A shell script is a computer program designed to be run by the Unix/Linux shell. A shell is a command-line interpreter and typical operations performed by shell scripts include file manipulation, program execution, and printing text.
Types of Shell scripts are:
- The Bourne Shell
- The C Shell
- The Korn Shell
- The GNU Bourne-Again Shell
Why do we use shell scripts?
- Shell scripts can be used to customize your environment
- Will gain experience with shell variables, shell script operators, and logic structures
Previewing the Application:
- Shell scripts can be used to customize your environment
- Will gain experience with shell variables, shell script operators, and logic structures
Using Comments
Comments are important!
- Provide useful documentation to both the programmer and to others who need to understand or debug the code
- To use, start the comment line with a #
The Programming Shell
- Bourne shell (sh)
- Korn shell (ksh)
- Bourne Again shell (bash)
- POSIX shell (sh)
The different C-type shells follow −
- C shell (csh)
- TENEX/TOPS C shell (tcsh)
The original Unix shell was written in the mid-1970s by Stephen R. Bourne while he was at the AT&T Bell Labs in New Jersey.
Variables:
Variables are symbolic names that represent values stored in memory
Three types of variables:
– Configuration variables store information about the setup of the OS
– Environment variables hold information about your login session
– Shell variables are created at the command prompt or in shell scripts and are used to temporarily store
information
Shell Variables
- Variables that you can define and manipulate for use with program commands in a shell
- Observe basic guidelines for handling and naming shell variables
Shell Operators
Bash shell operators are in four groups:
- Defining
- Evaluating
- Arithmetic
- Redirection
Defining Operators
- Used to assign a value to a variable
- The most common is = (equal sign)
- Use quotation marks with strings
- Backquote says to execute the command inside the backquotes and store the result in the variable
Useful Links on Unix
- Bell Labs − Gives an overview and history of the UNIX operating system. The Creation of the UNIX Operating System.
- BSD UNIX − FreeBSD is an advanced UNIX operating system for modern server, desktop, and embedded computer platforms.
- Linux Online − Linux is a free Unix-type operating system originally created by Linus Torvalds with the assistance of developers around the world.
- Unix @ Wikipedia − A brief description of the Unix Operating system.