Bash: How to read file in while loop containing multiple conditions?

Question

After reading a ton of documentation and multiple SO solutions, I still cannot get the right syntax for multiple conditions in a while loop with the read line command.

I tried the following stack overflow solution below to no avail.

Bash scripting, multiple conditions in while loop

!/bin/bash

while (( $(read -r line) && "$1" != 1 ))
do
  #Do stuff
done < myFile.txt

The error I am getting is ((: && 8 != 1 : syntax error: operand expected (error token is "&& 8 != 1 "

Also tried read -r line instead of wrapping it in the $ sign and still did not work.

Where did I go wrong?

EDIT: Ignore the logic of my original post. That will eventually get changed. I'm just trying to find out the proper syntax.

Answer 1

I think that the correct syntax here would be:

#!/bin/bash

while read -r line && (( "$1" != 1 ))
do
    # some stuff
done < myFile.txt

The read command needs to go outside of the (( )) if you want the loop to run as long as read is successful. That said, I'm not sure about the conditions you're using, as $1 is the first argument passed to your script, which won't ever change in the script you've shown us.

Answer 2

The syntax of a while loop is while compound_list do_group ¹ .

Where a compound_list is any valid sequence of commands basically.

Your current attempt is trying to stick commands/etc. inside an arithmetic expression which is why you are getting a syntax error. Neither of the bits of your compound_list are arithmetic expressions though (the second one could be but doesn't need to be).

So you want something more like this

while read -r line && [ "$1" != 1 ]

or to keep the arithmetic expression instead of the [ like this

while read -r line && (( "$1" != 1 ))

That being said, and as Tom Fenech indicated in his comment on the post, nothing in the code you've shown us will ever cause the value of $1 to change. So what exactly is that bit of the test supposed to be doing?

The problem you had following the advice in the answer you linked is that you didn't pay attention to where the various types of brackets/parentheses/etc. went and what they meant (the answer didn't explain them) and none of the conditions in that question were commands themselves.