Bash variable concatenation fails inside loop

Question

Given the following statements:

ac_reg_ids="-1" #Starting value
(mysql) | while read ac_reg_id; do
    echo "$ac_reg_id" #variable is a result of a mysql query. Echoes a number.
    ac_reg_ids="$ac_reg_ids, $ac_reg_id" #concatenate a comma and $ac_reg_id, fails.
done
echo "ac_reg_ids: $ac_reg_ids" #echoes -1

Now according to this answer: https://stackoverflow.com/a/4181721/1313143

Concatenation should work. Why doesn't it, though? What's different within the loop?

Just in case it could matter:

> bash -version
> GNU bash, version 4.2.8(1)-release (i686-pc-linux-gnu)

Update

Output with set -eux:

+ echo 142
142
+ ac_reg_ids='-1, 142'
+ read ac_reg_id

It looks fine to me. Is that the exact code that fails, copy-and-pasted, not typed by hand, nothing added or removed? — John Kugelman
– John Kugelman, Commented Feb 27, 2013 at 18:40
what version of bash are you using? I have just tried this using 4.2.24, and the output of your sample code is exactly what you would expect, if ac_reg_id variable is not set, last line prints -1, — Alex
– Alex, Commented Feb 27, 2013 at 18:44
Please run set -eux before that piece of code, run it again and paste output here. — emi
– emi, Commented Feb 27, 2013 at 18:46
@Alex I added version. It's a bit older than yours but I don't see how it could matter in such small difference. — MarioDS
– MarioDS, Commented Feb 27, 2013 at 18:48

that other guy · Accepted Answer · 2016-12-02 16:19:45Z

5

Like shellcheck would helpfully have pointed out, you're modifying ac_reg_ids in a subshell.

Rewrite it to avoid the subshell:

ac_reg_ids="-1" #Starting value
while read ac_reg_id; do
    echo "$ac_reg_id" 
    ac_reg_ids="$ac_reg_ids, $ac_reg_id"
done < <( mysql whatever )  # Redirect from process substution, avoiding pipeline
echo "ac_reg_ids: $ac_reg_ids"

edited Dec 2, 2016 at 16:19

answered Feb 27, 2013 at 18:56

that other guy

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3 Comments

MarioDS Over a year ago

I'll see if it does the trick. Thanks for pointing out that website, didn't know about it yet. Could you give more information about how "< <" works? I'm pretty new with bash.

that other guy Over a year ago

There is no < <. There is < which redirects from a file as I'm sure you already know, and there's <(cmd) which expands to a filename which when read produces the output from cmd. Any variation on this, such as << (cmd), <<(cmd) or < < cmd is invalid.

MarioDS Over a year ago

Okay I got it now. Thank you.

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