Bash edit value from subprocess?

Question

Okay, so I'm having trouble dealing with Bash and it's lack of global variables. The problem I'm having is that inside the while loop, I'm having to do a for loop, and inside that for loop I'm going to be calling a script and I need to dynamically change the variable $NAME. Unfortunately, I won't know what it is until I've tried a few different options. The variable never wants to change after it's been set in the original process. I've done a lot of research about this today and I've found that it's because reading a file in such a file opens a subshell and it can't edit values in it's parent process. I've tried making several files to source to export the variable dynamically, but that doesn't work either.

EDIT: THIS EXAMPLE IS TRIVIAL, BUT IT'S THE SAME LOGIC I'M USING AT WORK. CAN'T SHOW WORK CODE.

#!/usr/bin/env bash
FILE=$1
SCRIPT="FOO"
NAME="TEST"

#create script
cat > "${SCRIPT}" <<EOF
HELLO WORLD, $NAME
EOF

while read line; do
echo $line
echo $SCRIPT
  for i in `seq 1 10`; do
    (
    NAME="MEGAMAN"
    echo $NAME
    )
  done
done < "$FILE"
#echo $NAME
cat > "${SCRIPT}" <<EOF
HELLO WORLD, $NAME
EOF

My output:

jose@jose-desktop:~/workspace/Script $ ./Main FOO 
HELLO WORLD, TEST
FOO
MEGAMAN
MEGAMAN
MEGAMAN
MEGAMAN
MEGAMAN
MEGAMAN
MEGAMAN
MEGAMAN
MEGAMAN
MEGAMAN **this output is fine

And the file it outputs:

HELLO WORLD, TEST **This file should say MEGAMAN.

"Lack of global variables"? Huh? **All** variables are global in bash unless you declare them otherwise. — Charles Duffy, Sep 20 '16 at 02:04
...your problem is that you're running code in a subprocess, so the global variables you set there are cleared when that subprocess exits. See also [BashFAQ #24](http://mywiki.wooledge.org/BashFAQ/024). Same as with globals in any other language if you're forking off a subprocess. — Charles Duffy, Sep 20 '16 at 02:05
...the simplest solution is just not to do that -- which is to say, to ensure that the code that sets your variables is in-process. If you replaced your `( )`s with `{ }`s, you'd be set. — Charles Duffy, Sep 20 '16 at 02:08
Aside: All-caps variable names are, [per POSIX specification](http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html), used for variables with meaning to the shell and operating system; the specification guarantees that applications can use any variable name with at least one lower-case letter without modifying the behavior of the shell or operating system. The spec section is on environment variables, but since setting a shell variable with a name already used by an environment variable overwrites the latter, the convention applies there as well. Consider `$name` vs `$NAME`. — Charles Duffy, Sep 20 '16 at 02:13
Aside (2): `seq` is neither specified by POSIX nor shipped as part of bash. Consider using C-style for loops: `for ((i=0; i<10; i++))`, which avoids the fork/exec overhead to run an external tool. — Charles Duffy, Sep 20 '16 at 02:15

score 1 · Answer 1 · answered Sep 20 '16 at 01:58

1

It is impossible to change a variable from a child process, be that a subshell or some other shell script. If you have to do something like that, you have to find a way for the subprocess to communicate with the parent. For example, if you are invoking an external script and you want to take a value computed by that script and store it in a variable in the parent script, you could have the script output that value in its stdout and then capture it in the parent process, e.g.

# child-script.sh:
myvar=.... # compute some value
echo "$myvar" # put that at the end of the script

and in the parent, replace the line

./child-script.sh

by

myvar="$(./child-script.sh)"

answered Sep 20 '16 at 01:58

redneb

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But what if I need to pass in the variable, the variable that I need to pass in is the variable from the for loop. I suppose in this case it'd be the numbers 1 through 10? I need that to be interpolated into the script as $NAME – user3505901 Sep 20 '16 at 02:09
@user3505901, `$()` runs a subshell. Subshells inherit all the state of their parent, including variables' current values. So yes, you can run `myvar=$(something involving "$i")` if you want to go that route (though I wouldn't do so unless you can't possibly avoid the subshell altogether). – Charles Duffy Sep 20 '16 at 02:10

Bash edit value from subprocess?

1 Answers1