whats the last “-” hyphen means in this command?












1














In this npm installation tutorial, we need to execute following command :



# curl -sL https://rpm.nodesource.com/setup_6.x | sudo -E bash -


What the last hyphen "-" after bash means ?



I've seen a lot of command with this, and couldn't find myself a logical explanation and neither find how to reformulate a google search for it, it is the output of the piped command ? please advice.



Thank you,










share|improve this question



























    1














    In this npm installation tutorial, we need to execute following command :



    # curl -sL https://rpm.nodesource.com/setup_6.x | sudo -E bash -


    What the last hyphen "-" after bash means ?



    I've seen a lot of command with this, and couldn't find myself a logical explanation and neither find how to reformulate a google search for it, it is the output of the piped command ? please advice.



    Thank you,










    share|improve this question

























      1












      1








      1







      In this npm installation tutorial, we need to execute following command :



      # curl -sL https://rpm.nodesource.com/setup_6.x | sudo -E bash -


      What the last hyphen "-" after bash means ?



      I've seen a lot of command with this, and couldn't find myself a logical explanation and neither find how to reformulate a google search for it, it is the output of the piped command ? please advice.



      Thank you,










      share|improve this question













      In this npm installation tutorial, we need to execute following command :



      # curl -sL https://rpm.nodesource.com/setup_6.x | sudo -E bash -


      What the last hyphen "-" after bash means ?



      I've seen a lot of command with this, and couldn't find myself a logical explanation and neither find how to reformulate a google search for it, it is the output of the piped command ? please advice.



      Thank you,







      linux command-line bash pipe






      share|improve this question













      share|improve this question











      share|improve this question




      share|improve this question










      asked 1 hour ago









      Omar BISTAMI

      205




      205






















          2 Answers
          2






          active

          oldest

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          2














          In man bash, at the end of the single-character options there is:-



          --    A -- signals the end of options and disables further option processing.
          Any arguments after the -- are treated as filenames and arguments. An
          argument of - is equivalent to --.


          If you have quoted the complete command, I can see no reason to use - after bash in this instance, but it does no harm.






          share|improve this answer





















          • Thank you for your answer, Yes i have quoted the complete command. so anything after - or -- will not be seen as an option but as a file name or arguments, could you please give an example where it is useful ?
            – Omar BISTAMI
            56 mins ago










          • It's really to allow for a script whose name begins -, an unlikely requirement, but -/-- makes it possible.
            – AFH
            6 mins ago



















          1














          Bash behaves in somewhat non-standard way when it comes to -.



          POSIX says:




          Guideline 10:

          The first -- argument that is not an option-argument should be accepted as a delimiter indicating the end of options. Any following arguments should be treated as operands, even if they begin with the - character.



          […]



          Guideline 13:

          For utilities that use operands to represent files to be opened for either reading or writing, the - operand should be used to mean only standard input (or standard output when it is clear from context that an output file is being specified) or a file named -.




          And




          Where a utility described in the Shell and Utilities volume of POSIX.1-2017 as conforming to these guidelines is required to accept, or not to accept, the operand - to mean standard input or output, this usage is explained in the OPERANDS section. Otherwise, if such a utility uses operands to represent files, it is implementation-defined whether the operand - stands for standard input (or standard output), or for a file named -.




          But then man 1 bash reads:




          A -- signals the end of options and disables further option processing. Any arguments after the -- are treated as filenames and arguments. An argument of - is equivalent to --.




          So for Bash - means neither standard input nor a file, hence somewhat non-standard.



          Now your particular case:




          curl -sL https://rpm.nodesource.com/setup_6.x | sudo -E bash -



          I suspect the author of this command may not realize - is equivalent to -- in this case. I suspect the author wanted to make sure bash will read from its standard input, they expected - to work according to the guideline 13.



          But even if it worked according to the guideline, - would be unnecessary here because bash detects when its standard input is a pipe and acts accordingly (unless -c is given etc.).



          Yet - doesn't work according to the guideline, it works like --. Still -- is unnecessary here because there are no arguments after it.



