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    In this article we will explain the usage of important linux commands with examples. We have divided all the commands into categories and we will illustrate them one by one using a linux operating system.
    I have executed the commands using a raspberry pi. You can execute the commands listed on this article using virtualbox simulation environment. You can also schedule-a-project with project code PL000001 to execute commands used in this article.

Article contents

File System Commands

System Info Commands

• date, cal, uptime, whoami, uname -a, cat /proc/ df, du

Process Management Commands

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File System Commands

    ls is a basic linux file system command to list down the files and directories in the current directory. ls will not show the timing of change of the file and directory content and their permissions. You can see the permissions using ls -al. In the illustration below you can see Calculator is having read, write and executable permission where as add.c is having read and write permission. ls -lt will list the content same as ls -al but it will sort the formatted listing by time modification. As you can see from the illustration below Calculator is modified last at 04.41 hence listed at the top and add.c is listed at the bottom.

pi@raspberrypi:~/C $ ls
add.c  Calculator  divide.c  libcalc.so  multiply.c  subtract.c
add.o  common.h    divide.o  main.c      multiply.o  subtract.o
pi@raspberrypi:~/C $ ls -al
total 68
drwxr-xr-x  2 pi pi 4096 Jun 28 04:41 .
drwxr-xr-x 31 pi pi 4096 Jun 27 07:50 ..
-rw-r--r--  1 pi pi   42 Jun 28 04:24 add.c
-rw-r--r--  1 pi pi  860 Jun 28 04:37 add.o
-rwxr-xr-x  1 pi pi 8380 Jun 28 04:41 Calculator
-rw-r--r--  1 pi pi   94 Jun 28 04:27 common.h
-rw-r--r--  1 pi pi   49 Jun 28 04:25 divide.c
-rw-r--r--  1 pi pi  952 Jun 28 04:37 divide.o
-rwxr-xr-x  1 pi pi 7968 Jun 28 04:34 libcalc.so
-rw-r--r--  1 pi pi  343 Jun 28 04:40 main.c
-rw-r--r--  1 pi pi   51 Jun 28 04:25 multiply.c
-rw-r--r--  1 pi pi  872 Jun 28 04:37 multiply.o
-rw-r--r--  1 pi pi   47 Jun 28 04:24 subtract.c
-rw-r--r--  1 pi pi  872 Jun 28 04:37 subtract.o
pi@raspberrypi:~/C $ ls -lt
total 60
-rwxr-xr-x 1 pi pi 8380 Jun 28 04:41 Calculator
-rw-r--r-- 1 pi pi  343 Jun 28 04:40 main.c
-rw-r--r-- 1 pi pi  952 Jun 28 04:37 divide.o
-rw-r--r-- 1 pi pi  872 Jun 28 04:37 multiply.o
-rw-r--r-- 1 pi pi  872 Jun 28 04:37 subtract.o
-rw-r--r-- 1 pi pi  860 Jun 28 04:37 add.o
-rwxr-xr-x 1 pi pi 7968 Jun 28 04:34 libcalc.so
-rw-r--r-- 1 pi pi   94 Jun 28 04:27 common.h
-rw-r--r-- 1 pi pi   49 Jun 28 04:25 divide.c
-rw-r--r-- 1 pi pi   51 Jun 28 04:25 multiply.c
-rw-r--r-- 1 pi pi   47 Jun 28 04:24 subtract.c
-rw-r--r-- 1 pi pi   42 Jun 28 04:24 add.c
pi@raspberrypi:~/C $

    pwd is the command to see your current working directory. in the illustration below it shows I am in /home/pi/C directory. cd ../ will move one directory down where as cd dir Change directory to dir. Similaryly ../../ will change directory to two levels down.
    mkdir dir will create a new directory dir where as rmdir dir will delete directory dir from your current directory. rm file will delete a file. Also note that I have created a directory New and created a file inside it using touch command and then tried to delete the New directory. But it failed saying directory is not empty. In this case we use -rf option to force directory deletion.
    cp is the command used to move a file or directory from one directory to another. In the illustration I have moved the subtract.c from current directory to /home/pi/try directory. mv command is used to rename a file. In the illustration file.c is renamed as file1.c.

pi@raspberrypi:~/C $ pwd
/home/pi/C
pi@raspberrypi:~/C $ cd ..
pi@raspberrypi:~ $ pwd
/home/pi
pi@raspberrypi:~ $ cd C
pi@raspberrypi:~/C $ pwd
/home/pi/C
pi@raspberrypi:~/C $
pi@raspberrypi:~/C $ cd ../../
pi@raspberrypi:/home $ pwd
/home
pi@raspberrypi:/home $
pi@raspberrypi:/home $ cd pi/C
pi@raspberrypi:~/C $ mkdir NewDir
pi@raspberrypi:~/C $ ls
add.c  add.o  Calculator  common.h  divide.c  divide.o  libcalc.so  main.c  multiply.c  multiply.o  NewDir  subtract.c  subtract.o
pi@raspberrypi:~/C $ rmdir NewDir/
pi@raspberrypi:~/C $ ls
add.c  add.o  Calculator  common.h  divide.c  divide.o  libcalc.so  main.c  multiply.c  multiply.o  subtract.c  subtract.o
pi@raspberrypi:~/C $
pi@raspberrypi:~/C $ rm Calculator
pi@raspberrypi:~/C $ ls
add.c  add.o  common.h  divide.c  divide.o  libcalc.so  main.c  multiply.c  multiply.o  subtract.c  subtract.o
pi@raspberrypi:~/C $
pi@raspberrypi:~/C $ mkdir New
pi@raspberrypi:~/C $ ls
add.c  add.o  common.h  divide.c  divide.o  libcalc.so  main.c  multiply.c  multiply.o  New  subtract.c  subtract.o
pi@raspberrypi:~/C $ cd New
pi@raspberrypi:~/C/New $ touch file.c
pi@raspberrypi:~/C/New $ vi file.c
pi@raspberrypi:~/C/New $ vi file.c
pi@raspberrypi:~/C/New $ cd ..
pi@raspberrypi:~/C $ rmdir New/
rmdir: failed to remove 'New/': Directory not empty
pi@raspberrypi:~/C $
pi@raspberrypi:~/C $ rm -rf New
pi@raspberrypi:~/C $ ls
add.c  add.o  common.h  divide.c  divide.o  libcalc.so  main.c  multiply.c  multiply.o  subtract.c  subtract.o
pi@raspberrypi:~/C $ ls
add.c  add.o  common.h  divide.c  divide.o  libcalc.so  main.c  multiply.c  multiply.o  subtract.c  subtract.o
pi@raspberrypi:~/C $ cp subtract.c /home/pi/try/
pi@raspberrypi:~/C $ cd /home/pi/try/
pi@raspberrypi:~/try $ ls
a.out  file.c  subtract.c
pi@raspberrypi:~/try $ mv file.c file1.c
pi@raspberrypi:~/try $ ls
a.out  file1.c  subtract.c
pi@raspberrypi:~/try $

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