Working more effectively with Shell – basic commands

Let's learn a few commands that 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 the 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 content for pattern "passwd"
    $ 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 the /etc/ folder, such as /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 the manual page location of any command, with the 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 username:

    $ whoami

• This command displays the username of the logged in user:

    $ su

• The su (switch user) command will make the user the administrator but you should know the administrator's password. The sudo (superuser do) command will run the command with administrator privileges. The user should have been added to 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 operating system details, such as UNIX/Linux or the distribution that is installed with 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 localhost.localdomain 3.10.0-693.el7.x86_64 #1 SMP Tue Aug 22 21:09:27 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
    
  

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

    $ cat /proc/version   // detailed info about distribution
    $ cat /etc/*release
    # lsb_release -a
    .
  

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

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

• Printing some text on screen for showing results to the user, or to ask for 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 already exist, 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"
  

    $ echo "Hello World" > hello.sh

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

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

• The following command will copy the Hello World string to the hello.sh file:
• The following command will display the content of the file on screen:

    $ cat hello.sh