-x : permission where d stands for directory, rwx stands for owner privilege, r-x stands for the group privilege and r-x stands for others permission respectively.

Here r stands for read, w for write and x for executable.

2=> link count

root=>owner

root=>group

4096=> directory size

2010-04-29=>date of creation

05:17=> time of creation

bin=>directory file(in blue)

The color code of the files is as follows:

Blue: Directory file

White: Normal file

Green: Executable file

Yellow: Device file

Magenta: Picture file

Cyan: link file

Red: Compressed file

File Symbol

-(Hyphen) : Normal file

d=directory

l=link file

b=Block device file

c=character device file

19.  Using the rm command: When used without any option the rm command deletes the file or directory ( option -rf) without any warning. A simple mistake like rm / somedir instead of rm /somedir can cause major chaos and delete the entire content of the /(root) directory. Hence it is always advisable to use rm command with the -i(which prompts before removal) option. Also there is no undeleting option in Linux.

20.  Copying hidden files: cp .* (copies hidden files only to a new destination)

21. dpkg -l : To get a list of all the installed packages.

22. Use of ‘ > ‘ and ‘ >> ‘ : The ‘ > ‘ symbol ( input redirector sign) can be used to add content to a file when used with the cat command. Whereas ‘ >> ‘ can be used to append to a file. If the ‘ >> ‘ symbol is not used and content is added to a file using only the ‘>’ symbol the previous content of the file is deleted and replaced with the new content.

e.g: $ touch text (creates an empty file)

$ cat >text

This is text’s text. ( Save the changes to the file using Ctrl +D)

$cat >> text

This is a new text. (Ctrl + D)

Output of the file:

This is text’s text.

This is a new text.

23.  To count the number of users logged in : who |wc –l

24.  cat:  The cat command can be used to trickly in the following way:

- To count no. of lines from a file : cat <filename> |wc -l

- To count no. of words from a file : cat <filename> |wc -w

- To count no. of characters from a file : cat <filename> |wc –c

25.  To search a term that returns a pattern: cat <filename> |grep [pattern]

26.  The ‘tr’ command: Used to translate the characters of a file.

tr ‘a-z’ ‘A-Z’ <text >text1 : The command for example is used to translate all the characters from lower case to upper case of the ‘text’ file and save the changes to a new file ‘text1′.

27.  File permission using chmod: ‘chmod’ can be used directly to change the file permission of files in a simple way by giving the permission for root, user and others in a numeric form where the numeric value are as follows:

r(read-only)=>4

w(write)=>2

x(executable)=>1

e.g. chmod 754 text will change the ownership of owner to read, write and executable, that of group to read and executable and that of others to read only of the text file.

28.  more: It is a filter for paging through text one screenful at a time.

Use it with any of the commands after the pipe symbol to increase readability.

e.g. ls -ll |more

29.  cron : Daemon to execute scheduled commands. Cron enables users to schedule jobs (commands or shell scripts) to run periodically at certain times or dates.

1 * * * * echo “hi” >/dev/tty1 displays the text “hi” after every 1 minute in tty1

.—————- minute (0 – 59)

| .————- hour (0 – 23)

| | .———- day of month (1 – 31)

| | | .——- month (1 – 12) OR jan,feb,mar,apr …

| | | | .—– day of week (0 – 7) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat

* * * * * command to be executed

30.  fsck: Used for file system checking. On a non-journaling file system the fsck command can take a very long time to complete. Using it with the option -c displays a progress bar which doesn’t increase the speed but lets you know how long you still have to wait for the process to complete.

e.g. fsck -C

31.  To find the path of the command: which command

e.g. which clear

32. Setting up alias: Enables a replacement of a word with another string. It is mainly used for abbreviating a system command, or for adding default arguments to a regularly used command

e.g. alias cls=’clear’ => For buffer alias of clear

33.  The du (disk usage) command can be used with the option -h to print the space occupied in human readable form. More specifically it can be used with the summation option (-s).

e.g. du -sh /home summarizes the total disk usage by the home directory in human readable form.

34.  Two or more commands can be combined with the && operator. However the succeeding command is executed if and only if the previous one is true.

e.g. ls && date lists the contents of the directory first and then gives the system date.

35.  Surfing the net in text only mode from the terminal: elinks [URL]

e.g: elinks www.google.com

Note :that the elinks package has to be installed in the system.

