Working more effectively with shell – basic commands

Let us learn a few commands, which are required very often, such as man, echo, cat and similar:

• Enter the following command. It will show the various types of manual pages displayed by the man command:

  $ man man
  

  From the following table, you can get an idea about various types of man pages for the same command:

  Section number	Subject area
  1	User commands
  2	System calls
  3	Library calls
  4	Special files
  5	File formats
  6	Games
  7	Miscellaneous
  8	System admin
  9	Kernel routines

• We can enter the man command to display corresponding manual pages as follows:

  $ man 1 command
  $ man 5 command
  

• Suppose we need to know more about the passwd command, which is used for changing the current password of a user, you can type the command as follows:

  $ man command
    man -k passwd   // show all pages with keyword
    man –K passwd  // will search all manual pages for pattern
  $ man passwd
  

  This will show information about the passwd command:

  $ man 5 passwd
  

  The preceding command will give information about the file passwd, which is stored in /etc /passwd.

• We can get brief information about the command as follows:

  $ whatis passwd
  

  Output:

  passwd (1ssl)        - compute password hashes
  passwd (1)           - change user password
  passwd (5)           - the password file
  

• Every command we type in the terminal has an executable binary program file associated with it. We can check the location of a binary file as follows:

  $ which passwd
  /usr/bin/passwd
  

  The preceding line tells us that the binary file of the passwd command is located in the /usr/bin/passwd folder.

• We can get complete information about the binary file location as well as manual page location of any command by following:

  $ whereis passwd
  

  The output will be as follows:

  passwd: /usr/bin/passwd /etc/passwd /usr/bin/X11/passwd /usr/share/man/man1/passwd.1.gz /usr/share/man/man1/passwd.1ssl.gz /usr/share/man/man5/passwd.5.gz
  

• Change the user login and effective user name:

  $ whoami
  

  This command displays the user name of the logged in user:

  $ su
  

  The su command (switch user) will make the user as the administrator; but, you should know the administrators, password. The sudo command (superuser do) will run the command with administrator's privileges. It is necessary that the user should have been added in the sudoers list.

  # who am i
  

  This command will show the effective user who is working at that moment.

  # exit
  

• Many a times, you might need to create new commands from existing commands. Sometimes, existing commands have complex options to remember. In such cases, we can create new commands as follows:

  $ alias ll='ls –l'
  $ alias copy='cp –rf'
  

  To list all declared aliases, use the following command:

  $ alias
  

  To remove an alias, use the following command:

  $ unalias copy
  

• We can check about the operating system details such as UNIX/Linux or the distribution that is installed by the following command:

  $ uname
  

  Output:

  Linux
  

  This will display the basic OS information (UNIX name)

• Linux kernel version information will be displayed by the following:

  $ uname –r
  

  Output:

  3.13.0-32-generic
  

• To get all the information about a Linux machine, use the following command:

  $ uname –a
  

  Output:

  Linux ubuntu 3.13.0-32-generic #57~precise1-Ubuntu SMP Tue Jul 15 03:50:54 UTC 2014 i686 i686 i386 GNU/Linux
  

• The following commands will give you more information about the distribution of Linux:

  $ cat /proc/version   // detailed info about distribution
  $ cat /etc/*release
  # lsb_release -a   // will tell distribution info for Ubuntu
  

  The command cat is used for reading files and displayed on the standard output.

• Sometimes, we need to copy a file or directory in many places. In such situations, instead of copying the original file or directory again and again, we can create soft links. In Windows, a similar feature is called as creating a shortcut.

  $ ln  -s  file file_link
  

• To learn about the type of file, you can use the command file. In Linux, various types of files exist. Some examples are as follows:
  • Regular file (-)
  • Directory (d)
  • Soft link (l)
  • Character device driver (c)
  • Block device driver (b)
  • Pipe file (p)
  • Socket file (s)
• We can get information about a file using the following command:

  $ file   fil_name  // show type of file
  

• Printing some text on the screen for showing results to the user or to ask details is an essential activity.
  • The following command will create a new file called file_name using the cat command:

    $ cat > file_name
    line 1
    line 2
    line 3
    < Cntrl + D will save the file    >
    

  But this is very rarely used, as many powerful editors are already existing, such as vi or gedit.

  • The following command will print Hello World on the console. The echo command is very useful for Shell script writers:

    $ echo "Hello World"
    

  • The following command will copy the string Hello World to the hello.c file:

    $ echo "Hello World" >   hello.c
    

    The command echo with > overwrites the content of the file. If content already exists in the file, it will be deleted and new content will be added in the file. In a situation, when we need to append the text to the file, then we can use the echo command as follows:

    $ echo  "Hello World" >> hello.c  will append the text
    

  • The following command will display the content of the file on screen:

    $ cat hello.c