Linux Essential Commands

1> ps (process status) - Display the characteristics of a process (i.e. terminal number, time required, PID no and command name)

•$ps –f Full listing showing PPID of each process

•$ps –u username Displays processes of user ‘username’

•$ps –a Processes of all users

•$ps –e processes including user and system processes

2> wait - Waits until all background processes are completed and then exits.

•$wait

3> sleep - Used to execute commands after certain amount of time by sleeping for given seconds.

•$sleep 10 ; ps

•ps command will be executed after 10 seconds

4> exit - Used to quit the shell.

•$exit

1> ls - It is used to lists files in the current working directory

•Syntax :- $ls [option]

Option  Meaning

-a          List all files including hidden file starting with ‘.’

-d          List directories - with ‘*’

-i           List file’s inode index number

-l           List with long format showing all user permissions

2> rm - It is used to remove the file.

•Syntax :- $rm filename

•$rm –i file1

•-i will ask to remove file

3> mv - It is used to move a file from one location to another. (Rename the file)

•Syntax :- $mv [option] source destination

•Example :- $mv abc.txt aaa.txt

To rename the file abc.txt to aaa.txt

•-i (interactive) : the mv command with –i option ask for confirmation

•Example :- $mv –i abc.txt aab.txt

4> cp - It is used to copies a file to destination file. •If second file doesn’t exist, then first it creates one and content is copied to it. But if it exists then it is simply overwritten without any warning.

•Syntax - $cp sourcefile destinationfile

•Example :- $cp xyz.txt zzz.txt

join - It is used for joining lines of two files on the basis of common field in each file called as a key.

•Syntax:- join [option] file1 file2

•Example:- join student1 student2

5> split - It is used to split the large file into smaller files. Default size is 1000 lines per file.

•Syntax :- split –n filename tagname

-n number of lines in each smaller file

-tag name by default the split builds the output files named xaa, xab, xac ….. If tag name specified, it replaces the x with that tag name.

•Example:- split -10 student

6> head - It is used to read the first ten lines of file. (10 lines by default). The number of lines to be displayed may be specified in the head command.

•Syntax :- $head –n filename

-n  means how many lines to be displayed

7> tail - It is used to print last few number of lines of a file  (10 lines by default).

•Syntax:- tail {[+/-] n [lbc]} filename

+n Displays all lines starting from nth line

-n Displays n lines from end of the file

l Indicate lines

b Indicates blocks

c Indicates characters

8> touch - It is the easiest way to create new, empty files.

•Syntax:- $touch filename

1> mkdir command - It is used to create a new directory in a current directory

Syntax : $ mkdir <directory name>

2> cd command – It is used to change directory. We can use it to change to any directory by specifying a valid absolute or relative path.

Syntax : $cd <directory name>

3> diff - This command is used to show difference between two text files. It also tells which line in one has to be changed to make the two files identical.

Syntax : $diff filename1 filename2

a - Add text to file

c – Changes to be made in the file

d – Delete lines

< - Lines from the first file

> - Lines from the second file

4> comm - This command compares two sorted files.

•It compares each line of first file with its corresponding line in the second file and generates three column output.

•The first column lists the lines only in first file

•The second column lists the lines only in second file

•The third column list the lines in both files.

Syntax : $comm filename1 filename2

5> cmp - This command is used to compare files. It uses two filenames as an argument and display the difference on the terminals.

•$cmp filename1 filename2

6> pr - Convert text files for printing. The pr command does minor formatting of files on the terminal screen or for a printer. It does not modify the original file.

Syntax : $ pr filename

7> chmod - It is used to change or set three permissions (read, write and execute) for all the three categories of user.

•Types of users – user, group and other

•chmod can be operated in two ways –

–Symbolic or alphabetical notation

–Octal or absolute notation

A> Symbolic or alphabetical notation

Option               Symbols   Meaning

Category          u                Owner of the file and directory

                          g                Members of the group belonging to the user

                          o                All other system users

                          a                All the users

Permission       r                 To assign read permission to a file

                         w                To assign write permission to a file

                         x                 To assign execute permission to a file

Operations      +                 Assign permission

                         -                 Removes permission

                        =                 Assign absolute permission

Syntax : - $chmod <category> <operation><permission><filename>

Ex.- $chmod ugo+x abc

Assign the execute permission for user/owner, group and other

B>Octal or absolute notation

•This method uses a number to specify each set of permissions for the file.

•It assigns permissions in three digits.

•First digit for owner, second digit for group and third digit for other.

•Digit range is 0 to 7.

Number             Assignment

0                        No permission

1                        Execute permission

2                        Write permission

4                        Read permission

5(4+1)               Read & execute permission

6(4+2)               Read & write permission

7(4+2+1)          Read., write & execute permission

Syntax : - $chmod <Three Digit Octal Number> <filename>

Ex.- $chmod 750 abc

Assign all permissions for user, read & write for group and no permission for other

1> tr - It is the abbreviation of translate or transliterate, indicating its operation of replacing or removing specific characters in its input data set.

•Syntax: $tr [option] SET1 SET2

•-d - delete characters in SET1, do not translate

•-s - replace each input sequence of a repeated character that is listed in SET1 with a single occurrence of that character

2> cut - It is used to copy columns to the standard output file. It is used to cut portion of a file.

•Syntax : $cut [option] filename

•-f - print only these fields

•-c - select only these characters

3> paste - This command is used to join files horizontal by outputting lines consisting of lines from each file specified, separated by tab as delimiter.

•Syntax: $paste [option] [files]

4> spell - It is a spell-checking program which scans a text file for misspelled words and print each misspelled word on its own line.

•Syntax: $spell [option] filename

5> sort - This command is used to sort a file.

•Syntax: $sort [option] filename

•-o - write result to FILE

•-n – sort a file numerically

•-r sort in reverse order

6> grep - It is used to search a pattern/word inside files

•Syntax: $grep <option> <pattern> <filename>

•-c – display the count of the number of occurrences of the pattern

•-l – display the list of the filenames which contains a pattern

•-n – display line numbers along with the lines containing a pattern

7> more - If the file is too large for its contents to fit in one screen, it will scroll off your screen. Use more command to display file contents page wise.

•Syntax: $ more [option] filename