Linux Commands

The syntax for all Linux command is as follow.

command [- options] [arguments]

The 'command' is a Linux command. Options are always followed by a hyphen (-) sign. Only one hyphen is used if a user want to use more than one options. Options are used to change the way of working of command. Options are followed by arguments. The Linux commands perform operations on the arguments.  

$ ls -l /bin

In the above example 'ls' is a command. '-l' is an option and '/bin' is an argument.  

1. man command

Stands for Manual-gives help about Linux commands

Examples

$ man cat

Above command will display information about cat command.

$ man -k file

-k stands for keyword. Gives information about commands related to the files

2. ls command

ls - used to display the list of sub-directories and files in the directory.

Examples

$ ls

Displays the list in current directory.

$ ls /bin

Displays the list in /bin directory

$ ls -l

Displays the long list-more detailed information

3. cat command

cat - used to create, show, copy and conatenate files.

Examples
$ cat > abc.txt

Creates a fie named abc.txt.

$ cat abc.txt

Displays the content of abc.txt file.

$ cat abc.txt > xyz.txt

Copies the content of abc.txt to xyz.txt-overwrites the content of xyz.txt.

$ cat abc.txt >> xyz.txt

Copies the content of abc.txt to xyz.txt-append the content of abc.txt to the end of xyz.txt.

$ cat abc.txt xyz.txt >con.txt

Concatenate the content of abc.txt and xyz.txt and write to the con.txt.

4. date command
Used to check and set date.

Examples

$ date
Print the system date in default format,
$ date “Date is %d-%m-%y”
Print the date in specified format.
$ date –d “May 20 2019”
Print the specified date instead of system date.

$ date –d “next sun”

Print the date of next Sunday.

$ date –d “1 month ago”

Displays the date one month ago from the current date.

Displaying dates from a file. First we need to create a file having dates. 

$ cat >myfile

        Dec 11 2019 16:50:22 

        Nov 18 2011 

        April 22 1960

      Ctrl + d

$ date –r myfile
Above command displays the dates from the file 'myfile'

$ date +%Y%m%d -s "20200110"

$ date -s "01 JAN 2020 11:14:00" 

Both the commands above sets the date to the specified date.

5. mkdir command

This command is used to create new directories.

Examples

$ mkdir mydir

Above command will create a directory named 'mydir' in the current directory.

$ mkdir mdir1 mdir2

Above command  will create two directories in the current directory.

6. cd command

The cd command is used to change the current working directory.

Examples

$ cd mydir

Above command will change the current working directory to 'mydir'.

$ cd or cd ~

Both the commands will change the current working directory to user's home directory.

$ cd ..

This command moves to the parent directory of current working directory.

$ cd /

Move to the root directory.

$ cd  /bin

Above command will change the current working directory to the /bin directory.

7. pwd command

Stands for Print Working Directory. This command is used to show the present working directory.

Examples

$ pwd

 

8. rmdir command

This command is used to delete empty directories.

Examples

$ rmdir mydir

Above command will delete ‘mydir’ directory from the file system if it is empty.

$ rmdir –r mydir

Above command will delete the directory even if it is not empty. –r means recursively-recursively delete files in the directory.

9. touch command

This command updates the access time and modification time of file. It can also create a file if file does not exist.

Examples

$ touch myfile

This command will change the time stamp of ‘myfile’ file if this file exists. Otherwise it will create this file.

$ touch -a myfile

Changes only Access Time.

$ touch -m myfile

Changes only Modification Time.

10. cal command

'cal' stands for calendar. This command is used to display the calendar.

Examples

$ cal

This command displays the current month of current year.

$ cal 1992

This command will display the whole calendar of year 1992.

$ cal 1 2012

This command will display the first month of the year 2012.

$ cal –m

Above command will display Monday as the first day of the week.

11. tput command

'tput' command is used to control the terminal settings. 

Examples

$ tput clear

This command clears the terminal screen.

$ tput cup 5 6

‘cup’ stands for cursor position. Moves the cursor to the 6^th column of the 5^th line.

$ tput  line

This command displays the number of lines.

$ tput bold

This command set the bold attribute.

$ tput sgr0

This command reset all the attributes.

$ tput smul

Above command starts the underline mode.

$ tput rmul

Above command ends the underline mode.

