why the command; ls -l file_doesnot_exists > /dev/null 2>&1 is working while; ls -l 2>&1 file_doesnot_exists > /dev/null is not

Question

I'm just wondring how to know which instruction we normally put first in a shell command ? for example, why the command:

ls -l file_doesnot_exists > /dev/null 2>&1

is working, while this command :

ls -l 2>&1 file_doesnot_exists > /dev/null

is not

Answer 1

Note:

• The question is tagged linux, which means that ls is not a built-in alias of PowerShell's own Get-ChildItem cmdlet (which applies on Windows only) and instead refers to the standard /bin/ls Unix utility.

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• In PowerShell there is no difference between your commands, because the order of redirections does NOT matter.

  • All targeted output streams always retain their identity: any redirection of a stream affects any other redirections to the same stream in the same command.

  • Therefore, neither of your commands work as intended and produce no output, because while 2>&1 redirects the error stream (2) into the success output stream (1), the latter's output is ultimately discarded, due to > /dev/null (> is the same as 1>), including the redirected error-stream output.

• By contrast, in POSIX-compatible shells such as Bash, the order of redirections DOES matter:

  • ls -l 2>&1 file_doesnot_exists > /dev/null WORKS as intended:

    • 2>&1 redirects stderr output to the original stdout.
    • The later redirection of stdout (> is the same as 1>) has no effect on 2>&1
    • The net effect is that only stderr lines print to stdout, while stdout lines are discarded (by the redirection to /dev/null).

  • By contrast, ls -l file_doesnot_exists > /dev/null 2>&1 DOES NOT and produces no output:

    • > /dev/null redirects stdout to /dev/null, i.e. effectively discards stdout output.

    • Because 2>&1 comes later in the command, 1 refers to the already redirected stdout, so that stderr output too is discarded.

  • See this answer for more information.