UNIX / Linux Commands

In this post, I am going to share very useful and commonly used basic UNIX/Linux commands.

Basic UNIX Commands

man

The man command displays the online manual pages. Reads the manual page for a command.
Syntax: man command_name

ls

This command is used to list all the files and directories.

ls – l

It is called as the long list. It displays or lists all the files with their preferences. 

ls – a

This command is used to view all the hidden files.

mkdir

This command is used to create a directory. 

Syntax: mkdir dirname.

cd

This command is used for changing the directory

cd . 

Changes the home directory.

cd .. 

Changes the parent directory.

pwd

This command displays the path of the current directory.

cp

Copies the contents of one file to another. 

Syntax: cp file1 file2

mv 

This command is used to move or rename the files. 

Syntax: mv file1 file2 

rm 

This command is used for removing or deleting a directory. 

Syntax: rm file

rmdir 

This command is used for removing or deleting a directory. 

Syntax: rmdir directory

cat

This command is used to display a file.
Syntax: cat filename

cat: 

This command is used to concatenate the files. 

Syntax: cat file1 file2 >file3

more 

This command is used to display a file a page at a time. 

Syntax: more filename

head

This command is used to display the first few lines of a file. 

Syntax: head -n/+n filename
-n: n lines from the beginning are printed.
+n: n-1 lines are skipped or start from the nth line.

tail

This command is used to display the last few lines of a file. 

Syntax: tail -n filename
-n: n lines from the end are displayed.

wc

This command is used to count the number of lines/words/characters in the file. 

Syntax: wc <option(s)> filename
-w: To do a word count.
-l: To find out how many lines the file has.

grep

This command is used to grap regular expressions and patterns. It is used to search for a particular word in a file.
Syntax: grep <option(s)> keyword filename
-i: Ignores the case of the word.
-v: Prints the lines that do not match.  

pipe

It is used to join two or more commands where the output of command1 is the input of command2.
Syntax: command1| command2

sort

This command is used to sort the content or data. 

Syntax: sort <option(s)> filename

-n: Numeric sort. 

-r: Reverse sort. 

-d: Directory sort.

who

Lists users currently logged in. 

Syntax: who >filename
?: Matches one character.
*: Matches any number of records.

what is

Offers a brief description of a command.

apropos 

Matches command with the specified keyword in their man pages.
Syntax: apropos keyword. 

ps

This command is used to list the current processes.

chmod 

This command is used to change the access rights for the named file. 

Syntax: chmod [options] filename
-u: User
-g: Group
-o: Others
-a: All
-r: Read
-w: Write
-x: Execute
-+: Add permission
--: Take away permission

df

This command is used to report the space left on the file system. 

Syntax: %df

du

The number of kilobytes used by each subdirectory. 

Syntax: %du

nl

Appends line number. No number for empty files.

egrep

Useful for multiple patterns matching can be made file-based by –f option. 

Syntax: egrep <options> filename
-c+: One or more occurrences of character. 

-c?: Zero or one occurrence of a character. -a/b: Either a or b.
-(a): Regular expression.

fgrep 

Fast or fixed grep. It does not accept regular expression. In this one command is separated from the other by new line.
Syntax: fgerp ‘abc newline xyz’

tee

Directs the output to the standard output device. Syntax: tee <option> filename
-a: Appends to a file.

find 

This command is used to search for a file.
Syntax: find <option> filename
-atimen: True if file was accessed n days ago 

-ctimen: True if file was created n days ago. 

-namepattern: True if filename matches the pattern. 

-print: Print names of files found.

Cmp

This command is used to compare two files.

Syntax: cmp file1 file2 [skip1 [skip2] ] 

ReturnType:
0-files are identical 

1-files are different. 

>1-an error occurred.

Diff

This command is used to find differences between two files. 

Syntax: diff [options] from-file to-file.
Return Type:

-0 no difference
-1 some differences were found
-2 trouble.