Difference between arrays with " " and () in bash script?

Question

I have two examples of a bash script. All I do is just use sed to replace some strings in a file. The file is specified when the script is run. So, you would run it like ./replace_file.sh input_file.

hug="g h 7"

for elm in $hug
do
  echo "replacing $elm"
  sed -i'_backup' "s/$elm/BOO/g" $1
done

and

hug=('g' 'h' '7')

for elm in $hug
do
  echo "replacing $elm"
  sed -i'_backup' "s/$elm/BOO/g" $1
done

The first script above, echos the following:

replacing g
replacing h
replacing 7

But, the second one stops after replace g. It is unclear to me why. Can someone please shed some light?

Thank you.

for elm in ${hug[@]} in the case of the array... You can quote "${hug[@]}" to preserve whitespace in the elements (if they have any -- you don't) Attempting to call an array without braces, e.g. {...[@]}, you are simply referencing the first element of the array, 'g' in your case. — David C. Rankin
– David C. Rankin, Commented Feb 13, 2016 at 7:48
hug="g h 7" doesn't create an array at all; it just creates a string with whitespace in it. — chepner
– chepner, Commented Feb 13, 2016 at 16:32

David C. Rankin · Accepted Answer · 2016-02-13 07:57:08Z

2

As set forth in the comment, when you attempt to iterate over the elements in the array by calling for elm in $hug, you are simply referencing the first element in the array. To iterate over the elements in the array, you use the array syntax, e.g. ${array[@]} to access each element sequentially (and you can quote the array to preserve whitespace in the elements.) In your case you need only:

#!/bin/bash

hug=('g' 'h' '7')

for elm in ${hug[@]}
do
echo "replacing $elm"
# sed -i'_backup' "s/$elm/BOO/g" $1
done

Output

$ bash arrayiter.sh
replacing g
replacing h
replacing 7

answered Feb 13, 2016 at 7:57

David C. Rankin

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3 Comments

Charles Duffy Over a year ago

Needs quotes around the ${hug[@]} to avoid string-splitting. Try hug=( "foo bar" "baz qux" ) to observe the bug in this as currently written.

David C. Rankin Over a year ago

Now I'm confused. I noted you should quote to preserve whitespace (prevent word-splitting), but his elements are 'g', 'h', and '7'. so I left it as a note. Did I miss something?

chepner Over a year ago

If the elements of the array do not contain whitespace, then it doesn't matter if you quote it or not; the result is the same. But if even one item does contain whitespace, it is crucial that you quote the expansion.

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