FOR loops & WHILE statements

Introduction to Bash Scripting

Alex Scriven

Data Scientist

Basic FOR Loop structure

Python:

for x in range(3):
    print(x)

0
1
2

for (x in seq(3)){
    print(x)
}

[1] 1
[1] 2
[1] 3

The basic structure in Bash is a similar:

for x in 1 2 3

do
    echo $x
done

1
2
3

Bash has a neat way to create a numeric range called 'brace expansion':

for x in {1..5..2}

do
    echo $x
done

1
3
5

Another common way to write FOR loops is the 'three expression' syntax.

for ((x=2;x<=4;x+=2))
do 
    echo $x
done

2
4

Bash also allows pattern-matching expansions into a for loop using the * symbol such as files in a directory.

For example, assume there are two text documents in the folder /books:

for book in books/*
do  
    echo $book
done

books/book1.txt
books/book2.txt

Remember creating a shell-within-a-shell using $() notation?

You can call in-place for a for loop!

Let's assume a folder structure like so:

Structure of books directory

We could loop through the result of a call to shell-within-a-shell:

for book in $(ls books/ | grep -i 'air')

do  
    echo $book
done

AirportBook.txt
FairMarketBook.txt

Similar to a FOR loop. Except you set a condition which is tested at each iteration.

Iterations continue until this is no longer met!

Use the word while instead of for
Surround the condition in square brackets
- Use of same flags for numerical comparison from IF statements (such as -le)
Multiple conditions can be chained or use double-brackets just like 'IF' statements along with && (AND) or || (OR)
Ensure there is a change inside the code that will trigger a stop (else you may have an infinite loop!)

Here is a simple example:

x=1

while [ $x -le 3 ];

do
    echo $x
    ((x+=1))
done

1
2
3

Beware the infinite WHILE loop, if the break condition is never met.

x=1
while [ $x -le 3 ]; 
do
    echo $x
    # don't increment x. It never reaches 3!
    # ((x+=1)) 
done

This will print out 1 forever!

Introduction to Bash Scripting