          In my opinion the last - changes nothing. The command would work without it.



          To see how -- and - can be useful in general, study the example below.





          cat in my Kubuntu obeys both guidelines and I will use it to demonstrate usefulness of - and --.



          Let a file named foo exist. This will print the file:



          cat foo


          Let a file named --help exist. This won't print the file:



          cat --help


          But this will print the file named --help:



          cat -- --help


          This will concatenate the file named --help with whatever comes from the standard input:



          cat -- --help -


          It seems you don't really need --, because you can always pass ./--help which will be interpreted as a file for sure. But consider



          cat "$file"


          when you don't know beforehand what the content of the variable is. You cannot just prepend ./ to it, because it may be an absolute path and ./ would break it. On the other hand it may be a file named --help (because why not?). In this case -- is very useful; this is a lot more robust command:



          cat -- "$file"





          share|improve this answer























          • Thank your for your answer , is there any examples that can help explain it more in bash context ?
            – Omar BISTAMI
            50 mins ago










          • @OmarBISTAMI I have expanded my answer.
            – Kamil Maciorowski
            13 mins ago











          Your Answer








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          2 Answers
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          2 Answers
          2






          active

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          active

          oldest

          votes






          active

          oldest

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          2














          In man bash, at the end of the single-character options there is:-



          --    A -- signals the end of options and disables further option processing.
          Any arguments after the -- are treated as filenames and arguments. An
          argument of - is equivalent to --.


          If you have quoted the complete command, I can see no reason to use - after bash in this instance, but it does no harm.






          share|improve this answer





















          • Thank you for your answer, Yes i have quoted the complete command. so anything after - or -- will not be seen as an option but as a file name or arguments, could you please give an example where it is useful ?
            – Omar BISTAMI
            56 mins ago










          • It's really to allow for a script whose name begins -, an unlikely requirement, but -/-- makes it possible.
            – AFH
            6 mins ago
















          2














          In man bash, at the end of the single-character options there is:-



          --    A -- signals the end of options and disables further option processing.
          Any arguments after the -- are treated as filenames and arguments. An
          argument of - is equivalent to --.


          If you have quoted the complete command, I can see no reason to use - after bash in this instance, but it does no harm.






          share|improve this answer





















          • Thank you for your answer, Yes i have quoted the complete command. so anything after - or -- will not be seen as an option but as a file name or arguments, could you please give an example where it is useful ?
            – Omar BISTAMI
            56 mins ago










          • It's really to allow for a script whose name begins -, an unlikely requirement, but -/-- makes it possible.
            – AFH
            6 mins ago














          2












          2








          2






          In man bash, at the end of the single-character options there is:-



          --    A -- signals the end of options and disables further option processing.
          Any arguments after the -- are treated as filenames and arguments. An
          argument of - is equivalent to --.


          If you have quoted the complete command, I can see no reason to use - after bash in this instance, but it does no harm.






          share|improve this answer












          In man bash, at the end of the single-character options there is:-



          --    A -- signals the end of options and disables further option processing.
          Any arguments after the -- are treated as filenames and arguments. An
          argument of - is equivalent to --.


          If you have quoted the complete command, I can see no reason to use - after bash in this instance, but it does no harm.







          share|improve this answer












          share|improve this answer



          share|improve this answer










          answered 1 hour ago









          AFH

          13.9k31938




          13.9k31938












          • Thank you for your answer, Yes i have quoted the complete command. so anything after - or -- will not be seen as an option but as a file name or arguments, could you please give an example where it is useful ?
            – Omar BISTAMI
            56 mins ago










          • It's really to allow for a script whose name begins -, an unlikely requirement, but -/-- makes it possible.
            – AFH
            6 mins ago


















          • Thank you for your answer, Yes i have quoted the complete command. so anything after - or -- will not be seen as an option but as a file name or arguments, could you please give an example where it is useful ?
            – Omar BISTAMI
            56 mins ago










          • It's really to allow for a script whose name begins -, an unlikely requirement, but -/-- makes it possible.
            – AFH
            6 mins ago
















