How to store the output of command in a variable without creating a subshell [Bash <v4]

Question

ksh has a really interesting construct to do this, detailed in this answer: https://stackoverflow.com/a/11172617/636849

Since Bash 4.0, there is a builtin mapfile builtin command that should solve this problem: http://www.gnu.org/software/bash/manual/html_node/Bash-Builtins.html

But strangely, it doesn't seem to work with process substitution:

foo () { echo ${BASH_SUBSHELL}; }
mapfile -t foo_output <(foo) # FAIL: hang forever here
subshell_depth=${foo_output[0]} # should be 0

But how to do this in Bash v3.2 ?

Answer 1

Here's another way to do it, which is different enough that it warrants a separate answer. I think this method is subshell-free and bash sub-process free:

ubuntu@ubuntu:~$ bar () { echo "$BASH_SUBSHELL $BASHPID"; }
ubuntu@ubuntu:~$ bar
0 8215
ubuntu@ubuntu:~$ mkfifo /tmp/myfifo
ubuntu@ubuntu:~$ exec 3<> /tmp/myfifo
ubuntu@ubuntu:~$ unlink /tmp/myfifo
ubuntu@ubuntu:~$ bar 1>&3
ubuntu@ubuntu:~$ read -u3 a
ubuntu@ubuntu:~$ echo $a
0 8215
ubuntu@ubuntu:~$ exec 3>&-
ubuntu@ubuntu:~$

The trick here is to use exec to open the FIFO in read-write mode with an FD, which seems to have the side-effect of making the FIFO non-blocking. Then you can redirect your command to the FD without it blocking, then read the FD.

Note that the FIFO will be a limited-size buffer, probably around 4K, so if your command produces more output than this, it will end up blocking again.

Answer 2

This question comes up very often while looking how to just capture output of any "printing" command into variable. So for anyone looking it's possible (since bash v3.1.0) with:

printf -v VARIABLE_NAME "whatever you need here: %s" $ID

If you tweak your scripts for speed then you can use pattern of setting some global variable at the end of functions instead of just "echoing" it - use this with care, it's sometimes criticized as leading to hard to maintain code.