36.  The ps command displays a great more deal of information than the kill command does.

37.  To extract a no. of lines from a file:

e.g  head -n 4 abc.c is used to extract the first 4 lines of the file abc.c

e.g  tail -n 4 abc.c  is used to extract the last 4 lines of the file abc.c

38.  Any changes to a file might cause loss of important data unknowingly. Hence    Linux creates a file with the same name followed by ~ (Tilde) sign without the recent changes. This comes in really handy when playing with the configuration files as some sort of a backup is created.

39.   A variable can be defined with an ‘=’ operator. Now a long block of text can be assigned to the variable and brought into use repeatedly by just typing the variable name preceded by a $ sign instead of writing the whole chunk of text again and again.

e.g  ldir=/home/my/Desktop/abc

p abcd $ldir copies the file abcd to /home/my/Desktop/abc.

40. To find all the files in your home directory modified or created today:

e.g: find ~ -type f -mtime 0

41. BASH ( Bourne-Again SHell ) is the most common shell in Linux. It’s freeware shell.

            cat /etc/shells -> To find the available shells in your system 
            /bin/shell-name  -> For temporary changing of the shell

/bin/sh :  To change to the sh shell.

                To return back to the bash shell: /bin/bash

42. To print your home directory location:

echo $HOME

43. To print colouful text

            echo -e ” \ 033[31m Hello  World” Note : There is no space between the \ and 0
            Output : 
            Hello World ( in Red )

            Set foreground color:
            echo -e ” \ 033[3xm” ( where x = 0 – 7 ) Note : There is no space between the \ and 0
            Set background color :
            echo -e ” \ 033[4ym” ( where y = 0 – 7 ) Note : There is no space between the \ and 0

44. The sort command can be used to sort lines of text files. It can be used for sorting of both numbers and characters or words.

For example if your file myfile.txt contains the following data :

Raju
Khanal
aayush

If you want to sort it the following command gives you the sorted list

sort myfile.txt
Output :
aayush
Khanal
Raju

45. The cut command is used for removing sections from each line of files. For instance consider a file myfile.txt with the following data

IIT2009009:Raju:Khanal
IIT2009010:Pankaj:Bhansali

cut -c1-10 myfile.txt
Output :
IIT2009009
IIT2009010

cut -c4,8 myfile.txt
Output:
20
20

cut -d: -f2 myfile.txt
Output:
Raju
Pankaj

46. The paste command is used for the merging the lines of files.

paste <  file1 > < file2 >
Output shows that the two files are combined line-by-line with tabs separating the two.

47. The join Command is used for joining lines of two files on a common field.

File1 data 
IIT2009009 30
IIT2009010 40

File2 data :
IIT2009009 40
IIT2009010 50

join File1 File2
Output :
IIT2009009 30 40
IIT2009010 40 50

Note : Make sure the files are in sorted order.

48. uniq command is used to omit the repeated lines.

Myfile.txt data
Raju Khanal
Raju Khanal
Aayush Khanal

uniq -u myfile.txt -> For non-duplicated lines use the -u flag.
Output :
Aayush Khanal

uniq -c myfile.txt -> For counting the number of times each one appears -c flag is used.
Output :
2 Raju Khanal
1 Aayush Khanal

49. spell : It is a spell checking program which prints each misspelled word on a line of its own. When used with the -n option it gives the line numbers before lines.

spell -n myfile..txt gives the line number and the misspelled word in the myfile..txt file.

50. finger command : The finger command displays information about the system users.

51. Say you want to add a user but don’t know the command to do it. Don’t worry with the power of the Linux command line you don’t have to mug up the commands. You can use the man command with the option -k to search by keyword user for relevant commands.
Note : The man -k is equivalent to apropos command which is used to search the short manual page descriptions for keywords and display any matches.

52. The general forms of the sed command are as follows:

Substitution sed:  ‘s/regexp/replacement/g’ < file >
Deletion sed:  ‘< start >,< end > d’ < file >

sed ‘s/raju/aayush/g’ myfile.txt :
=> To change all occurrences of the word ‘raju’ to ‘aayush’ in the myfile.txt.txt filewhere “s” means substitute, and the “g” means make a global  change. If you leave off the “g” only the first occurrence on each line is changed

sed ’2,3d’ myfile.txt 
=> To remove lines starting from line number 2 and ending in line number 3 including the third line

53. With the ‘awk’ command, you can substitute words from an input file’s lines for words in a template or perform calculations on numbers within a file.
 Ex : Consider a file with the following data

Raju is 57 Kg and Aayush is 60 Kg.
Pankaj is 60 Kg and Pramod is 55 Kg.

awk ‘{ print $1 ” and ” $6  }’ < filename >
Output :
Raju and Aayush
Pankaj and Pramod

awk ‘{ print “Average of both are ” ( $3 + $8 ) / 2 }’ < filename >
Output :
Average of both are 58.5
Average of both are 57.5

54. expr is used for evaluating expressions.

