Use output of a command for another

Question

In my local machine my executable is stored at

.stack-work/install/x86_64-osx/lts-13.0/8.6.3/bin/nonosolver-exe

In my cloud instance it is stored at

./.stack-work/install/x86_64-linux-tinfo6/lts-13.0/8.6.3/bin/nonosolver-exe

Thus the executable will always be in ./.stack-work/install/{??}/nonosolver-exe, depending on the machine and version of GHC im using. So in my make file I'm using

find ./.stack-work/install -name nonosolver-exe

to find my executable.

How do I take the result of find to start a daemon with setsid (taken from here):

setsid {path to executable} >/dev/null 2>&1 < /dev/null &

I have tried (taken from here) :

find ./.stack-work/install -name nonosolver-exe -exec setsid {} >/dev/null 2>&1 < /dev/null &

to no avail

Mad Physicist · Accepted Answer · 2020-03-11T16:20:35.143

3

Shell

Use $(...) to insert the stdout of one command into another as a string:

setsid "$(find ./.stack-work/install -name nonosolver-exe)" >/dev/null 2>&1 < /dev/null &

This will cause unexpected problems if you end up finding a number of matches other than one. You can store into a variable to do error checking first:

exe_path="$(find ./.stack-work/install -name nonosolver-exe)"
# check non-empty and no newlines
setsid "${exe_path}" >/dev/null 2>&1 < /dev/null &

Makefile

The makefile equivalent to the bash $(...) construct is $(shell ...). And variables are expanded with $(...), not ${...}:

setsid "$(shell find ./.stack-work/install -name nonosolver-exe)" >/dev/null 2>&1 < /dev/null

Or

EXE_PATH = $(shell find ./.stack-work/install -name nonosolver-exe)
setsid "$(EXE_PATH)" >/dev/null 2>&1 < /dev/null

edited Mar 11 '20 at 16:20

answered Jan 19 '19 at 06:47

Mad Physicist

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your solution works great on the command line, but when i put it into a makefile the "${..}" expression resolves to "". Any ideas? – iluvAS Jan 19 '19 at 06:56
Shell variables and make variables don't have quite the same syntax. I'll update – Mad Physicist Jan 19 '19 at 06:58
EXE_PATH works, the makefile output was `EXE_PATH=./.stackwork/.../nonosolver-exe`, but the process didn't start. I then tested with `echo "$(EXE_PATH)"` and its showing `echo ""` so i'm guessing `setsid` didn't get to EXE_PATH either. – iluvAS Jan 19 '19 at 07:11
I used `setsid "$(shell find ./.stack-work/install -name nonosolver-exe)" >/dev/null 2>&1 < /dev/null &` and its working now, thank you – iluvAS Jan 19 '19 at 07:18
What about pipes? to xargs? – BladeMight Jan 19 '19 at 14:12
@BladeMight. What about them? Why start a third process when two will do? – Mad Physicist Jan 19 '19 at 14:13
Use them instead of $(...), just as an alternative. – BladeMight Jan 19 '19 at 14:14
@MadPhysicist I posted an answer using pipe & xargs. – BladeMight Jan 19 '19 at 14:23

score 2 · Answer 2 · answered Jan 19 '19 at 14:21

2

Using the pipe and xargs:

find ./.stack-work/install -name nonosolver-exe -print0 | xargs -0 -I{} setsid "{}" >/dev/null 2>&1 < /dev/null &

answered Jan 19 '19 at 14:21

BladeMight

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1

This would work equally well in a makefile as it does in the shell. Very +1 – Mad Physicist Jan 19 '19 at 14:28

Use output of a command for another

2 Answers2