          Thank you for your answer, Yes i have quoted the complete command. so anything after - or -- will not be seen as an option but as a file name or arguments, could you please give an example where it is useful ?
          – Omar BISTAMI
          56 mins ago




          Thank you for your answer, Yes i have quoted the complete command. so anything after - or -- will not be seen as an option but as a file name or arguments, could you please give an example where it is useful ?
          – Omar BISTAMI
          56 mins ago












          It's really to allow for a script whose name begins -, an unlikely requirement, but -/-- makes it possible.
          – AFH
          6 mins ago




          It's really to allow for a script whose name begins -, an unlikely requirement, but -/-- makes it possible.
          – AFH
          6 mins ago













          1














          Bash behaves in somewhat non-standard way when it comes to -.



          POSIX says:




          Guideline 10:

          The first -- argument that is not an option-argument should be accepted as a delimiter indicating the end of options. Any following arguments should be treated as operands, even if they begin with the - character.



          […]



          Guideline 13:

          For utilities that use operands to represent files to be opened for either reading or writing, the - operand should be used to mean only standard input (or standard output when it is clear from context that an output file is being specified) or a file named -.




          And




          Where a utility described in the Shell and Utilities volume of POSIX.1-2017 as conforming to these guidelines is required to accept, or not to accept, the operand - to mean standard input or output, this usage is explained in the OPERANDS section. Otherwise, if such a utility uses operands to represent files, it is implementation-defined whether the operand - stands for standard input (or standard output), or for a file named -.




          But then man 1 bash reads:




          A -- signals the end of options and disables further option processing. Any arguments after the -- are treated as filenames and arguments. An argument of - is equivalent to --.




          So for Bash - means neither standard input nor a file, hence somewhat non-standard.



          Now your particular case:




          curl -sL https://rpm.nodesource.com/setup_6.x | sudo -E bash -



          I suspect the author of this command may not realize - is equivalent to -- in this case. I suspect the author wanted to make sure bash will read from its standard input, they expected - to work according to the guideline 13.



          But even if it worked according to the guideline, - would be unnecessary here because bash detects when its standard input is a pipe and acts accordingly (unless -c is given etc.).



          Yet - doesn't work according to the guideline, it works like --. Still -- is unnecessary here because there are no arguments after it.



          In my opinion the last - changes nothing. The command would work without it.



          To see how -- and - can be useful in general, study the example below.





          cat in my Kubuntu obeys both guidelines and I will use it to demonstrate usefulness of - and --.



          Let a file named foo exist. This will print the file:



          cat foo


          Let a file named --help exist. This won't print the file:



          cat --help


          But this will print the file named --help:



          cat -- --help


          This will concatenate the file named --help with whatever comes from the standard input:



          cat -- --help -


          It seems you don't really need --, because you can always pass ./--help which will be interpreted as a file for sure. But consider



          cat "$file"


          when you don't know beforehand what the content of the variable is. You cannot just prepend ./ to it, because it may be an absolute path and ./ would break it. On the other hand it may be a file named --help (because why not?). In this case -- is very useful; this is a lot more robust command:



          cat -- "$file"





          share|improve this answer























          • Thank your for your answer , is there any examples that can help explain it more in bash context ?
            – Omar BISTAMI
            50 mins ago










          • @OmarBISTAMI I have expanded my answer.
            – Kamil Maciorowski
            13 mins ago
















          1














          Bash behaves in somewhat non-standard way when it comes to -.



          POSIX says:




          Guideline 10:

          The first -- argument that is not an option-argument should be accepted as a delimiter indicating the end of options. Any following arguments should be treated as operands, even if they begin with the - character.



          […]



          Guideline 13:

          For utilities that use operands to represent files to be opened for either reading or writing, the - operand should be used to mean only standard input (or standard output when it is clear from context that an output file is being specified) or a file named -.




          And




          Where a utility described in the Shell and Utilities volume of POSIX.1-2017 as conforming to these guidelines is required to accept, or not to accept, the operand - to mean standard input or output, this usage is explained in the OPERANDS section. Otherwise, if such a utility uses operands to represent files, it is implementation-defined whether the operand - stands for standard input (or standard output), or for a file named -.




