A shell is a program that acts as an interface between a user and the operating system kernel. It allows users to execute programs and utilities on the kernel through commands. Shell scripts allow running sequences of commands as a single command, automating repetitive or difficult tasks, and are portable across Unix-like systems. While shell scripts are easy to use and portable, they have slower execution speeds than programming languages and launch a new process for each command. A simple example encrypts and decrypts a file by putting tr commands in shell scripts and executing them.

1. Shell
Scripting

2. What is Shell
 A shell is a program that acts as an interface
between a user and the kernel. It allows a user
to give commands to the kernel and receive
responses from it. Through a shell, we can
execute programs and utilities on the kernel.
Hence, at its core, a shell is a program used to
execute other programs on our system.
 It is command line interpreter

3. What is Shell

4. Why use of shell scripts?
– To avoid repetition:
If you do a sequence of steps with
standard Unix commands over and over,
why not do it all with just one command?
– To automate difficult tasks:
Many commands have subtle and difficult
options that you don’t want to figure out or
remember every time.

5. Advantage of shell scripts
are many advantages of shell
scripting some of them are
– To run sequence of commands as a single
command
– Easy to use
– To automate the frequently performed
operations
– Portable (It can be executed in any Unix-like
operating systems without any modifications)

6. Disadvantage of shell scripts
some of them are
– 1. Slow execution speed compared to any
programming Languages
– 2. A new process launched for almost every
shell command executed.

7. A Simple Example (1)
 tr abcdefghijklmnopqrstuvwxyz
thequickbrownfxjmpsvalzydg < file1 > file2
– “encrypts” file1 into file2
 Record this command into shell script files:
– myencrypt
#!/bin/sh
tr abcdefghijklmnopqrstuvwxyz
thequickbrownfxjmpsvalzydg
– mydecrypt
#!/bin/sh
tr thequickbrownfxjmpsvalzydg
abcdefghijklmnopqrstuvwxyz

8. A Simple Example (2)
chmod the files to be executable;
otherwise, you couldn’t run the scripts
obelix[3] > chmod u+x myencrypt mydecrypt
Run them as normal commands:
obelix[4] > ./myencrypt < file1 > file2
obelix[5] > ./mydecrypt < file2 > file3
obelix[6] > diff file1 file3
Remember: This is needed
when “.” is not in the path