#!/bin/bash

That works in bash 3.2 for me. So does this:

[[ $file = *.txt ]]

Maybe, the error is somewhere else in your code.

File extensions are just a text based convention. Renaming a file to end in .PDF does not make the file a valid PDF.

You're really saying there's no way for a posix shell to take the text to the right of the last "." character and then do simple string comparison on the result?

Also why can't you just use normal globbing to feed arguments to your command? Why do you need to involve flow control?

https://stackoverflow.com/q/965053 ?

for i in ``ls *.png``; do something $i; done;

(not formatting correctly)

Is this sort of what you mean? You can tune the loop, but essentially you build a list of the files you want to do something against, then loop through it.

Are you actually using Bash? You might be in a different shell. You can check with echo $SHELL

Let's not forget that Linux filenames are case sensitive, so ".txt" ≠ ".TXT" ≠ ".Txt"

Maybe a little more context of what you want to run would help here. Find would work.

find . -name "*.png" -exec whateveryouwanttodohere {} \;

Or you could find, and then pass the arguments to xargs:

find . -name "*.png" -print | xargs whateveryouwanttodohere

ls | grep txt$ will return only files ending "txt"

If someone posts “[[ $file == *.txt ]]” I’m going to fucking scream because THAT DOES NOT WORK. IT’S NOT VAILD BASH CODE.

This is valid bash code.

Do you understand how string substitution works? In this example, "file" is the name of a variable, and $file substitutes its value. If you have not set the value of file, then it won't work.

Edit: you should, as a rule of thumb, quote your variables. So "$file" instead of just $file. Quoting prevents some weird behavior with whitespace and special characters in the value. But either way, this is valid code and will work in the general case.

How about instead of starting with a list of all files in the directory, you use find to filter just the ones ending in .txt (or .TXT, it's case insensitive)

find $dir -maxdepth 1 -type f -iname "*.txt"