How can I assign command output to a variable in GNU make target rule?

Question

At a BASH prompt, I can do the following:

~/repo$ HISTORY_LOG=$(git log $(get_old_version)..$(get_new_version)); [[ ! -z ${HISTOR_LOG} ]] && ( echo "Some header"; echo "${HISTORY_LOG}" )

Where git log is demonstrably simplified version of what I actually have.

In a make file I have the following command as part of a target:

$(OUTPUT): $(INPUT)
        ...
        echo "Some header" > $(LOG_FILE)
        git log $(shell get_old_version)..$(shell get_new_version) >> $(LOG_FILE)

How can I rewrite the make target to behave like the bash command?

If I do the following line-feeds are being stripped:

$(OUTPUT): $(INPUT)
        ...
        HISTORY_LOG="$(shell git log $(shell get_old_version)..$(shell get_new_version))" ; \
           [ -z "$${HISTORY_LOG}" ] && \
             true || \ 
             (echo "Some header" ; echo "$${HISTORY_LOG}" )

when run looks like:

~/repo $ make
commit 2b4d87b0e64d129028c1a7a0b46ccde2f42c5e93     Author: Jamie   <[email protected]>   Date:   Mon Jun 25 18:46:27 2012 -0400              Issue #468: This sucker's been sped up.

and what I prefer would be:

~/repo $ make
commit 2b4d87b0e64d129028c1a7a0b46ccde2f42c5e93     
Author: Jamie   <[email protected]>
Date:   Mon Jun 25 18:46:27 2012 -0400 

    Issue #468: This sucker's been sped up.

I think the issue is the that make executes commands in /bin/sh and not /bin/bash. Regardless I'm looking for a portable solution if there is one.

Is there some reason you want to assign the output to a variable? Note that the variable passes out of scope at the end of the command. — Beta
– Beta, Commented Jul 3, 2012 at 17:41
Because I want to add a preamble only if the output is non-empty. The command generating the output is a bit unwieldy, I don't want to put it in the make file twice. And the fact the variable passes out of scope at the end of the command is desirable. An edit shows the a header is also printed if the output is non-empty. — Jamie
– Jamie, Commented Jul 3, 2012 at 17:47
GNUMake allows you to choose the shell, but not being a shell expert I'd probably just store the output in a temporary file. — Beta
– Beta, Commented Jul 3, 2012 at 17:55

William Pursell · Accepted Answer · 2012-07-03 20:31:23Z

5

Make's shell is eating your newlines. Just stop using it. Instead of $( shell get_old_version ), escape the $:

$$( get_old_version )

answered Jul 3, 2012 at 20:31

William Pursell

214k49 gold badges279 silver badges317 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Jamie Over a year ago

The escaping idea worked; but the spot you picked wasn't the culprit it was the $(shell git log ...) that needed the $$(git log ...) treatment. Thanks.

Collectives™ on Stack Overflow

How can I assign command output to a variable in GNU make target rule?

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related