Rules for expanding a bash array

Question

According to my bash man page: If the word is double-quoted ${name[@]} expands each element of name to a separate word.

This usually leads to exactly the expected behavior:

$ a=("foo 1" "bar 2")
$ for i in "${a[@]}"; do echo $i; done
foo 1
bar 2
$ size(){ echo $#;};size "${a[@]}"
2
$ [ "${a[@]}" = "foo 1 bar 2" ]&&echo ok
bash: [: too many arguments

But sometimes it does not:

$ [[ "${a[@]}" == "foo 1 bar 2" ]] && echo ok
ok
$ case "${a[@]}" in "foo 1 bar 2") echo ok; esac
ok

In those cases it seems to get evaluated into a single string. - This makes sense, but is a bit surprising. - I expected it to be equal to "foo 1" "bar 2" and therefore cause a syntax error.

Is there a rule in which context it is evaluated which way? (I couldn't find the right section in the bash man page.)

Additional question: Is there a case in which "${a[@]}" is handled differently from ${a[@]}?

Remove the " and use ${a[@]} instead of "${a[@]}".

Micha Wiedenmann
– Micha Wiedenmann

2013-06-07 10:57:37 +00:00
Commented Jun 7, 2013 at 10:57 — Micha Wiedenmann
– Micha Wiedenmann, Commented Jun 7, 2013 at 10:57
case ${a[@]} in "foo 1 bar 2") echo ok; esac -> ok

michas
– michas

2013-06-07 11:11:44 +00:00
Commented Jun 7, 2013 at 11:11 — michas
– michas, Commented Jun 7, 2013 at 11:11
Use set -xv to view how bash processes the commands.

choroba
– choroba

2013-06-07 11:47:45 +00:00
Commented Jun 7, 2013 at 11:47 — choroba
– choroba, Commented Jun 7, 2013 at 11:47

devnull · Accepted Answer · 2013-06-07 12:16:25Z

2

No word splitting or glob expansion is performed for [[.

From Bash FAQ:

Since word splitting is not performed when expanding shell variables in all
operands of the [[ command, this allows users to quote patterns as they wish
when assigning the variable, then expand the values to a single string that
may contain whitespace.

Additionally, refer to Word Splitting:

Word splitting is not performed on expansions inside Bash keywords such as [[ ... ]] and case.
Word splitting is not performed on expansions in assignments. Thus, one does not need to quote anything in a command like these:
- foo=$bar
- bar=$(a command)
- logfile=$logdir/foo-$(date +%Y%m%d)
- PATH=/usr/local/bin:$PATH ./myscript

answered Jun 7, 2013 at 12:16

devnull

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Comments

cforbish · Accepted Answer · 2013-06-07 11:04:55Z

-1

The man page did say to a separate word and did not say into one word. The a array "${a[@]}" is not the same thing as "foo 1 bar 1". This only returns 1 big string:

for i in "foo 1 bar 1"; do echo $i; done

answered Jun 7, 2013 at 11:04

cforbish

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1 Comment

michas Over a year ago

Exactly, In the first example it gets evaluated into separate words. In the second example it gets evaluated into one single word. - That is surprising and the reason for the question.

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