Part 3: Introduction to Shell Scripting

Shell Scripting Basics

Shell Script Directives
- #!/bin/sh
- #!/bin/bash
Python script Directive
- #!/usr/bin/env python3
‘Hello World’ example shell script
- Create the shell script: $ touch hello_world.sh
- Turn your touch file into a bash script: $ echo '#! /bin/bash >> hello_world.sh'
  - At first, echoing the bash shebang takes place
  - >> - bash output redirection operator - appending output to file
- $ echo 'echo hello world' >> hello_world.sh - printing the statement and redirecting it to the file
- Making the bash script file executable
  - Check the current permission settings: ls -l hello_world.sh
  - Making it executable for all users: chmod +x hello_world.sh
- Run your bash script: $ ./hello_world.sh

In the current folder you’ll be able to see greet.sh which can be run in your terminal using the command $ bash greet.sh or $ ./greet.sh

Output:

Enter your name:
Welcome
Congratulations! You just created and ran your first shell script using Bash on IBM Skills Network.

Similarly, you can also play around with the file greetnew.sh

Generally, it is not a good idea to grant permission to a script for all user, groups and other alike. It is more appropriate to limit the execute permission to only the owner

To change the permissions for greet.sh, run the following command
- $ chmod u+x greet.sh

Filters, Pipes and Variables

What are Pipes? Put simply, pipes are commands in Linux which allow you to use the output of one command as the input of another.

Pipes “	” use the following format:
`[command 1] \| [command 2] \| [command 3] ... \| [command n]`

Some commands, such as tr, only accept “standard input” as input (not strings or filenames):

tr(translate) - replaces characters in input text
- Syntax: tr [OPTIONS] [target characters] [replacement characters] In cases like this, we can use piping to apply the command to strings and file contents.
With strings, you could, for example, use echo in combination with tr to replace all vowels in a string with underscores, as follows:
- $ echo "Linux and shell scripting are awesome\!" | tr "aeiou" "_"
  - Output: L_n_x _nd sh_ll scr_pt_ng _r_ _w_s_m_!
- To perform the complement of this use tr -c
Defining Shell Variables
- $ var_name=value (There are no spaces around ‘=’), below is the example
  - $ GREETINGS="Hello"
  - $ echo $GREETINGS
    - Output: Hello
- To clear a variable use $ unset var_name
Environment Variables
- Extended Scope
  - export var_name
    - $ export GREETINGS
    - $ env | grep "GREE"
      - Output: $ GREETINGS=Hello - Greetings is now an environment variable

Useful features of Bash Shell

Metacharacters are special characters
- # - precedes a comment
- ; - command separator
- * - filename expansion wildcard
  - $ ls /bin/ba* outputs /bin/bash
- ? - single character wildcard in filename expansion
  - $ ls /bin/?ash outputs /bin/bash /bin/dash
Quoting
- \ - escape special character interpretation
  - $ echo "\$1 each" - escapes the default interpretation of $ and outputs $1 each
- " " - interpret literally, but evaluate metacharacters
  - echo "$1 each" outputs each
- ' ' - interpret literally
  - echo '$1 each' outputs $1 each
I/O Redirection It refers to a set of features used for redirecting
- > - redirect output to file also creates if it doesn’t exist
- >> - Append output to file
- 2> - Redirect standard error to file
- 2>> - Append standard error to file
- < - Redirect file contents to standard output

Command Substitution

A feature of the shell, which helps save the output generated by a command in a variable.

It can also be used to nest multiple commands, so that the innermost command’s output can be used by outer commands.

$ myip=$(hostname -i)
- echo $myip outputs your ip
Counting the total number of files in your directory
- $ ls | wc -l
Find the total disk space usage
- $ df -h | grep overlay
  - df -h - disk usage for all individual filesystems including the total usage across the server under the head overlay
  - You can get the overall disk usage if you grep for overlay from the output of df -h
List five largest files
- ls -lS /bin | head -6
  - The -S option option of ls command sorts the files from largest to smallest

For the file present with the name wish.sh execute the following commands

Making the script executable for everyone - $ chmod +x wish.sh
Run the script with two arguments - $ ./wish.sh Ramesh Sannareddy
You should see the below output
- Hi Ramesh Sannareddy
- Ramesh is your firstname
- Sannareddy is your lastname

Let us create a bash script named dirinfo.sh that takes the directory name as an argument and prints the total number of the the directories and the number of files in it.

We will make use of the find command with -type option which will list only files or directories depending upon the usage of d switch or f switch respectively.

-d - operator to check if the given directory exists or not

The command wc -l will count the lines.

Make the script executable
Run the script with a directory as an argument

Scheduling Jobs using Cron

$ crontab -e
Cron is the general name of the tool that runs scheduled jobs consisting of shell commands or shell scripts.
Cron interprets ‘crontab files’ and submits jobs to cron
Job Syntax - min hour date_of_month month day_of_week command
- 30 15 * * 0 date >> sunday.txt - append the current date to file sunday.txt at 15:30 every sunday

Scheduling a shell script. Run the file diskusage.sh that prints the current time and the current disk usage statistics.

Make the script executable
$ crontab -e
$ 0 0 * * * /home/project/disksusage.sh >>/home/project/diskusage.log
Check if the job is added to the crontab by running the following command - $ crontab -l

The -r option causes the current crontab to be removed.

Caution: This removes all your cron jobs. Be extra cautious when you use this command on a production server. $ crontab -r

Have Fun …