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Basics of Shell Scripting

15 Aug, 2015 Linux / Unix

A Unix shell is a command-line interpreter or shell that provides a
traditional user interface for the Unix operating system and for
Unix-like systems. Users direct the operation of the computer by
entering commands as text for a command line interpreter to
execute or by creating text scripts of one or more such commands.Shell scripting plays a major role in pentesting.

To create a shell script, we need to start with a text editor. You can use any of the text editors in Linux including vi,sublime,atom etc  but I prefer using Sublime text editor.

We start by entering the shebang or #!. This tells the operating system that whatever follows the shebang is the interpreter we want to use for our script. We then follow the shebang with /bin/bash indicating that we want the Unix system to use the BASH shell.

Next, we enter echo, a command in Linux that tells the system to print in the terminal. In this case, we want the system to echo back to us “Hello world”.

  • echo “Hello world”

Save it and close the text editor

bash

Next we set the executable permission,using CHMOD command.When we create a file, it’s not necessarily executable, not even by us, the owner.Now we change the permission as

  • chmod +x filename.sh

chmod4

where + refers the addition of permission and x refers the executable permission

To run our simple script, we simply type:

  • ./filename.sh

By this we can add several linux commands to automate the daily or complex tasks like backup

Some Linux commands:

  • mv target destination —Moving a file to another location
  • cp target destination —Copying a file to another location
  • Chmod option filename —Changing file permissions
  • cat filename —displaying the contents of the file
  • More — Does the same thing as cat
  • TOP — Displays all the running processes
  • Who–Displays current logged users
  • Whoami–Displays the current user name
  • PS–Displays the user processes
  • SU–Switches the current user to superuser account