Set a parent shell's variable from a subshell

Question

How do I set a variable in the parent shell, from a subshell?

a=3
(a=4)
echo $a

Answer 1

You can output the value in the subshell and assign the subshell output to a variable in the caller script:

# subshell.sh
echo Value

# caller
myvar=$(subshell.sh)

If the subshell has more to output you can separate the variable value and other messages by redirecting them into different output streams:

# subshell.sh
echo "Writing value" 1>&2
echo Value

# caller
myvar=$(subshell.sh 2>/dev/null) # or to somewhere else
echo $myvar

Alternatively, you can output variable assignments in the subshell, evaluate them in the caller script and avoid using files to exchange information:

# subshell.sh
echo "a=4"

# caller
# export $(subshell.sh) would be more secure, since export accepts name=value only.
eval $(subshell.sh)
echo $a

The last way I can think of is to use exit codes but this covers the integer values exchange only (and in a limited range) and breaks the convention for interpreting exit codes (0 for success non-0 for everything else).