Redirect stdout/stderr of a background job from console to a log file?

Apparently I misread your question the first time, so here's my updated answer:

After you sent your program to the background, you first have to find its PID

pgrep foo.sh

Then you could use gdb to attach to that process

gdb -p <PID>

In gdb you then change where this program writes to

p dup2(open("/path/to/file",577, 420), 1)
p dup2(1, 2)

then you detach from the process and quit gdb

detach
quit

A little explanation

• 577 is equivalent to O_CREAT|O_WRONLY|O_TRUNC
• 420 is equivalent to S_IRUSR|S_IWUSR|S_IRGRP|S_IROTH
• So the call to open opens the file and truncates it to 0 bytes if it exists or creates a new one with the right file permissions if it doesn't exist
• The first call to dup2 duplicates the file descriptor returned by the call to open to file descriptor 1 (which is stdout)
• The second call to dup2 duplicates the file descriptor 1 to 2 (which is stderr)