Commands

We have a 3 different types of commands………………

1. Internal commands :- An internal command is a built-in command into the shell. The shell does not start a separate process to run internal commands. Ex- "cd" command

2. Aliases :- An alias is a user defined command as per need. Type alias on the command line  to see the provided default aliases. Ex- alias newcommand=’oldcommand’,

alias ll=’ls -l – color=auto'    Aliases are executed before anything else.

3. External commands :- When we type the name of a command, the shell first checks if it is a built-in command,executes it. If we write a command with its pathname beginning with like /,/usr/bin then shell does not search the command. It executes it with its path. If the command is not built-in or not specified absolute path then shell looks in its search path for an executable program or script with the given name.The search path is a list of directories that the shell look through for a command whose name matches with the command. External commands require the shell to fork and exec a new subprocess, this takes time. That’s why external commands are slow.

[root@TEST /]# type cd   //To check the type of command.//

$PATH variable is used to share or define the path of external commands. To find out command location use which.Ex:- Type “which ls” To run a command from the current directory you have to include ./ in front of command. The dot stands for the current directory, and by running it as ./ you’ll tell bash to look for the command in the current directory. 

STANDARD COMMANDS LINE TOOLS

1. Tilde (~) :- Shows that user is in home directory.

                         [root@TEST~]#

2. pwd :- Print working directory

                 [root@TEST ~]# pwd

                  /root

3. cd :- Change Directory

              [root@TEST ~]# cd /opt

              [root@TEST opt]#

4. To see the available files or directories in present directory or a specifid location

     [root@TEST opt]# ls                          // Shows in list form.//

    [root@TEST opt]# ls -a              // list in alphabetic order.//

   [root@TEST opt]# ll        // list with detailed information.//

  [root@TEST opt]# ll -t       // list in time ascending order.//

 5. 

     [root@TEST opt]# touch file1         // Create a empty file name “file1”//

     [root@TEST opt]# cat file1              // Display the file data in read format.//

    [root@TEST opt]# mkdir dir1           // Create a directory in present directory.//

   [root@TEST opt]# mkdir -p dir1/dir2/dir3      // Create multiple directory in one time.//

  [root@TEST opt]# mkdir /opt/dir1              // Create directory in /opt //

6. VI Editor:- We use it to edit files at command line.

    [root@TEST opt]# vi file1                      // Its open the file in coomand line//

    Press “i”:- To insert data or write in file.

    Press “Esc + / + word”:- To search a word location in file.

    Press “Esc + : + wq” :- To write and exit from file.

    Press “Esc + : + set number” :- To show the line number in vi editor.

7. 

    [root@TEST opt]# rm file1                 // To delete a file.//

    [root@TEST opt]# rm -f file1              // Delete file without ask you. F= forecefully//

    [root@TEST opt]# rm -rf                   // Delete directory within directory forcefully.//

8.

    [root@TEST opt]# find file OR dir-name          // Search in present working directory. //

    [root@TEST opt]# find /opt file/dir-name     // Search in specified /opt directory for file/dir. //

    [root@TEST opt]# locate file/dir-name           // Show you the exact path of file/dir. //

The above locate command uses a database to search the files or directory. This database get updated once a day. To update the database of locate command type

    [root@TEST ~]# /etc/cron.daily/mlocate OR

    [root@TEST ~]# updatedb -v        //v will show you files.//

9. head/tail :- Bydefault shows 10 top/last lines of file. You can change the number of line to display.

  [root@TEST ~]# head -n15 /etc/yum.repos.d/CentOS- Base.repo           // Show 15 top lines  //

 [root@TESt ~]# tail -n15 /etc/yum.repos.d/CentOS-Base.repo           // Show 15 last lines //

   [root@TEST~]# less /etc/yum.repos.d/CentOS-Base.repo

              //move up/down with page up/down key. less can open .gz file without mount it. //

[root@TEST ~]# more /etc/yum.repos.d/CentOS-Base.repo  

                                  //Scroll from top to down one page per screen. //

Grep command is used to fetch specific data from file or input. Ex:-

[root@TEST opt]# grep root /etc/passwd

root:x:0:0:root:/root:/bin/bash

operator:x:11:0:operator:/root:/sbin/nologin

[root@TEST opt]# ps -A | grep chrome

4389 ? 01:25:02 chrome

4429 ? 00:00:00 chrome-sandbox

4430 ? 00:00:00 chrome

4439 ? 00:00:00 chrome-sandbox

4442 ? 00:00:01 chrome

4575 ? 01:03:08 chrome

In the above command ps shows the information about the active processes. Option -A shows the all running processes. “grep chrome” shows the running processes of chrome.

[root@TEST opt]# ps -u username               // shows the processes runnning under username.//

To kill a process use process id or kill all processes running by that application use:

[root@TEST opt]# kill pid-no              //pid-no shows by ps -option command in 1st column.//

[root@TEST opt]# pkill process-name           // kill all processes running by process-name.//