Ex: expr 10 + 3 gives 13.

Note: For multiplication it is \* and not *.

55. bc , the , Linux calculator program can be used to perform arithmetic calculation.
For instance if you want to multiply 10 and 3, just use the bc command and then give the expression 10 * 3 to get the desired output.
Note : In this case the multiplication symbol is the traditional “*” instead of “\*” as we used in expr .

56. whatis is a command that displays the manual page descriptions. If you explore your Linux system, you will find a lot of programs and you may not know what they do.

Ex : whatis cat
Output :
cat (1) – concatenate files and print on the standard output.

Note : man -f cat gives the same output.

57. The tee command reads from standard input and write to standard output and files.

ls -l | tee dir_list => For directing the output of ls -l to dir_list file.

When used with the -a option it is used for appending ( remember the >> operator )

58. zcat will read gzipped files without needing to uncompress them first.

zcat myfile.txt.gz => To read the gzipped file myfile.txt.gz

59. The file command is used to determine the file type.
A directory known as abc if cannot be identified from its color, can be identified from the file command.

file abc
Output :
abc: directory

60. The diff command is used to compare files line by line. When used with the -y option ( side by side ) the output is obtained in two columns differentiating the two files increasing readability.
Note : The sign “|” in the output indicates that there is a difference whereas the sign “>” and “<” indicate what is left out or added.

61. The cat command provides three useful options -v for displaying non-printing characters , -t prints “^I” for each Tab in the file and -e prints a “$” at the end of each line to indicate a NULL character.

cat -vet < filename >

62. mkdir used with the -p option creates nested directories.

63. The tac command concatenates and prints files in reverse.

64. The dict command can be used to find out the meaning of any word.

dict encumbered
gives you the meaning, thesarus words and sample sentences of the word encumbered.

Note: The dict package is neccessary for the usage of the dict command.

65. The style command analyses the surface characteristics of a document giving the sentence info, word usage, sentence beginnings and so on.

66. The locate command can be used to find files by name. It is an alternative to the find command and can be really useful.

locate < filename > gives the path of that filename.

67. useradd -G developers, ftp, admin  naveen

                Create new user naveen and add Groups developers, ftp, and admin

68. usermod -a -G  developers  naveen

                Adding group to an existing user naveen.

General

69.  free –m   Show amount of (remaining) RAM (-m displays in MB)

70.  chsh -s /bin/bash   for changing login bash fro user

71.  mount -o remount,rw /dev/root   /system remount with rw permissions

72.  : set hlsearch à highlight search for Vim editor

73.  watch -n 60 my_command -Execute my_command repeatedly at 60-second intervals (the default interval is 2 seconds).

74.  lsof  -List the opened files

75.  Fore ground and Background process

any_command & Run any command in the background

or

Run a command, after that we use < ctrl > Z to put the process in Suspend State and use the                                                                         command bg to put the process in background

jobs - List my background or stopped processes and show their job numbers.

         fg  job_number  - Bring a background or stopped process to the foreground.

         bg  job_number  - Place a process in the background, so it is exactly as if it had been started with & 

disown -h %1  - The disown prevents a SIGHUP to the process if my terminal dies.

IP  Cmd’s

Display Current Config for all NIC's: ifconfig
Display Current Config for eth0: ifconfig eth0
Assign IP: ifconfig eth0 IPAddress (192.168.1.1)
Ping: ping -c 3 IPAddress (192.168.1.1)
Assign multiple IP's: ifconfig eth0:0 IPAddress (192.168.1.1)
Assign second IP: ifconfig eth0:1 IPAddress (192.168.1.1)
Disable network card: ifconfig eth0 down
Enable network card: ifconfig eth0 up
View current routing table: route "or" route -n
View arp cache: arp "or" arp -n
Assign IP/Subnet: ifconfig eth0 192.168.1.2 netmask 255.255.255.0
Assign Default Gateway: route add default gw IPAddress (192.168.1.1)
Trace Route: traceroute www.whatismyip.com
Trace Path: tracepath www.whatismyip.com
DNS Test: host www.whatismyip.com
Advanced DNS Test: dig www.whatismyip.com
Reverse Lookup: host 66.11.119.69
Advanced Reverse Lookup: dig -x 66.11.119.69
ip route add default via <ipaddressofyourmobile>