Should I use cat or echo?

cat or echo are used to generate the text that will be written somewhere

When should I use cat?

• cat takes text FROM a file
• the only reason to use cat is so that you can use a heredoc and you usually want to use a heredoc if there is much to write
• when you need to read stdin and need to write to stdout
• when you actually need to conCATenate stuff e.g. you need to write several files in one go, or mix files and stdin

When should I use echo?

• when you're using an old shell without printf
• *should* or *can* ? Maybe when you're quite certain of the output - no vars and no backslashes and no leading hyphens - and the brevity over printf matters, then, maybe, you should use echo

When should I use ls?

• NEVER: Globs are much more simple AND correct: for file in *.txt

• if you are caught between a rock and a REALLY hard place then ls might be the only choice you have to find out certain information, like where a symlink points. but it's really a last resort and in the vast majority of cases not needed at all