Linux Commands

Today I think to post some linux commands which we are using daily. I hope it will be helpful to you. I am posted some examples how to do and what actually command do.

Note : 

Linux command can take a number  of options. These options can be passed in both shorthand form or expanded form. with shorthand form use single hyphen – and with expanded form use double hyphen -  -, say you can get help option to a command as -h or – -h

1 -> history -  this command will show you all the command you have typed in past

Example:

$ history – this will list all command used in past

$ history | grep netbeans - given that we have piped the result of history to grep it will return only those command , which contain word history in them

2 -> man – this command show complete detail of a command

Example:

$ man man - it will tell about how to use man itself

$ man sudo - will tell you all about sudo command

$ man dpkg - will tell you all about dpkg command

3 -> help - show the available options for a command

Example:

$ cd /netbeans-6.9.1/bin - move to netbeans bin directory

$ ./netbeans – -help - you want to start netbeans and want to know available options. It has listed down all options for you

General options:

  –help                         - show this help

 –jdkhome <path>     -  path to Java(TM) 2 SDK, Standard Edition

  -J<jvm_option>         - pass <jvm_option> to JVM

  –cp:p <classpath>   - prepend <classpath> to classpath

  –cp:a <classpath>   -append <classpath> to classpath

4  -> ls - Listing files and folders

Examples:

$ ls - will list all folder and files but not hidden files or folder

$ ls -a - will list hidden files and folders also

$ ls -B - will not list the backup file ending with ~

$ ls -l - will do complete listing with all details like owner, date, permission etc. but will not show hidden file

$ ls -l -a -B - will do all the above

5 -> mkdir - creating directory

Example:

$ mkdir myfolder – to make directory called myfolder

$ mkdir -p myfolder1/myfolder2/myfolder3 – the -p option tell to create nested directory with parent child relation. Thus myfolder3 will be created in myfolder2 a

$ mkdir -p myfolder - the p option means parent , if the directory already exist do not throw error but use the existing directory as parent

$ mkdir -pv -m 777 myfolder - it will not complain if directory exist, set the permission to 777 and also print the task it performing on terminal

6 -> cd - changing directory

Examples:

$ cd myfolder - take you inside myfolder or it will show inside the folder

$ cd .. - take you out from current folder to the previous folder

$ cd ~ - take you out of every previous folder to the root of terminal

7-> touch - it will create a file if not present and if already present will modify its modification or access time depending on argument you pass to it, but not overwrite its content

Examples :

$ touch test1 - will create test1 file

$ touch test1 test2 test3 test4 - will create test2 test3 test4 as new file. Since test1 is already there it will just modify its access time

$ touch -a test1 - will change access time of test1 file

$ touch -m test1 - will change modification time of file test1

$ touch -am test1 - will change both access as well as the modification time

$ touch -d '1 May 2005 10:22' test1 - with d option you can set any date you want but maintain the formate of string, say '1 05 2005 10:22' otherwise '2005 1 May 10:22' will give error. however, you can ignore time if you want

$ touch -d '1 May 2005' test1 - it will set only the date, if you do not pass year then it will take current year i,e '1 May' will also work

$ touch -r test2 test1 – the -r is reference option i,e it will set datetime of file test2 to test1 file

8 -> rm - removing file or folder, by default it remove only files to remove a folder, you need to pass additional options

Example :

$ rm my_file - will remove my_file

$ rm -f my_file - will not throw error if my_file do not exist i,e it will ignore the removal

$ rm -fi my_file - Will ignore if file is not there and if here ask you to press Y to delete the file i,e it will become interactive

$ rm -r my_folder - The -r option must be passed to delete a folder other while it will not delete it

$ rm -rf my_folder - will delete the folder and will not complain if the folder do not exist

9 -> rmdir - it will list empty directory

Examples:

$ rmdir dir1 dir2 dir3 - it will delete the empty directory dir1 dir2 dir3

$ rmdir -p dir1/dir2/dir3/dir4 - It will delete the parent also if deleting the child make it empty, so first dir4 will be deleted then dir3 then dir2 and so on

10-> mv - It will move file or directory to other location. if the location of source destination is same the file or directory will be get renamed

Examples :

$ mv myfile.txt myfile.rb -  it will rename myfile.txt to myfile.rb i,e the extension name is changed.