          But then man 1 bash reads:




          A -- signals the end of options and disables further option processing. Any arguments after the -- are treated as filenames and arguments. An argument of - is equivalent to --.




          So for Bash - means neither standard input nor a file, hence somewhat non-standard.



          Now your particular case:




          curl -sL https://rpm.nodesource.com/setup_6.x | sudo -E bash -



          I suspect the author of this command may not realize - is equivalent to -- in this case. I suspect the author wanted to make sure bash will read from its standard input, they expected - to work according to the guideline 13.



          But even if it worked according to the guideline, - would be unnecessary here because bash detects when its standard input is a pipe and acts accordingly (unless -c is given etc.).



          Yet - doesn't work according to the guideline, it works like --. Still -- is unnecessary here because there are no arguments after it.



          In my opinion the last - changes nothing. The command would work without it.



          To see how -- and - can be useful in general, study the example below.





          cat in my Kubuntu obeys both guidelines and I will use it to demonstrate usefulness of - and --.



          Let a file named foo exist. This will print the file:



          cat foo


          Let a file named --help exist. This won't print the file:



          cat --help


          But this will print the file named --help:



          cat -- --help


          This will concatenate the file named --help with whatever comes from the standard input:



          cat -- --help -


          It seems you don't really need --, because you can always pass ./--help which will be interpreted as a file for sure. But consider



          cat "$file"


          when you don't know beforehand what the content of the variable is. You cannot just prepend ./ to it, because it may be an absolute path and ./ would break it. On the other hand it may be a file named --help (because why not?). In this case -- is very useful; this is a lot more robust command:



          cat -- "$file"





          share|improve this answer























          • Thank your for your answer , is there any examples that can help explain it more in bash context ?
            – Omar BISTAMI
            50 mins ago










          • @OmarBISTAMI I have expanded my answer.
            – Kamil Maciorowski
            13 mins ago














          1












          1








          1






          Bash behaves in somewhat non-standard way when it comes to -.



          POSIX says:




          Guideline 10:

          The first -- argument that is not an option-argument should be accepted as a delimiter indicating the end of options. Any following arguments should be treated as operands, even if they begin with the - character.



          […]



          Guideline 13:

          For utilities that use operands to represent files to be opened for either reading or writing, the - operand should be used to mean only standard input (or standard output when it is clear from context that an output file is being specified) or a file named -.




          And




          Where a utility described in the Shell and Utilities volume of POSIX.1-2017 as conforming to these guidelines is required to accept, or not to accept, the operand - to mean standard input or output, this usage is explained in the OPERANDS section. Otherwise, if such a utility uses operands to represent files, it is implementation-defined whether the operand - stands for standard input (or standard output), or for a file named -.




          But then man 1 bash reads:




          A -- signals the end of options and disables further option processing. Any arguments after the -- are treated as filenames and arguments. An argument of - is equivalent to --.




          So for Bash - means neither standard input nor a file, hence somewhat non-standard.



          Now your particular case:




          curl -sL https://rpm.nodesource.com/setup_6.x | sudo -E bash -



          I suspect the author of this command may not realize - is equivalent to -- in this case. I suspect the author wanted to make sure bash will read from its standard input, they expected - to work according to the guideline 13.



          But even if it worked according to the guideline, - would be unnecessary here because bash detects when its standard input is a pipe and acts accordingly (unless -c is given etc.).



          Yet - doesn't work according to the guideline, it works like --. Still -- is unnecessary here because there are no arguments after it.



          In my opinion the last - changes nothing. The command would work without it.



          To see how -- and - can be useful in general, study the example below.





          cat in my Kubuntu obeys both guidelines and I will use it to demonstrate usefulness of - and --.



