Bash: Retain variable's value after while loop

Question

I need to retain the value of TEMP_FLAG after my while loop ends.

#!/bin/bash

TEMP_FLAG=false

# loop over git log and set variable
git log --pretty="%H|%s" --skip=1 |
    while read commit; do 

        # do stuff like parsing the commit...

        # set variable     
        TEMP_FLAG=true 
    done

echo "$TEMP_FLAG" # <--- evaluates to false :(

I know that my issue is caused by piping the git log to the while loop, which spawns a subshell that doesn't give back my updated variable.

However, is there a way get my intended behavior without changing the pipe?

You cannot use pipe if you want to retain variables outside. Use process substitution — anubhava, Dec 04 '15 at 22:09
At all? I've seen things like `done <<< (something)` that seem to provide a way to receive variables that have been changed in a piped while loop — Danny Delott, Dec 04 '15 at 22:11
@DannyDelott right, but that doesn't use the pipe, which is key. Pipes create subshells and a subshell cannot modify its parent's environment — Eric Renouf, Dec 04 '15 at 22:12
@anubhava: Does that line imply removing the original `git log ... | ` — Danny Delott, Dec 04 '15 at 22:13
BTW, this is BashFAQ #24: http://mywiki.wooledge.org/BashFAQ/024 — Charles Duffy, Dec 04 '15 at 22:22
My issue was using `sh` instead of `bash` to launch my script, along with the rule about piping. — Danny Delott, Dec 04 '15 at 22:25

score 3 · Accepted Answer · answered Dec 04 '15 at 22:14

3

When you use a pipe you are automatically creating subshells so that the input and output can be hooked up by the shell. That means that you cannot modify the parent environment, because you're now in a child process.

As anubhava said though, you can reformulate the loop to avoid the pipe by using process substitution like so:

while read commit; do
    TEMP_FLAG=true
done < <( git log --pretty="%H|%s" --skip=1 )

printf "%s\n" "$TEMP_FLAG"

answered Dec 04 '15 at 22:14

Eric Renouf

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This gives me: `line 36: syntax error near unexpected token \`<' ` – Danny Delott Dec 04 '15 at 22:15
@DannyDelott, that means your shell isn't bash. Be sure your script starts with `#!/bin/bash`, not `#!/bin/sh`, and that you aren't running it with `sh yourscript`. – Charles Duffy Dec 04 '15 at 22:17
Hm, I'm using `#!/bin/bash` at the top of my script. – Danny Delott Dec 04 '15 at 22:18
@DannyDelott, and you're starting the script how? – Charles Duffy Dec 04 '15 at 22:18
I am running it with `sh myscript.sh` though, how should it be run? – Danny Delott Dec 04 '15 at 22:18
1

`bash myscript.sh`, though you shouldn't use a `.sh` extension for something that isn't a POSIX sh script. (Common practice is for executable commands to have no extension at all -- you don't run `ls.elf`, after all -- but if something is a shell *library*, then it should be `.bash` if it's for bash, `.ksh` for ksh, etc., and `.sh` only for POSIX sh). – Charles Duffy Dec 04 '15 at 22:19
Thanks! Works as expected now. – Danny Delott Dec 04 '15 at 22:20

Bash: Retain variable's value after while loop

1 Answers1