Skip to main content

/docs/images/banner.jpg

Operating System

Shellscript

Shell script​

  • Shell script is a computer program designed to be run by the Unix/Linux shell which could be one of the following:

    • The Bourne Shell
    • The C Shell
    • The Korn Shell
    • The GNU Bourne-Again Shell

  • A shell is a command-line interpreter and typical operations performed by shell scripts include file manipulation, program execution, and printing text.

  • Shell scripts are also employed extensively in the default installations of Unix-like operating systems.

  • Shell scripts are executed by a shell process.

  • Shell scripts often serve as convenient wrappers around UNIX commands.

  • They automate sequences of UNIX commands that would otherwise be time-consuming to type into the keyboard.

  • A shell script is a text file containing shell commands.

Shell Script Example​

# This is a comment!
echo Hello World        # This is a comment, too!

  • The first line tells Unix that the file is to be executed by /bin/bash.

  • This is a comment. The second line is also a comment.

  • The third line actually does something. It asks that the words Hello World be printed.

  • The #! characters are magic to Unix. They are the characters that tell Unix that what follows is the name of the program (or programs in the case of a pipeline).

  • The # character begins a comment, and Unix ignores the rest of the line.

  • The echo command prints its arguments, in this case, the words "Hello World".

  • The echo command is one of the most basic and frequently used commands in Linux.

Executing Shell Scripts​

  • To execute the script from the current directory, you can run:
$ ./hello.sh
  • this wont run unless you give it permission to run
$ chmod +x hello.sh

Shell Variables​

  • A shell variable is a special variable that is set by the shell and is required by the shell in order to function correctly.

  • Some of these variables are environment variables whereas others are local variables.

  • A local variable is a variable that is present within the current instance of the shell.

  • It is not available to programs that are started by the shell.

  • They are set at the command prompt.

  • To set a local variable, use the following syntax −

variable_name=variable_value
  • To access a local variable, use the following syntax −
$variable_name
  • Example
NAME="Zara Ali"
echo $NAME