Linux Commands

Command	Description
General Commands
clear	Clears the Linux/Unix terminal screen
date	Displays the system date and time
cal	It Displays calander
type	Tells about type of command (whether internal or external)
alias	Creates alias of existing command
pwd	Shows your current working directory
man	Shows various options associated with a particular command
who	Display the user details who is currently login
File and Directory related Commands
touch	Creates the empty file(s) if it does not exist, Otherwise change the timestamp of file
cat	1.creates the file and save the content 2.Display the file contents 3.Append the content in existing file
file	Displays wether its file or directory
more	Pagewise display with %
less	Pagewise display without %
mkdir	Creates the new directory
cd	To change the directory
rm	To remove or delete the files / directories
rmdir	To remove a empty directory
ls	To list all files and directories of target directory
cp	Copy file(s) from source to destination
mv	Move the file(s) from source to destination
ln	To make hard or soft link netween the files
Text Processing Commands
paste	To combine the content of file files line by line and printing the files content horizontally
wc	Displays the no of lines words and characters of any data
head	Displays the lines from top to bottom as per option given
tail	Displays the lines from bottom to top as per option given
cut	Used to retrieves specific characters or fields from data or file
tr	Used on out put of some other command to translate characters
sort	Used to sort data, by default sorting happens as per the English disctionary
grep	Used to pattern search in the given file
uniq	Removes adjacent repeated lines in given data of file
cmp	Used to compare two files, Its pinpoints at only first diffrence
comm	Used to compare two files but can be apply only on sorted files
Process and Memory Related Commands
ps	Displays the list of process submitted to kernel
pstree	Displays the process in the form of tree diagram
bg	Displays the status of process running in background
fg	Used to bring any background job in foreground
nice	Execute a command with updated scheduling priority
kill	Used to send signal to perticular process
nohup	Execute the command in background even after user get log out
df	Print the system disk space usage and available
du	Estimate file space usage in current directory
Compression Utilities Commands
gzip	-To compress and decompress the file(s) -Compressed file(s) gets .gz extension
gunzip	Used to decompress the file having extension of .gz Works same as gzip -d file.gz
zip	-Use to compress file(s) to avoid compatibility problemsand with non-Linux users.
unzip	-To extract the contents of a zip file.
zcat	Displays the contents of compressed files
zmore	Used to view content pagewise of compressed file
tar	-Used to create a tape archive file -Extract from archive file
Find Command
Find is one of the powerful utility of Unix/Linux used for searching the files or group of in a directory hierarchy. Syntax :      $ find [pathname] [expression] Examples on the find utility : 1). How to find file 'abc.txt' from home directory downwards and print them ? $ find ~ -name abc.txt -print or $ find  ~  –type  f  –name  abc.txt  –print 2). How to find which ends with any digits and begin with 'abc' from home    directory downwards and print them ? $ find ~ -name "abc[0-9]" -print 3). How to find a directory name 'dirname' from home directory downwards and        print them ? $ find ~ -type d -name dirname -print 4). How to search a character special file 'abc.txt' from current directory ? $ find . -type c -name abc.txt -print 5). How to find file in current directory , which has inode value 76538 ? $ find . –inum 76538 –print 6). How to find all files from current directory , which have 2 links ? $ find . –links 2 –print 7). How to find all files from current directory , which have more than 2 links ? $ find . –links  +2 –print 8). How to find all files from current directory , which have less than 4 links ? $ find . –links  -4 –print 9). How to find all files from home directory downwards, which have permissions        755 ? $ find ~ –perm 755 –print 10). How to search a block special file ‘abc.txt’ from home directory downwards        and print them? $ find  ~  –type  b  –name  abc.txt  –print 11). How to search a file ‘abc.txt’ from home directory downwards whose owner        is ‘mukesh’ ? $ find ~ \( –user mukesh –a –name abc.txt \) –print 12). How to search all files from home directory downwards, which have size       1 MB $ find ~ –size 1M –print 13). How to search a file 'file.txt' from current directory downwards and execute         'rm' command  on the result ? $ find . –name file.txt –exec rm {} \; 14). How to search a file 'file.txt' from current directory  and execute 'rm'        command on the result after confirmation ? $ find . –name file.txt –ok rm {} \;   ( or ) $ find  .  –name file.txt –exec rm –i {} \; 15). How to search the file 'abc.txt' in the current directory and three levels down to the current directory ? $ find -maxdepth 4 -name "abc.txt" 16). How to print the files in the sub-directories between level 1 and 5 ? $ find -mindepth 2 -maxdepth 6 -name "abc.txt" 17). How to find the empty files in current directory? $ find . maxdepth 1 -empty 18). How to find all files in current directory? find . -maxdepth 1 -type f
Communication Commands
write command Send a message to another user who is logged in. Syntax $ write <user_name> Example mithun.s@mks-laptop:~$ write mukesh.kumar write: mukesh.kumar is logged in more than once; writing to pts/0 hi mukesh ,how are you ? in above command mithun.s sending message to mukesh.kumar as 'hi mukesh ,how are you ?' Now mukesh.kumar will get this on his terminal as below mukesh.kumar@mks-laptop:~$ Message from mithun.s@mks-laptop on pts/2 at 13:32 ... hi mukesh mesg command allow and disallow to other user to send message to you. Syntax $ mesg y[or n] Example $ mesg  -- Displays the status as below is y Disallow other users to send you message. $ mesg n $ mesg     -- Now it displays the status as below is n To allow other users to send you message type $ mesg y Note : Even if user disables the message he/she would receive the messages from the root. wall command Send a message to all the logged in user who having message recieving permission as yes. Syntax $ wall [-n] [-t TIMEOUT] [file] -n : no banner -t : timeout ,default time out is 300 sec Example $ wall hello world Broadcast message from root (pts/1) (Thu Feb 27 15:06:04 IST 2014): hello world ftp command Ftp (File Transfer Protocol) is the user interface to the Internet standard File Transfer Protocol. The program allows a user to transfer files to and from a remote network site. Syntax  $ ftp [hostname] put command to copy one file from the local machine to the remote machine $ put filename get command To copy one file from the remote machine to the local machine $ get filename To copy multiple files from the local machine to the remote machine;  you are prompted for a y/n answer before transferring each file $ mput file1 [file2...] copy all from the local machine to the remote machine,which are starting from 'f'. $ mput f* To copy multiple files from the remote machine to the local machine;  you are prompted for a y/n answer before transferring each file $ mget file1 [file2...] To copy all files from remote machine to the local machine which are starting from 'f'. $ mget f* To exit the FTP environment (same as quit) $ bye To terminate a connection with another computer System will give “Good Bye” message then type bye $ close To request a list of all available FTP commands $ help or $ ? User can also type ‘?’ symbol for to list all FTP command ![Command] Syntax $ ![Command [arg...]] Used to execute any command in local machine It shifts the control from ftp prompt to local machine $ ! This will shift the control from ftp prompt to local machine. Now user can perform any operation in local system by using the command prompt. To shift the control back to ftp prompt, use exit command.

