How can I tell what device or file name I am redirecting to

Question

From within a shell script (sh/bash), how can I tell what device or file name I am redirecting to?

For example, I have a shell script named script.sh.

I run the shell script and redirect STDOUT to a file named out.txt, and redirect STDERR to a file named err.txt.

So, that would look like this:

$ ./script.sh > out.txt 2> err.txt

Now, from within the shell script, I can tell if STDOUT (or STDERR) is being redirected, or not:

if [ -t 1 ]; then...
if [ ! -t 1 ]; then...

but, once I know STDOUT (or STDERR) is being redirected, how can I tell where it is being rdirected to?

Answer 1

You could use lsof:

/usr/sbin/lsof -p $$ -a -d 1

The above would list just file descriptor 1 for just the current process:

COMMAND  PID USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
bash    5054 matt    1u   CHR  136,1      0t0    4 /dev/pts/1

Answer 2

You generally do things such as "watch" scripts, by watching them from the either the outside or from the script that called this subordinate script.

$ ./script.sh > out.txt 2> err.txt &
[1] 6280

Now we watch:

$ watch "ls -l out.txt err.txt"
Every 2.0s: ls -l out.txt err.txt                        Sun Feb  9 00:30:37 2014

-rw-rw-r--. 1 saml saml 0 Feb  9 00:30 err.txt
-rw-rw-r--. 1 saml saml 0 Feb  9 00:30 out.txt

Every 2 seconds (you can change this) the output will get updated:

Every 2.0s: ls -l out.txt err.txt                        Sun Feb  9 00:30:39 2014

-rw-rw-r--. 1 saml saml 200 Feb  9 00:30 err.txt
-rw-rw-r--. 1 saml saml 200 Feb  9 00:30 out.txt