How do I learn how to get quoting right in bash?

Question

I\'m constantly confused by the rules for quoting and evaluating when I\'m writing bash scripts. I know some of the basics, like the difference between \'\' and \"\" and ``,

Answer 1

Use singlequotes '' for quoting raw text (even backslashes do not escape anything in singlequotes):

> echo '\'
\
> echo '$PATH'
$PATH

Use doublequotes "" for quoting text which contains things the shell shall evaluate like variables ($bla), subshell calls ($(ls)), and evaluations ($((5 + 3))).

> echo "$PATH"
/home/alfe/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games
> echo "$(ls | tail -1)"
bla
> echo "$((5 + 3))"
8

Use backticks `` if for some reason you cannot use $() (e. g. in the rare cases where you have to you sh instead of bash. Normally use $() to gather the output of a subshell into the current command.

> echo "$(ls | tail -1) is the last file in the current dir."
bla is the last file in the current dir.

One of the main problems I run into with bash code of other people is missing doublequotes around something which often is just a word but in rare cases may be more than one word, or which can contain special characters. So use doublequotes wherever they are possible.

> a="four    spaces"
> echo $a
four spaces
> echo "$a"
four    spaces