bash script overrides hard coded variables in executed second script

Question

I'm calling Uncle. I'm attempting to manipulate variables that have hard coded values in a second bash script I am calling. I have no control over the script and am building a wrapper around it to adjust some build behavior before it finally kicks off a yocto build. I'm not sure what else to try after reading and trying numerous examples.

Examples of the situation:

build.sh calls build2.sh

IS_DEV=1 ./build2.sh #trying to override value

build2.sh

IS_DEV=0 # hardcoded value
echo $IS_DEV
# always results in 0.

I have also tried export IS_DEV=1 before calling build2.sh.

I'm sure this is pretty simple, but I cannot seem to get this to work. I appreciate any assistance. Is this possible? I'm using GNU bash, version 4.3.48(1)-release (x86_64-pc-linux-gnu) on Ubuntu 16.04.4 LTS.

Oh, I have also tried the sourcing technique with no luck.

IS_DEV=1 . ./build2.sh
IS_DEV=1 source ./build2.sh

Where am I getting this wrong?

Much appreciated.

This question, and the answers, would indicate you may be out of luck unless you can get support for this in the second script stackoverflow.com/questions/4609668/… — Brandon Miller
– Brandon Miller, Commented Jun 15, 2018 at 19:43
Possible duplicate of override variable in bash script from command line — Brandon Miller
– Brandon Miller, Commented Jun 15, 2018 at 19:44
@BrandonMiller yes I have looked at that post and tried the examples, no go. I'll see if I can gain the ability to tweak the second script. thx. — Frank Beard
– Frank Beard, Commented Jun 15, 2018 at 19:48
I'd always have written your build2 to have : "${IS_DEV:=0}", making 0 explicitly an overridable default. (BTW, all-caps variables are used by the shell and POSIX utilities for their own use; the namespace of variables with at least one lower-case letter are reserved for application use and guaranteed not to modify the shell's behavior; see relevant spec @ pubs.opengroup.org/onlinepubs/9699919799/basedefs/…, fourth paragraph) — Charles Duffy
– Charles Duffy, Commented Jun 15, 2018 at 20:07
@CharlesDuffy I wish I could directly edit the called script and clean it up, but I cannot. :( — Frank Beard
– Frank Beard, Commented Jun 15, 2018 at 20:09

tripleee · Accepted Answer · 2018-06-15 19:47:44Z

4

If you can't modify the script, execute a modified version of it.

sed 's/^IS_DEV=0 /IS_DEV=1 /' build2.sh | sh

Obviously, pipe to bash if you need Bash semantics instead of POSIX sh semantics.

If the script really hard-codes a value with no means to override it from the command line, modifying that script is the only possible workaround. But the modification can be ephemeral; the above performs a simple substitution on the script, then passes the modified temporary copy through a pipe to a new shell instance for execution. The modification only exists in the pipeline, and doesn't affect the on-disk version of build2.sh.

answered Jun 15, 2018 at 19:47

tripleee

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3 Comments

tripleee Over a year ago

See also tangentially Difference between sh and bash

Frank Beard Over a year ago

thank you! this saved me. I've never used sed, now I will. thx again.

Frank Beard Over a year ago

hey guys, the second part of this is that after using sed to rewrite an in-memory copy of the second script, I also need to "forward along" a big command line to the same script. what is the approach for that? example: sed 's/^IS_DEV=0 /IS_DEV=1 /' build2.sh | bash -d -a 1 --logs I would want all the "additional" cmd line args to pass to build2.sh and not as args to the bash shell.

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