What does the last “-” (hyphen) mean in options of `bash`?












15














In this tutorial we need to execute the following command:



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


What does the last - (hyphen) after bash mean?



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










share|improve this question




















  • 5




    Same question on Ask Ubuntu — with the same example!
    – 200_success
    Dec 30 '18 at 3:47






  • 2




    Downloading stuff from the network and piping it directly into sudo bash sounds really scary. Try looking for a tutorial that doesn't encourage such practices.
    – Henning Makholm
    Dec 30 '18 at 16:43










  • Its the npm tutorial, but i do agree with you...
    – Omar BISTAMI
    Dec 30 '18 at 18:01








  • 1




    If you need to search for things with symbols in them, try symbolhound.com.
    – Joe
    Jan 1 at 5:04
















15














In this tutorial we need to execute the following command:



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


What does the last - (hyphen) after bash mean?



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










share|improve this question




















  • 5




    Same question on Ask Ubuntu — with the same example!
    – 200_success
    Dec 30 '18 at 3:47






  • 2




    Downloading stuff from the network and piping it directly into sudo bash sounds really scary. Try looking for a tutorial that doesn't encourage such practices.
    – Henning Makholm
    Dec 30 '18 at 16:43










  • Its the npm tutorial, but i do agree with you...
    – Omar BISTAMI
    Dec 30 '18 at 18:01








  • 1




    If you need to search for things with symbols in them, try symbolhound.com.
    – Joe
    Jan 1 at 5:04














15












15








15


3





In this tutorial we need to execute the following command:



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


What does the last - (hyphen) after bash mean?



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










share|improve this question















In this tutorial we need to execute the following command:



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


What does the last - (hyphen) after bash mean?



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







linux command-line bash pipe






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Dec 28 '18 at 23:33









Kamil Maciorowski

24.6k155277




24.6k155277










asked Dec 28 '18 at 22:03









Omar BISTAMI

927




927








  • 5




    Same question on Ask Ubuntu — with the same example!
    – 200_success
    Dec 30 '18 at 3:47






  • 2




    Downloading stuff from the network and piping it directly into sudo bash sounds really scary. Try looking for a tutorial that doesn't encourage such practices.
    – Henning Makholm
    Dec 30 '18 at 16:43










  • Its the npm tutorial, but i do agree with you...
    – Omar BISTAMI
    Dec 30 '18 at 18:01








  • 1




    If you need to search for things with symbols in them, try symbolhound.com.
    – Joe
    Jan 1 at 5:04














  • 5




    Same question on Ask Ubuntu — with the same example!
    – 200_success
    Dec 30 '18 at 3:47






  • 2




    Downloading stuff from the network and piping it directly into sudo bash sounds really scary. Try looking for a tutorial that doesn't encourage such practices.
    – Henning Makholm
    Dec 30 '18 at 16:43










  • Its the npm tutorial, but i do agree with you...
    – Omar BISTAMI
    Dec 30 '18 at 18:01








  • 1




    If you need to search for things with symbols in them, try symbolhound.com.
    – Joe
    Jan 1 at 5:04








5




5




Same question on Ask Ubuntu — with the same example!
– 200_success
Dec 30 '18 at 3:47




Same question on Ask Ubuntu — with the same example!
– 200_success
Dec 30 '18 at 3:47




2




2




Downloading stuff from the network and piping it directly into sudo bash sounds really scary. Try looking for a tutorial that doesn't encourage such practices.
– Henning Makholm
Dec 30 '18 at 16:43




Downloading stuff from the network and piping it directly into sudo bash sounds really scary. Try looking for a tutorial that doesn't encourage such practices.
– Henning Makholm
Dec 30 '18 at 16:43












Its the npm tutorial, but i do agree with you...
– Omar BISTAMI
Dec 30 '18 at 18:01






Its the npm tutorial, but i do agree with you...
– Omar BISTAMI
Dec 30 '18 at 18:01






1




1




If you need to search for things with symbols in them, try symbolhound.com.
– Joe
Jan 1 at 5:04




If you need to search for things with symbols in them, try symbolhound.com.
– Joe
Jan 1 at 5:04










2 Answers
2






active

oldest

votes


















31














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































    6














    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
      Dec 28 '18 at 22:27






    • 1




      It's really to allow for a script whose name begins -, an unlikely requirement, but -/-- makes it possible.
      – AFH
      Dec 28 '18 at 23:17










    • Thank you for your answer.
      – Omar BISTAMI
      Dec 29 '18 at 19:15






    • 1




      @OmarBISTAMI Quoting a command will affect how the shell expands it, but won't affect any of the arguments which follow it. If you extend the quotes around legitimate arguments, then they become part of the command name which isn't what you want either. There are some commands which take file names as arguments, but don't use standard input by default. A contrived example allows you to sandwich input (from a terminal or a pipe) between two files. cat file1 - file2 > file3.
      – Joe
      Jan 1 at 5:31











