What you may want to consider doing is capturing
stderr to variable, and afterwards checking if it's not empty. Consider the example below:
$ foo=$(stat /etc/passwd 2>&1 > /dev/tty ) File: /etc/passwd Size: 1710 Blocks: 8 IO Block: 4096 regular file Device: fd00h/64768d Inode: 537594 Links: 1 Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root) Access: 2019-03-08 15:28:10.481688274 -0700 Modify: 2019-03-08 15:28:10.333669255 -0700 Change: 2019-03-08 15:28:10.341670283 -0700 Birth: - $ test "x$foo" = "x" && echo "empty" empty
$(...) runs a subshell and captures
stdout only, hence we want to duplicate the file descriptor 1 onto 2
2>&1 so that stderr now can be captured by command substitution, however we still want to see normal output on screen, hence afterwards we point
stdout at the controlling terminal
Now, what if we actually capture stderr into the variable ?
$ foo=$(stat nonexistent.txt 2>&1 > /dev/tty ) $ test "x$foo" = "x" && echo "empty" || echo "not empty" not empty $ printf "<Arbitrary Text>%s\n" "$foo" <Arbitrary Text>stat: cannot stat 'nonexistent.txt': No such file or directory
As you can see the because
stderr is captured we can wrap arbitrary text around it as we wish. The main reason for using a variable is that standard shell methods such as pipeline and command substitution operate on
stdout file descriptor 1, so there's not a lot we can do to parse
stderr on the fly. Of course, we could use a named pipe and redirect to it as
stat foobar.txt 2> /tmp/named_pipe.fifo, but the problem with that is blocking - information sent to pipes is buffered until consumed by another process, so your shell script will be stuck. Of course, this could be handled via starting a background process, but IMHO it's more complexity than necessary, and doing multiple processes is far better done in a programming language like Python where you can actually directly access multiprocessing calls.
[ are the same command, so you could just as well write
[ "x$foo" = "x" ] and use that in a more elaborate if-else statement