TIL: Looping in bash scripts and other things

I was in need of writing some bash scripts today to handle the automation of some of my test runs. in the past, most of my “bash scripting” was limited to putting a command into a file and ending it with .sh. Today I branched out a bit more and dealt with looping, command line arguments, and other fun stuff.

Arguments in bash

Args are pretty simple in bash scripts. Whatever you put in will be read as a string and assigned to a variable equal to the number of their position. For instance, if I were to run the script below using the command ./myscript.sh hello 123 butts:

1

echo "$3 $2 $1 $0"

Then it would echo back out butts 123 hello myscript.sh

Looping in bash

For loops are supported in bash scripts, pretty similar to how they work in python. It follows the for <var> in <something> format. That something could be an array or list of items or numbers. For instance:

1
2
3
4

for i in {1..20}
do
echo "$i"
done

will echo back the numbers 1 - 20 on a seperate line.

Passing in numbers

Because the script interprets all of the command line arguments as strings, I had to find a way to get numbers to be converted back to numbers. I wanted to be able to define the number of times an action would be looped by providing it as an argument.

The easy way of doing this is to perform a math operation on the value. That forces bash to treat the variable as an int. So to do so, you can use the format:

1
2

a=$(($1+0))
echo "$a"

This will convert your variable into an int. That could then be fed into your loop:

1
2
3
4
5

a=$(($1+0))
for i in $(seq 1 $a)
do
echo "$i"
done

Note that we switch to using a sequence here, with basically does the same thing as the number array.

Setting an array of strings

Arrays are created in () brackets with a single space separating items. No commas used.

1

a=( "string1" "string2" "string3" )

💚