cannot understand combined exec and redirection in bash script

Question

I know exec is for executing a program in current process as quoted down from here

exec replaces the current program in the current process, without forking a new process. It is not something you would use in every script you write, but it comes in handy on occasion.

I'm looking at a bash script a line of which I can't understand exactly.

#!/bin/bash
LOG="log.txt"
exec &> >(tee -a "$LOG")
echo Logging output to "$LOG"

Here, exec doesn't have any program name to run. what does it mean? and it seems to be capturing the execution output to a log file. I would understand if it was exec program |& tee log.txt but here, I cannot understand exec &> >(tee -a log.txt). why another > after &>?
What's the meaning of the line? (I know -a option is for appending and &> is for redirecting including stderr)

EDIT : after I selected the solution, I found the exec &> >(tee -a "$LOG") works when it is bash shell(not sh). So I modified the initial #!/bin/sh to #!/bin/bash. But exec &>> "$LOG" works both for bash and sh.

Does your system have the bash manual page installed? Start it and search for "exec " (exec followed by a space). — Jens, Jul 28 '16 at 09:41

score 4 · Accepted Answer · edited Aug 20 '16 at 14:18

4

From man bash:

exec [-cl] [-a name] [command [arguments]]

If command is not specified, any redirections take effect in the current shell, [...]

And the rest:

&>     # redirects stdout and stderr
>(cmd) # redirects to a process

See process substitution.

edited Aug 20 '16 at 14:18

agc

7,973
2
29
50

answered Jul 28 '16 at 09:29

Karoly Horvath

94,607
11
117
176

Could you explain in the context of the example? – sjsam Jul 28 '16 at 09:32
You're redirecting the output of the current shell. – Karoly Horvath Jul 28 '16 at 09:35
Thankyou. Nice explanation. – sjsam Jul 28 '16 at 09:50
1

Thank you, so in other words, exec &> redirects the output of the current shell script, and the following > redirects it to the log file. (Wow, this means I can redirect the script's output to a file inside the script!). – Chan Kim Jul 28 '16 at 12:46
I changed `#!/bin/sh` to `#!/bin/bash` because `exec &> >(tee -a "$LOG")` works only when it is bash. – Chan Kim Jul 28 '16 at 13:00

cannot understand combined exec and redirection in bash script

1 Answers1