$ mv -bvi myfile.txt myfile.rb - here we are passing some option b is to backup the file before renaming, i is to  ask your yes no before renaming and v means print what going on the console

$ mv test1 test_folder myfile.rb /home/charmal/my_folder - it will move the file test , myfile.rb and the test_folder to /home/charmal/my_folder

$ mv *  /home/charmal/my_folder - it will move everything in current folder to /home/charmal/my_folder

11-> grep - It is used to search a pattern specified by user in the given text or files or block passed from first command to it through the pipe symbol |

$ grep “my works” file1 file2 file3 - It will list all lines of these three file containing “my woks” in it in separate line, each line will be preceded with name of the file in which they appear. If you do not want the file name you can use -h option as below

$ grep -h “my works” file1 file2 file3 - It will not precede the matched line with the file in which they are found

$ grep -i “my works” file1 file2 file3 - it tell to ignore the case i,e “my woks” will also match

$ grep “my works” * - will search the text “my woks” in all the text file in the current directory. Search for a given string in all files recursively.

$ grep -r “my works” * - the -r option will make it to search the text “Linux is good” in all the text file recursively in the current directory as well as all it child directory

$ grep -c “my works” file1 file2 - It will not print each line but just return the count of match in each file

$ grep -n “my works” file1 file2 - It will also print the line number of the line containing the pattern in each file

$ grep -rni “my works” * - will recursively search each directory, print the line no and will ignore the case

$ ls -l | grep “foo” - it will list only those file and directory which containg the word foo

$ history | grep rails - will list history of only those command which contain the word rails

$ history | grep -in rails - will ignore case,  and print line no.

$ grep -i "the" demo_file - Search for a given string in a file (case in-sensitive search).

$ grep -A 3 -i "example" demo_text - Print the matched line, along with the 3 lines after it.

12 -> less - can be used to see content of a file .It allow you to display a large data in block and let you to scroll down, basically, it keep the screen at first line and let you scroll down. If you do not use this, screen will set to the last line and you have to scroll up to see the first line. you can exit the content by pressing q .

Examples:

$ less myfile.txt - It will show content of file from the top, need to scroll down to see more content. press q to exit the content

$ ps -ef | less - will show the result of ps -ef command from the top

13 -> more - It can be also used to display content of a file on terminal.

Examples:

$ more myfile - will show that much content of file which fit in screen

$ more -100 + 50 myfile - will show 100 lines of file starting from line 50

$ more +/”Iam charmal” -100 - will show 100 line of the file from the line which contain “Iam charmal”

$ more -s -u -100  +50 myfile -  s will cause to remove blank line from display and u will remove underlines

$ ps -ef | more -10 + 20 - will display 10 lines of the result of command ps -ef starting from the 20 line

14 -> cat - will be used to create a file, read a file, concenate content of many files

Example:

$ cat >> test_file1 - it will create test_file1 and when you press enter it will take you to the next line where you write to it, to exist press enter then press ctrl and d.I have pressed enter on last line to reach here again pressed eneter to come to next line and keep writing.All content Iam writing will get written to test_files1to exist writing we will press Enter followed by Ctrl D

$ cat test_file1 - it will show you content of file test_file1, so you can see the content you have written on the terminal. I have pressed enter on last line to reach here again pressed enter to come to next line and keep writing. All content I am writing will get written to test_files1 to exist writing we will press Enter followed by Ctrl D

charmal@charmal-kavirathna:~$ cat -n test_file1 #the -n option will print the line no also

·         I have pressed enter on last line to reach here

·         again pressed eneter to come to next line and keep writing

·         All content Iam writing will get written to test_files1

·         to exist writing we will press Enter followed by Ctrl D

$ cat >> test_file2 - will create a new file

This is the second file you added this line to test_file2

$ cat test_file1 test_file2 >> test_file3 - will create file test_file3 and append content of test_file1 test_file2 to it

$ cat test_file3 - it show the content of test_file3, you can see that this file contain content of both the files

