Linux

In bash, if you put:

ls /Users/*/.ssh/id_rsa 2>&1 > rsa-keys.log

...you're redirecting stderr to the stdout's destination while stdout is still sending output to the screen. So any permission errors encountered will go to the screen, not to rsa-keys.log.

From the bash manpage:

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Note that the order of redirections is significant. For example, the command

   ls > dirlist 2>&1

directs both standard output and standard error to the file dirlist, while the command

   ls 2>&1 > dirlist

directs only the standard output to file dirlist, because the standard error was duplicated from the standard output before the standard output was redirected to dirlist.

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Commands given to the shell are evaluated and processed in a specific order and fashion, and this is one quirk of that that many people are unaware of.