whats the best practice for conditionals in bash when checking multiple statements?

Question

Given your trying to check that a variable is not empty and not some other value as in the following code:

if [ ! -z "$foo" ] && [[ ${foo} != "bar" ]]; then

what is the best practice for accomplishing this. I've seen bash conditionals written several ways including the following...

if [[ ! -z "$foo" && ${foo} != "bar" ]]; then

I understand there is a difference when using the single brackets and the double, I'm more concerned with when to put the && or || inside the brackets or out.

You only need one conditional: if `foo` is empty, it's not going to equal `bar`. — chepner, Apr 24 '14 at 21:32

that other guy · Accepted Answer · 2014-04-24T21:44:28.507

7

Put &&/|| inside brackets for [[ ]]. Outside is also accepted.

Put &&/|| outside brackets for [ ]. Inside is NOT allowed.

This is due to the fact that && binds normal commands together based on return value, e.g.

wget file && echo "Success"

[, despite its funny name, is a regular command and obeys the same rules as e.g. wget or echo.

[ foo || bar ] is two commands, [ foo and bar ], neither of which are valid.

[[ .. ]] on the other hand is not a normal command but special shell syntax. [[ foo || bar ]] is a single command, and interpretted accordingly.

edited Apr 24 '14 at 21:44

answered Apr 24 '14 at 21:25

that other guy

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3

`[` is a builtin in Bash. – gniourf_gniourf Apr 24 '14 at 21:29
3

Yes it is, but that's an implementation optimization. – that other guy Apr 24 '14 at 21:32
@gniourf_gniourf `[[` is a builtin, `[` is an alias for the program `test`. – Swiss Apr 24 '14 at 21:33
2

@Swiss `[`/`test` is also a standalone program, one of several that `bash` implements as built-in commands (along with `kill`, `echo`, `printf`, and others) for efficiency. – chepner Apr 24 '14 at 21:34
3

@Swiss `[[` is a keyword and `[` is a builtin. Please check with `type [[` and `type [` in bash. – gniourf_gniourf Apr 24 '14 at 21:34
2

@thatotherguy not only an implementation optimization. `builtin [` has more options that `command [` (on my system). – gniourf_gniourf Apr 24 '14 at 21:36
btw, I don't want to argue whether builtin/command, we all know what what is. Just that it's not exactly clear in your answer (and the question is tagged bash): reading it without knowing would confort me into thinking that `[` is really the command `/usr/bin/[`. – gniourf_gniourf Apr 24 '14 at 21:38

score 2 · Answer 2 · edited Jun 20 '20 at 09:12

To complete the previous answers :

if [[ ! -z $foo && $foo != "bar" ]]; then ...
# [[ will execute the two conditions with "and" operator in a single instruction

Is equivalent of :

if [[ ! -z $foo -a $foo != "bar" ]]; then ...
# [[ will execute the two conditions with "and" operator in a single instruction

But not equivalent of :

if [[ ! -z $foo ]] && [[ $foo != "bar" ]]; then ...
# second [[ will be executed if the first success ($? = 0)

-a (and) and -o (or) will work with test and [.

See man test to get more details ;)

Otherwise, no need to protect your variables by doubles quotes with [[ and no need to use delimiters (${}) in this case.

Here is a reminder about the necessity (or not) to protect your variables with double quotes..

whats the best practice for conditionals in bash when checking multiple statements?

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