In Unix-like systems, there are two output paths that if left unmodified will send output to your screen. Standard error (or stderr) is the one that captures most failures and error conditions.
To pass the permission denied message in the stderr to the same output stream as "regular output" you must combine the two. In your example, in order for your
grep -v to properly operate on it, you combine stdout (standard output) and stderr with the arcane syntax you see.
From the advanced bash scripting guide :
If ‘|&’ is used, the standard error of
command1 is connected to command2's
standard input through the pipe; it is
shorthand for 2>&1 |. This implicit
redirection of the standard error is
performed after any redirections
specified by the command.
Also, as geirha points out, if you want to just get rid of stderr output, you would want to do something like
find -name 'myfile.*' 2> /dev/null
find -name 'myfile.*' > /tmp/errorlog
And note that if you have strings of commands, such as a
find passing its output to
xargs you would need to put the entire pipeline of commands in parentheses to capture the output from all components of the command. E.g.,
(find | egrep ^[RS].[0-9]+/.svg] | xargs head -1 ) 2> /dev/null
If you left out the parentheses, and did this instead --
find | egrep ^[RS].[0-9]+/.svg] | xargs head -1 2> /dev/null
you would still see permission denied errors from the find or egrep, but stderr would be redirected for xargs.
As you've seen, you would likely throw away the stderr only after viewing its contents during a test run.
Note that with
GNU find and as far as I can tell, any POSIX-compliant
-print option is implicit. You can still supply it explicitly if you like.