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    2 Answers
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    2 Answers
    2






    active

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    active

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    active

    oldest

    votes









    31














    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




























      31














      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


























        31












        31








        31






        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 Dec 28 '18 at 23:09

























        answered Dec 28 '18 at 22:30









        Kamil Maciorowski

        24.6k155277




        24.6k155277

























            6














            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
              Dec 28 '18 at 22:27






            • 1




              It's really to allow for a script whose name begins -, an unlikely requirement, but -/-- makes it possible.
              – AFH
              Dec 28 '18 at 23:17










            • Thank you for your answer.
              – Omar BISTAMI
              Dec 29 '18 at 19:15






            • 1




              @OmarBISTAMI Quoting a command will affect how the shell expands it, but won't affect any of the arguments which follow it. If you extend the quotes around legitimate arguments, then they become part of the command name which isn't what you want either. There are some commands which take file names as arguments, but don't use standard input by default. A contrived example allows you to sandwich input (from a terminal or a pipe) between two files. cat file1 - file2 > file3.
              – Joe
              Jan 1 at 5:31
















            6














            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
              Dec 28 '18 at 22:27






            • 1




              It's really to allow for a script whose name begins -, an unlikely requirement, but -/-- makes it possible.
              – AFH
              Dec 28 '18 at 23:17










            • Thank you for your answer.
              – Omar BISTAMI
              Dec 29 '18 at 19:15






            • 1




              @OmarBISTAMI Quoting a command will affect how the shell expands it, but won't affect any of the arguments which follow it. If you extend the quotes around legitimate arguments, then they become part of the command name which isn't what you want either. There are some commands which take file names as arguments, but don't use standard input by default. A contrived example allows you to sandwich input (from a terminal or a pipe) between two files. cat file1 - file2 > file3.
              – Joe
              Jan 1 at 5:31














            6












            6








            6






            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 Dec 28 '18 at 22:21









            AFH

            14k31938




            14k31938












            • 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
              Dec 28 '18 at 22:27






            • 1




              It's really to allow for a script whose name begins -, an unlikely requirement, but -/-- makes it possible.
              – AFH
              Dec 28 '18 at 23:17










            • Thank you for your answer.
              – Omar BISTAMI
              Dec 29 '18 at 19:15






            • 1




              @OmarBISTAMI Quoting a command will affect how the shell expands it, but won't affect any of the arguments which follow it. If you extend the quotes around legitimate arguments, then they become part of the command name which isn't what you want either. There are some commands which take file names as arguments, but don't use standard input by default. A contrived example allows you to sandwich input (from a terminal or a pipe) between two files. cat file1 - file2 > file3.
              – Joe
              Jan 1 at 5:31


















            • 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
              Dec 28 '18 at 22:27






            • 1




              It's really to allow for a script whose name begins -, an unlikely requirement, but -/-- makes it possible.
              – AFH
              Dec 28 '18 at 23:17










            • Thank you for your answer.
              – Omar BISTAMI
              Dec 29 '18 at 19:15






            • 1




              @OmarBISTAMI Quoting a command will affect how the shell expands it, but won't affect any of the arguments which follow it. If you extend the quotes around legitimate arguments, then they become part of the command name which isn't what you want either. There are some commands which take file names as arguments, but don't use standard input by default. A contrived example allows you to sandwich input (from a terminal or a pipe) between two files. cat file1 - file2 > file3.
              – Joe
              Jan 1 at 5:31
















            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
            Dec 28 '18 at 22:27




            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
            Dec 28 '18 at 22:27




            1




            1




            It's really to allow for a script whose name begins -, an unlikely requirement, but -/-- makes it possible.
            – AFH
            Dec 28 '18 at 23:17




            It's really to allow for a script whose name begins -, an unlikely requirement, but -/-- makes it possible.
            – AFH
            Dec 28 '18 at 23:17












            Thank you for your answer.
            – Omar BISTAMI
            Dec 29 '18 at 19:15




            Thank you for your answer.
            – Omar BISTAMI
            Dec 29 '18 at 19:15




            1




            1




            @OmarBISTAMI Quoting a command will affect how the shell expands it, but won't affect any of the arguments which follow it. If you extend the quotes around legitimate arguments, then they become part of the command name which isn't what you want either. There are some commands which take file names as arguments, but don't use standard input by default. A contrived example allows you to sandwich input (from a terminal or a pipe) between two files. cat file1 - file2 > file3.
            – Joe
            Jan 1 at 5:31




            @OmarBISTAMI Quoting a command will affect how the shell expands it, but won't affect any of the arguments which follow it. If you extend the quotes around legitimate arguments, then they become part of the command name which isn't what you want either. There are some commands which take file names as arguments, but don't use standard input by default. A contrived example allows you to sandwich input (from a terminal or a pipe) between two files. cat file1 - file2 > file3.
            – Joe
            Jan 1 at 5:31


















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