rkakodkar/basic_bash

Introduction to bash scripting

Bash Basics

Important concepts

Bash is a scripting language

A script it a list of things for the computer to do

Scripts allow you to automate work.

Scripts are text files. To tell the computer that the file is a bash script, put the following on the first line

#!/bin/bash

Running a script

bash my_script.sh

You can make the script exectuable

chmod +x my_script.sh

Then execute the script like this:

./my_script.sh

Debugging options

Verbose:

#!/bin/bash -v

Exercises

1. Environment variables

Environemnt variables are variables that are set outside your bash script, generally when you log in to the system or load a module. Run the script hello. What happens?

Take a look at the script with cat hello

Which environment variable is being used here?

List all the enviroment variables in your session with env

2. Variables

Variables can be used to make your scripts more reusable and easier to read. variables has an example of a string variable, a number variable and a variable set to the ouput of a command.

3. Arguments

Arguments are inputs given to a script. Run the script arguments with some arguements, for example: bash arguments one two dog cat fish Extra credit: Can you change the script to check that there are at least two arguments?

4. If statements

If statements are logic you can use to make your script do different things depending on the if condition. example_if takes a number as an argument and prints out if the number is less than or greater than 3. bash example_if 55

Extra credit: Can you change the script to only accept numbers?

5. Case statements

Case statements are an alternative to writing nested if statements (ifs inside ifs inside ifs ...). case is an example case statement. case also contains an infinite while loop. The case statement checks the input from the keyboard. If the input is bye the break is executed and code exits the infinite while loop.

6. Loops

The script loops has three different loop examples.

7. Functions

The directory animals contains a file with a list of animals, and a script hedgehog_finder. hedgehog_finder reads the list_of_animals file line by line and prints out if a hedgehog is detected. What do you think the ${1,,} does?

8. Some useful tools

grep searches within files for matching text. In the animals directory, try using grep to find different animals:

grep hedgehog list_of_animals
grep Hedgehog list_of_animals
grep -i hedgehog list_of_animals

You can use grep recursively:

grep -r hedgehog .
grep -ri hedgehog .
grep -rin hedgehog .

find finds

To find all filens with names that start with list*

find . -name list*

To find files in the current directory which were modified within the last 1 day:

find . -mtime -1

9. Pipes and redirects

Pipes let you take the output from one command and use it as the input to another.

Let's cd into the animals directory to play with pipes and redirects

To show only the first few lines of a file, use head

head list_of animals

To redirect this ouput to a file instead of the screen use >

head list_of animals > some_animals

Take the head of list_of_animals then sort into reverse alphabetical order

head list_of_animals | sort -r

Take the head of list_of_animals then sort into reverse alphabetical order and write that output to a file

head list_of_animals | sort -r > reversed

Using < sends data from a file to a command

This takes the list_of_animals file and runs sort on the file

sort < list_of_animals

This takes the list_of_animals file and runs sort -u on the file which sorts the file and removes duplicates, then outputs the result of this to a file 'unique_animals'

sort -u < list_of_animals > unique_animals

>> means append to a file

Reources:

The bash manual

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