为什么要命令； ls -l file_doesnot_exists > /dev/null 2>&1 正在工作； ls -l 2>&1 file_doesnot_exists > /dev/null 不是

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 , ie 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.

Answer 2

Assuming this is being ran in PowerShell Core ( since the tags indicate it is ), it's safe to assume you're confusing bash commands with PowerShell cmdlets. In PowerShell, you have to be mindful of Terminating , and Non-Terminating errors.

Simply put, the reason why ls -l 2>&1 file_doesnot_exists > /dev/null doesn't work is because, ls - alias for Get-ChildItem - produces a terminating error that halts the rest of the execution since it is missing an argument. This is demonstrated in a Try {...} catch {...} statement where the catch block " catches " terminating errors.

try {
    ls -l
}
catch {
    "it no workie"
}

Since ls is an alias to Get-ChildItem , the - L parameter defaults to -LiteralPath where it's now expecting a path and it's erroring out when ran since it wasn't provided.