$ echo `tput bold` Linux `tput sgr0`

This command prints the Linux in bold.

$ tput cuu 7

Moves the cursor 7 lines up.

$ tput cud 6

Moves the cursor 6 lines down.

$ tput rc

This command reset the cursor position.

$ tput setf 4

This command will set the text color to red.

$ tput setb 4  

This command will set the background color to red.

12. expr command

'expr' command is used to perform arithmetic operations on integer values and string opeartions. Arithmetic operators (+, -, *, / and %) , Relational operators (=, !=, <, >, <= and >=) and Logical operator (&, | and !) can be used with this command.

Examples

$ expr 4+5-2

The result of above command is 7.

$ expr 9 = 10

The above command returns 0 (false).

 $ expr 9 = 9

The above command returns 1 (true).

$ expr length Linux

Returns the length of string. The result of this command is 5. The string ‘Linux’ has 5 characters.

$ expr index Linux n

This command will return the index of character ‘n’ in string ‘Linux’. Output is 3.

$ expr substr Redhat 4 3

This command will return ‘hat’. ‘substr’ return the sub string from 4^th index to the length of 3.

13. bc command

‘bc’ stands for Basic Calculator. This command is used to perform basic calculation even on floating values.       

Write ‘bc’ on the command prompt. Then ‘bc’ takes the control and starts its own prompt. Here a user can perform arithmetic operations as given below.  

Examples

$ bc

5+5

10

Press ctrl+d to get out of ‘bc’ command.

$ echo “4.9+1.2”| bc

This command will add 4.9 and 1.2.

$ echo “a=6;++a;”|bc

The output of above command will be 7.

$ echo ‘if (4>5) print “Greater “ else print “Smaller”’|bc

14. uname command

This command is used display information about the hardware and operating system.

Examples

$ uname –a

Provides information about everything.

$ uname –m

This command provides the information about the machine.

$ uname –n

This command will display the hostname of the computer.

15. tr command

‘tr’ command is used to translate characters in a string.

Run this command on the command prompt. Then type strings and press enter. Press ctrl+d to exit from the ‘tr’ command.

Examples

$ tr  a o

Type strings here

Ctrl+d

$ echo “Hit”|tr i o

The output of this command will be ‘Hot’. The character ‘a’ will be replaced with ‘o.

$ cat myfile | tr '[A-Z]' '[a-z]'

Here ‘myfile’ is a file name. This command will convert all the upper case letters into lower case.

16. who command

‘who’ command is used to display the list of users who are logged in.

Examples

$ who

Above command will display the list of users.

$who am i

This command displays the information about the user who run this command.

17. cp command

This command is used to copy files from one directory to another.

Examples

$ cp /home/mydir1/myfile1.txt   /home/mydir2/

This command will copy the file ‘myfile1.txt form the ‘mydir1’ directory to ‘mydir2’ directory with same name.

$ cp /home/mydir1/myfile1.txt   /home/mydir2/myfile2.txt

This command will also copy the file but the file will be renamed to ‘myfile2.txt’.

18. mv command

This command is similar to the ‘cp’ command. The purpose of this command is to rename and move files.

Examples

$ mv /home/mydir1/myfile1.txt   /home/mydir2/

Above command will move the ‘myfile1.txt’ from the ‘mydir1’ to ‘mydir2’ directory.

$ mv /home/mydir1/myfile1.txt   /home/mydir2/myfile2.txt

This command will move and rename the file.

19. rm command

This command is used to remove one or more files.

Examples

$ rm myfile.txt

This command will remove myfile.txt from the storage.

$ rm –d mydir

Above command will remove the directory mydir.

$ rm file1 file2 file3

This command will remove multiple files.

$ rm –i myfile.txt

‘i’ stands for interactive. Confirms before deleting file.

20. find command

The ‘find’ command finds the file by name, type, owner, group, date or permission.

Examples

$ find file1.txt

The above command will search the file1.txt.

$ find –atime +8

This command will display the files accessed before 8 days.

$ find –atime -8

This command will display the files accessed after 8 days.

$ find $HOME –print

Lists all files and directories in home directory.

21. ln command

‘ln’ command is used to create soft link (like windows shortcut) or hard link of file and directories.

Examples

$ ln /etc/passwd  /user/home

This command will create a (hard) link of ‘passwd’ file in the user’s home directory.

