7.1 Scripting with Bash¶
Author: Dr. Alejandra Rougon
This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.
🔍 Learning Objectives¶
After completing this lesson you will learn:
What is a bash script
The basics of bash scripting
How to use variables in bash
How to make a for loop in bash
Sometimes you may need to run the same command or the same series of commands on many files. Or you may want to look not only for a string but for a list of strings, or iterate over some commands a number of times. In these cases, you can write scripts in bash, just like in other programming languages (e.g. python, perl). A script is a program that will be executed by an interpreter. In this case the interpreter will be bash. It doesn’t have to be compiled as programs written in other languages (e.g. C, C++).
The simplest script would be to print to the screen Hello world!. Let’s create a folder called Activity in the folder Documents and just create the following file and run it with sh.
$ cat > helloworld.sh
echo Hello world!
$ sh helloworld.sh
Hello world!
Good practices¶
It is a good practice to add the extension .sh to bash scripts. It is also a good practice to add a #! in the first line followed by the path to bash. In that way you can also run it by making it executable and without the command sh. And finally it is also recommended to add comments that explain your code. You can also add empty lines where needed so it looks neat. So our first bash script could be improved like this.
#!/bin/bash
# This is my first script and will
# print Hello world! to the screen
echo Hello world!
And now you can run it either with
sh helloworld.sh
or with
chmod +x helloworld.sh
./myfirstscript.sh
If you want to run a series of commands sequentially only add the next command in the following line.
#!/bin/bash
# This is my first script and
# will print Hello world! to the screen
echo Hello world!
echo This is my first file
echo It is stored in the following path
pwd
$ sh helloworld.sh
Hello world!
this is my first file
it is stored in the following path
/root/sandbox/Documents/Activity
Variables¶
A variable in bash shell scripting, as in other programming languages, is a memory location that is used to contain a number, a character, a string, etc. The most commonly used data type of variables are integer (46), string (apple), float (0.24), and boolean (FALSE). The data type of any variable has to be defined when the variable is declared for strongly type programming languages. However Bash is a weakly typed or loosely typed programming language and there is no need to define a data type.
Variables are declared with the = sign, and are called with the $ sign.
$ a=apple
$ echo $a
apple
It is recommended to add quotes. Single and double quotes have different meaning.
Double quotes are also called “weak quotes”. If you store a variable and you call it with double quotes it will give you the contents stored in the variable.
$ a=apple
$ b="Value of a is $a"
$ echo $b
Value of a is apple
Single or ‘strong quotes’ will give you a literal interpretation.
$ a=apple
$ b='Value of a is $a'
$ echo $b
Value of a is $a
when you are not calling a variable or you are not using regular expression it doesn’t make any difference to use single or double quotes.
a='apple'
or
a="apple"
Let’s create a script called variable.sh
#!/bin/bash
#This script prints Hello world!
#to the screen and uses a variable.
Greet='Hello world!'
echo "$Greet"
$ sh variable.sh
Hello world!
Loops¶
In order to iterate over commands we can use loops. The for loop is represented in the following examples:
for variable in 1 2 3 4 5 .. N
do
command1
command2
commandN
done
for variable in file1 file2 file3
do
command1 on $variable
command2
commandN
done
Let’s create a greetings script with a for loop. Now instead of printing Hello world! let’s print a greeting for each continent.
#!/bin/bash
#This script prints a greeting for each continent.
continents="Africa Americas Antartica Asia Australia Europe"
for item in $continents
do
echo "Hello $item"
done
The variable continents contains 6 items Africa Americas Antartica Asia Australia Europe. In the for loop the variable item will go through each one of the items of the variable continents , called with $continents .
Each one of them will be printed on each cycle of the loop with echo as they are being called with $item on each iteration.
$ sh hellocontinents.sh
Hello Africa
Hello Americas
Hello Antartica
Hello Asia
Hello Australia
Hello Europe
If we want to save the output of our script to a single file we would redirect the output with >>
#!/bin/bash
#This script prints a greeting for each continent and saves the output to the file Greetings.txt
continents="Africa Americas Antartica Asia Australia Europe"
for item in $continents
do
echo "Hello $item" >> Greetings.txt
done
If we want to save the output into different files we could do this
#!/bin/bash
#This script prints a greeting for each continent and saves the output into a different file for each continent.
continents="Africa Americas Antartica Asia Australia Europe"
for item in $continents
do
echo "Hello $item" > Greet.$item
done
🔑 In this lesson you have
What is a bash script
The basics of bash scripting
How to use variables in bash
How to make a for loop in bash