Unable to assign variable in Makefile

Question

In Makefile, when I am assigning bash variable to Makefile file, it is not working, empty string is assigned. In below code, variable LINE and variable y are empty.

SHELL = bash
target:
        while read -r CURRENT_LINE; do \
                echo "$$CURRENT_LINE" ; \ ##WORKING FINE, display line by line
                LINE=$(shell echo "$$CURRENT_LINE") ; \ ##NOT WORKING
                echo "line:-$(LINE)" ; \ ##empty
                LINE=$$(CURRENT_LINE) ; \ ##NOT WORKING
                echo "line:-$(LINE)" ; \ ##empty
                y=$(subst :, ,$$CURRENT_LINE) ; \ ##NOT WORKING
                echo "y:- $(y)"; \  ##empty

        done < sample_file.txt ;

`$(shell ...)` executes in make, not the shell, so it can't access shell variables. — Barmar, Jun 21 '22 at 09:43
@Barmar what would be correct syntax as I want to split the `CURRENT_LINE` by delimiter or space? How to access the value of `CURRENT_LINE` — piyush-balwani, Jun 21 '22 at 10:04
Why not read directly into multiple variables? `while read -r var1 var2 var3 ...` — Barmar, Jun 21 '22 at 10:06
If you want an array, see https://stackoverflow.com/questions/10586153/how-to-split-a-string-into-an-array-in-bash — Barmar, Jun 21 '22 at 10:08
As I am reading line by line from a file, so `CURRENT_LINE` is the current line that I am processing at a time. Now I want to split this line by delimiter. — piyush-balwani, Jun 21 '22 at 10:20
`while read -r CURRENT_LINE; do IFS=', ' read -r -a array <<< "$$CURRENT_LINE" ; echo "${array[0]}" ; done — piyush-balwani, Jun 21 '22 at 10:20
@piyush-balwani `array` is a shell variable, so `echo "$${array[0]}` — William Pursell, Jun 21 '22 at 10:22
Thanks @Barmar and @William. `while read -r -a array; do echo "$${array[1]}" done — piyush-balwani, Jun 21 '22 at 10:27

score 0 · Answer 1 · answered Jun 21 '22 at 15:47

The correct solution would be:

        while read -r CURRENT_LINE; do \
                LINE="$${CURRENT_LINE}" ;
                echo "LINE:$${LINE}"; \
        done

You need to distinguish between make variables and bash variables, and understand that make expands make variables before passing strings to the shell.

                echo "$$CURRENT_LINE" ; \ ##WORKING FINE, display line by line
                LINE=$(shell echo "$$CURRENT_LINE") ; \ ##NOT WORKING
                echo "line:-$(LINE)" ; \ ##empty

This has two errors: first, you're running $(shell ...), which starts a new shell when make is parsing the recipe... (notice that this is before the recipe runs, and therefore before CURRENT_LINE is set), so in this shell instance, $CURRENT_LINE will expand to empty. The second issue is that you're doing an echo of $(LINE), which is also expanded at make parse time to make's value of its LINE variable. You would need to use $$LINE, which make would expand to $LINE before passing to the shell, and then the shell would be able to expand to its value of $LINE.

For the next part:

                LINE=$$(CURRENT_LINE) ; \ ##NOT WORKING
                echo "line:-$(LINE)" ; \ ##empty

First line should be LINE=$$CURRENT_LINE or LINE=$${CURRENT_LINE}. Bash treats items in $(...) as commands, not variables. This also has a second error that the echo line echo's $(LINE) is again a make variable, which is empty.

                y=$(subst :, ,$$CURRENT_LINE) ; \ ##NOT WORKING
                echo "y:- $(y)"; \  ##empty

So again this is an order thing -- $(subst ...) is expanded at make parse time (before the recipe is actually passed to shell). What it's doing is it's substituting on the string $CURRENT_LINE, not the expanded version of this. Also, you're printing the make version of $(y) again.

Unable to assign variable in Makefile

1 Answers1