          Let a file named foo exist. This will print the file:



          cat foo


          Let a file named --help exist. This won't print the file:



          cat --help


          But this will print the file named --help:



          cat -- --help


          This will concatenate the file named --help with whatever comes from the standard input:



          cat -- --help -


          It seems you don't really need --, because you can always pass ./--help which will be interpreted as a file for sure. But consider



          cat "$file"


          when you don't know beforehand what the content of the variable is. You cannot just prepend ./ to it, because it may be an absolute path and ./ would break it. On the other hand it may be a file named --help (because why not?). In this case -- is very useful; this is a lot more robust command:



          cat -- "$file"





          share|improve this answer














          Bash behaves in somewhat non-standard way when it comes to -.



          POSIX says:




          Guideline 10:

          The first -- argument that is not an option-argument should be accepted as a delimiter indicating the end of options. Any following arguments should be treated as operands, even if they begin with the - character.



          […]



          Guideline 13:

          For utilities that use operands to represent files to be opened for either reading or writing, the - operand should be used to mean only standard input (or standard output when it is clear from context that an output file is being specified) or a file named -.




          And




          Where a utility described in the Shell and Utilities volume of POSIX.1-2017 as conforming to these guidelines is required to accept, or not to accept, the operand - to mean standard input or output, this usage is explained in the OPERANDS section. Otherwise, if such a utility uses operands to represent files, it is implementation-defined whether the operand - stands for standard input (or standard output), or for a file named -.




          But then man 1 bash reads:




          A -- signals the end of options and disables further option processing. Any arguments after the -- are treated as filenames and arguments. An argument of - is equivalent to --.




          So for Bash - means neither standard input nor a file, hence somewhat non-standard.



          Now your particular case:




          curl -sL https://rpm.nodesource.com/setup_6.x | sudo -E bash -



          I suspect the author of this command may not realize - is equivalent to -- in this case. I suspect the author wanted to make sure bash will read from its standard input, they expected - to work according to the guideline 13.



          But even if it worked according to the guideline, - would be unnecessary here because bash detects when its standard input is a pipe and acts accordingly (unless -c is given etc.).



          Yet - doesn't work according to the guideline, it works like --. Still -- is unnecessary here because there are no arguments after it.



          In my opinion the last - changes nothing. The command would work without it.



          To see how -- and - can be useful in general, study the example below.





          cat in my Kubuntu obeys both guidelines and I will use it to demonstrate usefulness of - and --.



          Let a file named foo exist. This will print the file:



          cat foo


          Let a file named --help exist. This won't print the file:



          cat --help


          But this will print the file named --help:



          cat -- --help


          This will concatenate the file named --help with whatever comes from the standard input:



          cat -- --help -


          It seems you don't really need --, because you can always pass ./--help which will be interpreted as a file for sure. But consider



          cat "$file"


          when you don't know beforehand what the content of the variable is. You cannot just prepend ./ to it, because it may be an absolute path and ./ would break it. On the other hand it may be a file named --help (because why not?). In this case -- is very useful; this is a lot more robust command:



          cat -- "$file"






          share|improve this answer














          share|improve this answer



          share|improve this answer








          edited 14 mins ago

























          answered 53 mins ago









          Kamil Maciorowski

          24.1k155176




          24.1k155176












          • Thank your for your answer , is there any examples that can help explain it more in bash context ?
            – Omar BISTAMI
            50 mins ago










          • @OmarBISTAMI I have expanded my answer.
            – Kamil Maciorowski
            13 mins ago


















          • Thank your for your answer , is there any examples that can help explain it more in bash context ?
            – Omar BISTAMI
            50 mins ago










          • @OmarBISTAMI I have expanded my answer.
            – Kamil Maciorowski
            13 mins ago
















          Thank your for your answer , is there any examples that can help explain it more in bash context ?
          – Omar BISTAMI
          50 mins ago




          Thank your for your answer , is there any examples that can help explain it more in bash context ?
          – Omar BISTAMI
          50 mins ago












          @OmarBISTAMI I have expanded my answer.
          – Kamil Maciorowski
          13 mins ago




          @OmarBISTAMI I have expanded my answer.
          – Kamil Maciorowski
          13 mins ago


















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