$ ln –s /etc/passwd  /user/home

This command will create a soft link of ‘passwd’ file in the user’s home directory.

22. more command

‘more’ command is used to display the output page by page.

Examples

$ more myfile.txt

This command will display the content of file page by page.

$ ls –lr | more

This command will display the list of file and directories page by page.

23. less command

‘less’ command is similar to the ‘more’ command. This command need to press ‘q’ to quit.

Examples

$ less myfile.txt

24. Redirecting Input and Output

Most of Linux commands take input from standard input and outputs to standard output. The input and output of the commands can be redirected to the files.

A. Input Redirection

Command can get input from a file.

Examples

 $ tr a A<myfile.txt

‘tr’ command cannot take input from a file so we need to redirect its input. In the above example ‘tr’ command will replace the ‘a’ with ‘A’ in the file myfile.txt

B. Output Redirection

By default command displays the output on the screen. The output of commands can be redirected to the files using > or >> signs.

$ ls –l>myfile.txt

This command will redirect the output to the file ‘myfile.txt’ instead of showing on the screen.

$ man ls>>myfile.txt

‘>’ overwrite the content of existing file. If a user wants to append the data at the end of file he needs ‘>>’ sign.

25. Piping

With the help of piping the output of one command can be the input of other command. A user can combine the features of different commands.

Examples

$ ls | wc –c

The output of ‘ls’ command will be the input of ‘wc’ command. So this command will count the number of files and directories in the current directory.

$ ls –lr | more

This command will show the output of ls command page by page.

$ sort myfile.txt | uniq

This command will sort the content of file myfile.txt and display the unique content of file.

26. tee command

The ‘tee’ command helps to redirect the output as well as display the output on the screen. This command is used with pipe.

Examples

$ ls –l | tee sfile.txt

Above command will display the output on the screen and it will also store the output in the file sfile.txt.

27. grep command

The ‘grep’ command is used search words in files using regular expression.

Examples

$ grep  linux myfile.txt

$ grep  ‘linux’ myfile.txt

This command displays the lines having ‘linux’.

$ grep –c  linux myfile.txt

This command will count the number of occurrence of word ‘linux’ in file myfile.txt.

$ grep -c /bin/tcsh /etc/passwd 

Lists the number of users who use tcsh shell.

$ grep ^linux myfile.txt

$ grep ‘^linux’ myfile.txt

This command finds ‘linux’ in the beginning of the line.

$ grep ‘linux$’ myfile.txt

This command finds ‘linux’ at the end of the line.

$ grep “[aeiou]” myfile.txt

This command displays the lines having any character among a, e, i, o or u.

28. head command

The ‘head’ command is used to print the first (Bydefault Top 10) line of one or more files.

Examples

$ head myfile.txt

This command will print the top 10 lines of the file myfile.txt

$ head -20 myfile.txt

This command will print the top 20 lines of the file myfile.txt

$ ls| head -5

This command will display the 10 files in the current directory.

29. tail command

This command displays the last (Bydefault 10) lines of the file.  

Examples

$ tail myfile.txt

This command will display the last 10 lines of the file.

$ tail -15 myfile.txt

This command will display the last 15 lines of the file.

$ ls | tail 5

This command will display last 5 files in the current directory.

$ head -15 myfile.txt | tail 10

This command will display lines between 5^th and 15^th line. 

30. cut command

The ‘cut’ command allows a user to extract selected columns or fields from one or more files.

Examples

$ cut –c2,4 mydata.txt

This command will display 2^nd  and 4^th column from the file mydata.txt.

$ cut –c3-5 mdata.txt

The above command will display columns from 3^rd  column to 5^th column.

$ cut –d”:” –f3,6 mydata.txt

This command will extract fields rather than columns. Delimiter is required to specify if default (tab) is not used.

31. paste command

The ‘paste’ command is used to combine the fields of one or more files.

Examples

$ paste file1 file2

This example will combine the line of  file1 and file2.

$ paste –d”:” file1 file2

The option ‘–d’ is used to specify the delimiter.

32. cmp command

The ‘cmp’ command is used to compare the content of files.

Exmaples

$ cmp file1 fil2

The above command will display the differences between both files.

$ cmp –s file1 file2 && echo “No changes”

This command will display ‘No changes’ if files are not different.

33. diff command