1. Tutorial of shell script
Professor: Dr. Shu-Ching Chen
TA: Samira Pouyanfar/Hector Cen
Spring 2018

2. Outline What is a shell script? What are they good for?
Different types of shell
Recommended text editors
How to write and execute shell scripts?
Variables
Operations (Arithmetic, comparison)
Flow control
Loops
I/O
References

3. What is a shell script?
A shell script is file containing a series of commands.
The shell reads this file and executes the commands sequentially as if they were inputted manually.

4. What are they good for?
Shell scripts are useful for a good number purposes:
Operating systems boot and initialization
Automation of maintenance tasks
Setup and deployment of applications
In general, to automate processes/tasks

5. Different types of shells
Shell scripts are interpreted by another programs
These programs establish differences as to how write the scripts to be interpreted
Most commonly known shells are:
sh: One of the first ones. Developed for UNIX at AT&T
bash: the default shell for many Linux distributions. It has many improvements over sh.
csh and tcsh: short for C-shell. C language style and approach. Not as popular as bash.

6. Recommended text editors
gedit
nano
vim
Notepad++ (Windows)
Sublime Text (cross-platform)

7. Recommended text editors – (1)
Vim: short for Vi Improved. Is a popular text- editor in unix-like environments
Open a file: $ vim test.sh
Command mode: Press esc to switch to command mode
Insert mode: Press i to switch to insert mode

8. Recommended text editors – (2)
Basic vim commands:
:w – Write file
:wq – Write file and quit
:w! – Force write
:n – Go to line n
:%s/old/new/g – searches for old and replaces with new within the entire file (g)
There’s much, much more…

9. Recommended text editors – (3)
Caveat for text editors in Windows
There are two types of line endings:
CR LF: used in Windows
LF: Unix (includes macOS)
If working in Windows (Notepad++ ), line ending should be set to LF to be compatible with Unix/Linux.

10. How to write and execute shell scripts – Permissions
Shell scripts need execution permission to be able to run:
$ chmod u+x test.sh
Permission granted for the user
No execution permission granted

11. How to write and execute shell scripts – Execute scripts
After permissions are granted, scripts can be executed
$ ./test.sh

12. How to write and execute shell scripts – Variables (1)
In shell scripts, variables are untyped.
Good for scripting if used carefully.
Consequences arise if not used properly
Can be local, environmental
Variables defined within the context of a function are local.
Environmental variables affect the behavior of the shell
An environment can have multiple variables
Examples: PATH, SHELL, HOSTNAME

13. How to write and execute shell scripts – Variables (2)
In shell scripts, variables can be defined by users:
Syntax: variable_name=value
value is assigned to the variable called variable_name.
Example:
course_code=CIS5027
Interpreter

14. How to write and execute shell scripts – Variables (3)
How to access the value of a variable:
Use $ followed by the variable name

15. How to write and execute shell scripts – Variables (4)
Output of a command can also be saved in a variable:

16. How to write and execute shell scripts – Operations (1)
Several operations can be performed in shell scripts
Arithmetic
Addition, subtraction, multiplication, division, mod
Comparison
equal, not equal, greater than, less than, etc…

17. How to write and execute shell scripts – Operations (2)
Several operations can be performed in shell scripts
Arithmetic

18. How to write and execute shell scripts – Operations (2)
Several operations can be performed in shell scripts
Comparison

19. How to write and execute shell scripts – Flow control
Scripts need to do more than just executing commands sequentially
Scripts can make decisions and perform different actions depending on true or false conditions

20. How to write and execute shell scripts – Loops (1)
Scripts can execute a sequence of commands repeatedly, until a given condition is satisfied
while loop
Condition

21. How to write and execute shell scripts – Loops (2)
Scripts can execute a sequence of commands repeatedly, until a given condition is satisfied
for loop
Condition

22. How to write and execute shell scripts – I/O (1)
Scripts can also read input from the user
Typically from the keyboard

23. How to write and execute shell scripts – I/O (2)
Scripts can also read input in the form of arguments
Arguments are passed to an script as follows:
$ ./test.sh 560
To access arguments within the script, you can use $1, $2,…,$n, where $1 is the first argument, $2 the second one, and so on…

24. How to write and execute shell scripts – I/O (3)
Loading contents from text files
Three files
File as input

25. How to write and execute shell scripts – I/O (4)
Output will be:

26. References http://www.freeos.com/guides/lsst/