I have pressed enter on last line to reach here again pressed enter to come to next line and keep writing. All content writing will get written to test_files1 to exist writing we will press Enter followed by Ctrl D. This is the second file

15 -> echo - you can use it to write a string to file or terminal itself

Example:

$ echo Iam Charmal - it will print the output to terminal Iam Charmal

$ echo Iam Arun to test_file >> test_file - it will append the string to the test_file, if test_file is not present >> operator will create it and the append to it

$ cat test_file - we will see the content of test_file

$ echo Iam adding one more line >> test_file - one more line is appended

$ cat test_file - you can see both the line

$ echo this “>” operator is dangerous as it will erase and write > test_file # see the danger of using >, if the file exist it will erase its content before appending

arun@arun-yadav:~$ cat test_file

this > operator is dangerous as it will erase and write # see earlier line is erased and only single line present

16 -> pstree - will return all the running process in tree structure with there name and process id

Example:

$ pstree - will list all the running process with all the user

$ pstree root - will show all the process running with root user

$ pstree arun - will list all the process running with the user arun

$ pstree -p arun - will print the PID i,e process id also with the name

$ pstree -pn  - will print the PID and also in the sorted order

$ pstree -pnh - the h option will highlight the current running process

$ pstree -pnhu - the u option will also print the user owning that process

$ pstree -pnhul - l option will not allow to truncate the process detail if it is too long

17 -> pgrep - It will return PID of the process

Examples:

$ pgrep firefox - will return PID of firefox

$ pgrep ruby - will return PID of ruby

$ pgrep skype - will return PID of skype

18 -> ps - it means process status and return all the running process with name and PID

Examples :

$ ps - will return currently active process

$ ps -e - will return all the running process

$ ps -ef - will return all running process with full detail

$ ps -ef | less - as process list is very long, we have piped the output of ps -ef to less command. It help you to display result in small blocks and let you to scroll down to see more

19 -> pkill - will kill the process passed to it

Examples:

$ pkill ruby

$ pkill firefox

20 -> kill - will kill the process whose PID is passed to it

Examples:

$ kill 485 - will terminate the process with PID 485. here the default signa 15 get passed, there may be chance that it fail to kill the process, say if it is making some system call

$ kill -9 485 - it will be guaranteed that the process will be called. If the process is making any system call, it will wait and kill it when it return from the call

21 -> killall - will kill the process along with its child whose name passed to it. helpful if you know the process name

Examples:

$ killall nautilus  -will kill the nautilus process

$ killall -e nautilus - it will tell to kill whose name exactly match with nautilus. It will be helpful if your process name is more that 15 character, since killall by default match only upto that, so will delete all other process also whose first 15 character matches. the -e option will prevent this.

$ killall -ew nautilus - the w option will tell to wait till all the process are killed, otherwise killall check it every second by default and resend the signal if found some is still running, but since some has already get  killed, some signal become unresponsive. Also killall can kill all the process, including other killall process, except itself

22 -> pwd - will print the current working directory

23 -> clear - It will remove all previous command and output from the console

24-> find command 

Examples:

$ find -iname "MyCProgram.c" - Find files using file-name ( case in-sensitve find)

$ find -iname "MyCProgram.c" -exec md5sum {} \; - Execute commands on files found by the find command

$ find ~ -empty - Find all empty files in home directory

25->SSH Command

Examples:

$ ssh -l jsmith remotehost.example.com - Login to remote host

$ ssh -v -l jsmith remotehost.example.com - Debug ssh client

$ ssh -V OpenSSH_3.9p1, OpenSSL 0.9.7a Feb 19 2003 – to Display ssh client version

26-> unzip command

Examples:

$ unzip test.zip - To extract a *.zip compressed file

$ unzip -l jasper.zip - View the contents of *.zip file (Without unzipping it)

Archive:  jasper.zip

  Length     Date   Time    Name

 --------    ----   ----    ----

    40995  11-30-98 23:50   META-INF/MANIFEST.MF

    32169  08-25-98 21:07   classes_

    15964  08-25-98 21:07   classes_names

    10542  08-25-98 21:07   classes_ncomp

Later i will post another commands.

THANK YOU