                                                     Commands in detail                
Clear Command
It will clear your linux/unix terminal screen

$ clear

date command
To display the system date and time ,it will give the below output

$ date
Fri Feb 21 00:56:30 IST 2014

cal command
 1.To display the calendar for current month

 $ cal

 2. To display the calendar for Jan month(month value 01) for 2014

 $ cal 01 2014

 3. To display the complete calendar of year 2007

 $ cal 2007

Type command
 To check whether clear command is internal or external

 $ type clear
  clear is /usr/bin/clear

 Above command showing one path in output, means it is stored as a file, so clear is
 external command.

 Now let us check whether alias command is internal or external

 $ type alias

 alias is a shell builtin

 It is displaying the message “alias is a shell built-in”, so alias is internal command.
 alias Command
 To create alias name of an existing command

  $ alias c=clear

 now you can use only command 'c' instead of clear ,but its scope remain for the
 current  terminal

Who command
To display the details of all users who are currently logged in

$ who          --  Displays the self details as below

mukesh.kumar tty7         2014-02-26 19:26 (:0)
mukesh.kumar pts/0        2014-02-26 23:24 (:0.0)

$ who i am
O/P: mukesh.kumar pts/5        2014-02-24 17:43 (:0.0)

man command
This command can be use to find the various option associated with particular command.
Syntax  :

 man <command name>

To see the various option associated with who command on terminal type below command

$ man who

pwd command
This command will show your present working directory.

$ pwd -- will show as below
/home/mukesh.kumar/Desktop/Ayush_patient_data

touch command
Used to create the empty file ,it is also used to change the timestamp of existing files
Syntax

$ touch [option] file_name(s)

Examples
To create multiple empty file at a time ,type below command

$ touch file1 file2 file3 file4

To change both the access and modification times to the current time of existing file file1

$ touch -am file1

To use the times of file1 for file2, use oprion -r(reference) as below

$ touch -r file1 file2

The -d and -t options allow the user to add a specific last access time, to change the last access time of file1 to 10 Mar 2014 09:30 would be enclosed in single quotes
and used as follows

$ touch -d '1 Mar 2014 09:30' file1

cat command
Cat can be used for the following purpose
1.To reading or display the content of file
2.New file creation
Syntax

$ cat [option] [filename]

Examples
To reading or display content of file1 ,write below command

$ cat file1

To display the content of more than one files

$ cat file.txt abc.txt f1.txt

The above command will display the content of files in same order ,first it will show the content of file.txt then below that abc.txt then in last f1.txt

To create a new file and writing some content in file.

$ cat > file1.txt
  ----
  ----
 ctr+d (to save all content in file1.txt)

Note : if file1.txt already exist then new data will get override.

