Commands

There are tons of commands in a Linux system. And I think it is not important to memorize all of them; well, it's kinda impossible. However, once one starts working with a system, one usually gets familiarized the frequently used commands. And of course, the "TAB" button is always helpful.

Command Pattern

All commands in UNIX follow this pattern

 command     -arg/opt     source          destination

If the arguments consist of more than one alphabet, it is written as --argument. Different commands have different structures. Some commands don’t need any arguments, some commands use source only, some commands only need destination. You’ll get used to the structures as you keep working with Linux.

pwd (Present Working Directory)

Syntax:                   command

pwd is used to view absolute path of current working directory that one’s working in.

[root@prime audit]# pwd

/var/log/audit

cd (Change Directory)

Syntax:                   command      destination

cd is used to change a directory. The destination may be absolute or relative path.

# cd  /var/log/audit/	Goes to /var/log/audit
# cd	Goes to the home directory of the user
# cd ..	Goes ‘UP’ one level

mkdir (Make Directory)

Syntax:                   command      -arg          destination

This command is used to create new directories/folders.

# mkdir  testdir	Creates a new directory ‘testdir’ in pwd
# mkdir  /tmp/testdir2	Creates a new directory ‘testdir2’ in /tmp
# mkdir -p /newdir/testdir3/testdir4	Creates all necessary directories in the path as necessary

touch (Creates a new empty file)

Syntax:                   command      -arg          destination

This command is used to create a new empty file.

# touch  testfile	Creates a new file ‘testfile’ in pwd
# touch  /tmp/testfile2	Creates a new file ‘testfile2’ in /tmp

ls (list)

Syntax:                   command      -arg          destination

This command is used to see the contents of a directory. Contents of a file cannot be viewed by ls.

# ls	Shows content of pwd
# ls –l	Shows detailed information about content of pwd
# ls –l /tmp	Shows detailed information about content of /tmp
# ls –a	Show all files (including hidden) in pwd
# ls –a /tmp	Show all files (including hidden) in  /tmp
# ls –lh	Shows file size in human readable format
# ls –alh	Shows all file size in human readable format

cp (Copy)

Syntax:                   command      -arg          source     destination

cp is used to copy a file from one location to another one. The address of the source/destination may be absolute or relative path.

# cp  f1  /tmp	Copies the file ‘f1’ from pwd to /tmp
# cp  f1  /tmp/f2	Copies the file ‘f1’ from pwd to /tmp and renames it as ‘f2’
# cp  /root/f2  /mnt/f32	Copies the file /root/f1 to /mnt and renames it as ‘f32’
# cp  -r  /root/testdir  /tmp/testdir	Copies the directory /root/testdir to /tmp. The ‘-r’ indicates the operation will be recursive i.e. all files and subdirectories in ‘testdir’ will also be copied.

mv (move)

Syntax:                   command      -arg          source     destination

cp is used to copy a file from one location to another one. The address of the source/destination may be absolute or relative path.

# mv  f1  f2	Renames ‘f1’ to ‘f2’
# mv  f1  /tmp	moves the file ‘f1’ from pwd to /tmp
# mv  f1  /tmp/f2	Moves the file ‘f1’ from pwd to /tmp and renames it as ‘f2’
# mv  /root/f2  /mnt/f32	Moves the file /root/f1 to /mnt and renames it as ‘f32’
# mv  /root/testdir  /tmp/testdir	Moves the directory /root/testdir to /tmp.

rm (Remove)

Syntax:                   command      -arg          destination

This command is used to delete files and directories. To delete directories, the option –r must be used.

# rm  f1	Removes file ‘f1’ in pwd
# rm  /root/f2	Removes file /root/f2
# rm  –r   testdir	Removes the directory ‘testdir’ in pwd
# rm  -rf   testdir2	Forcefully remove the directory ‘testdir2’. Does not ask for confirmation.