To append the data in existing file

$ cat >> file1.txt
  ----
  ----
 write some data and hit 'ctr+d' to save the file1.txt

file command
To check type of file ie: whether it is file or directory.
Syntax

$ file [option] [file/dirname]

more command
To display page wise ,useful when file having more pages which can not be display at a time on the computer screen.
The more command allow you to reading the content from page1

$ more file1.txt

less command
Similar to more command.
This command allows you to view a text file without fear of accidently modifying it, as you would with a text editor

$ less file1.txt

mkdir command
Creates the directory in the current directory, if they do not already exist.
Syntax

$ mkdir [option] [dirname]

Examples
To create a directory dir1 in current directory

$ mkdir dir1

To create a dir in a path ,suppose i would like to create a directory dir2 inside another directory name 'freelancer'
Then we need to give the path of the directory 'freelancer' followed by new directory 'dir2' as follow

$ mkdir /home/mukesh.kumar/Desktop/freelancer/dir2
       (or)
--in general

$ mkdir path/dir2

To create the hierarchy of directories ,use -p(parents) option

$ mkdir –p dir1/dir2/dir3

cd command
To change the shell working directory
Following command will change current directory to dirname if dirname is present in current directory

$ cd dirname

To change the directory in the mentioned path

$ cd path/dir1

To changes to the home directory

$ cd (or) $ cd ~

To changes to the root directory (i.e. />)

$ cd /

To changes to the Parent directory

$ cd ..

rm command 
Used to delete the file or directory.
Syntax

rm [option] [-r directories] [filename]

Examples
To remove multiple files

$ rm file1 file2 file3 [file4..]

To remove a directory recursively

$ rm -r dirname

To remove file interactively

$ rm -i filename.txt

rm: remove regular file `filename.txt'?

Note : if rm command used by using '-i' option it will always ask user to confirm by displaying a message.

Removing forcefully
Assume filename is not having write permission for the user. If the user is giving the below command

$ rm filename

It will display confirmation message before removing the file.
But if user is giving the command with ‘-f’ option as follows:

$ rm –f file_name

It will remove the file forcefully without displaying confirmation message

To remove a file whose name starts with a `-', for example `-foo',
use one of these commands:

$ rm -- -foo
   (or)
$ rm ./-foo

rmdir command
rmdir can be use to remove the empty directory

$ rmdir emptydir

ls (list) command
List information about the FILEs (the current directory by default).
Syntax

$ ls [option] [file]

Note : To check the various option ,use the following command

$ ls --help

Examples
To list all file and directories of current directory

$ ls

To list files and directories of target directory in long format
(includes accesspermissions, size of file, last modification date & time, no of links etc.)

$ ls -l

To list files and directories of target directory

$ ls dirname

To list all the files and directories (including hidden) of target directory

$ ls -a dirname

To list files and directories of target directory in reverse order.

$ ls -r dirname

Note : by default it shows in alphabetically.
To list files and directories of target directory with their inode value.

$ ls -i dirname

To list all the files and directories of target directory recursively

$ ls -R dirname

To list files and directories of target directory with information of format of file
(ie : directory name with ‘/’ as suffix, executable files with ‘*’ as suffix etc.)

$ ls -F dirname

To list with inode value

$ ls -i dirname

To list all the files and directories(including hidden files) of target directory in long format

$ ls -al dirname

To list files and directories with inode values recursively of target directory in long format

$ ls -iRl dirname

To list files and directories with last modified in reverse order

$ ls -ltr dirname

cp command 
To copy files and directories from source to destination.
Synatx

$ cp [option] source destination

Examples
To create copy of file1.txt with name file2.txt in same directory

$ cp file1.txt file2.txt

To create copy of multiple source file(s) with same name in source directory

$ cp file1.txt [file2.txt] dirname

To copy dir1 or content of dir1 in dir2

$ cp -r dir1 dir2

Note : if dir2 is not present : create dir2 in current dir,copy content of dir1 in dir2
            if dir2 present : COPY sub tree structure rooted at dir1 in dir2
           ( inside dir2 we will get dir1)
To copy files interactively. It will ask for confirmation by displaying a message to the user but only in the case of overwriting.

$ cp –i source destination

mv command
Used to rename source file/directory to destination name
Move source file/directory to destination directory
Syntax

$ mv [option] source destination

To rename source file with destination name

$ mv file1.txt file2.txt

To move source files in destination directory

$ mv file1.txt [file2.txt] dirname

To move dir1 or content of dir1 into dir2

$ mv -r dir1 dir2

Note : if dir2 is not present :rename dir1 with name dir2
           if dir2 present then move sub tree structure rooted at dir1 in dir2
To Move or rename interactively. It will ask for confirmation by displaying a message to the user but only in the case of overwriting.

$ mv –i source destination

ln( link ) command 
To create hard and soft link between the files
There are two types of link we can create

1.Hard Link :
While creating a link from source to Destination file.The changes done in the source file will be effected in destination file.
But when The source file is deleted the destination file remains the same.

2.Soft Link :
While creating a link using soft link from source to destination file → the changes done
in the source file will be effected in Destination file too.
But when the source file is deleted the destination file also gets invisible. i.e when u tried to check the destination file its says no such file or directory

To make hard link between the file

$ ln sourcefile destinationfile

To makes soft(or symbolic) link between files

$ ln -s source destination

Note : we can not create hard link for directory