Process substitution inside a subshell to set a variable












1















I'm trying to run a script remotely and use its standard output to populate a variable. I'm doing this to avoid temporary files.



Here's the pattern I'm trying:



var=$(bash <(curl -fsSkL http://remote/file.sh))

echo "var=${var}"


I'm testing this pattern without curl using cat:



var=$(bash <(cat ./local/file.sh))

echo "var=${var}"


This should be the same as far as syntax is concerned. ./local/file.sh contains echo hello, so I would expect var to contain the value hello, but alas, executing the above results in the following:



test.sh: command substitution: line 4: syntax error near unexpected token `('

test.sh: command substitution: line 4: `bash <(cat ./local/file.sh)'

var=


How can I accomplish my goal without using temporary files?






bash variable subshell process-substitution







share|improve this question



























    1















    I'm trying to run a script remotely and use its standard output to populate a variable. I'm doing this to avoid temporary files.



    Here's the pattern I'm trying:



    var=$(bash <(curl -fsSkL http://remote/file.sh))
    
echo "var=${var}"


    I'm testing this pattern without curl using cat:



    var=$(bash <(cat ./local/file.sh))
    
echo "var=${var}"


    This should be the same as far as syntax is concerned. ./local/file.sh contains echo hello, so I would expect var to contain the value hello, but alas, executing the above results in the following:



    test.sh: command substitution: line 4: syntax error near unexpected token `('
    
test.sh: command substitution: line 4: `bash <(cat ./local/file.sh)'
    
var=


    How can I accomplish my goal without using temporary files?






    bash variable subshell process-substitution







    share|improve this question

























      1












      1








      1








      I'm trying to run a script remotely and use its standard output to populate a variable. I'm doing this to avoid temporary files.



      Here's the pattern I'm trying:



      var=$(bash <(curl -fsSkL http://remote/file.sh))
      
echo "var=${var}"


      I'm testing this pattern without curl using cat:



      var=$(bash <(cat ./local/file.sh))
      
echo "var=${var}"


      This should be the same as far as syntax is concerned. ./local/file.sh contains echo hello, so I would expect var to contain the value hello, but alas, executing the above results in the following:



      test.sh: command substitution: line 4: syntax error near unexpected token `('
      
test.sh: command substitution: line 4: `bash <(cat ./local/file.sh)'
      
var=


      How can I accomplish my goal without using temporary files?






      bash variable subshell process-substitution







      share|improve this question














      I'm trying to run a script remotely and use its standard output to populate a variable. I'm doing this to avoid temporary files.



      Here's the pattern I'm trying:



      var=$(bash <(curl -fsSkL http://remote/file.sh))
      
echo "var=${var}"


      I'm testing this pattern without curl using cat:



      var=$(bash <(cat ./local/file.sh))
      
echo "var=${var}"


      This should be the same as far as syntax is concerned. ./local/file.sh contains echo hello, so I would expect var to contain the value hello, but alas, executing the above results in the following:



      test.sh: command substitution: line 4: syntax error near unexpected token `('
      
test.sh: command substitution: line 4: `bash <(cat ./local/file.sh)'
      
var=


      How can I accomplish my goal without using temporary files?






      bash variable subshell process-substitution




      bash variable subshell process-substitution






      share|improve this question













      share|improve this question











      share|improve this question




      share|improve this question










      asked 2 hours ago









      Sean AllredSean Allred

      8143816




      8143816






















          1 Answer
          1






          active

          oldest

          votes


















          7














          Those are the errors you get when trying to perform a process substitution in bash when the shell is running in POSIX mode. The bash shell does not support process substitutions in POSIX mode.



          bash will run in POSIX mode when either





          1. set -o posix has been used, or

          2. the shell is being invoked as sh.


          My hunch is that you have a script, test.sh, that you are running with sh test.sh or that has a #!/bin/sh hashbang line, and that your sh happens to be bash. Another possibility is that the script does not have #!-line at all, and it is being invoked by bash-as-sh in some other way.



          Instead, see to that your test.sh script is being invoked by bash.



          Example:



          $ cat script.sh
          
echo hello
          





          $ cat test.sh
          
var=$(bash <( cat script.sh ))
          
printf 'var="%s"n' "$var"
          





          $ bash -o posix test.sh
          
test.sh: command substitution: line 2: syntax error near unexpected token `('
          
test.sh: command substitution: line 2: `bash <( cat script.sh ))'
          
var=""
          





          $ bash test.sh
          
var="hello"





          share|improve this answer


























          • Yep, I was invoking as sh – I didn't realize POSIX didn't support these features. Thanks! As a short follow-up, is there a way to ensure bash-as-sh (if invoked incorrectly) will not run in POSIX mode? In other words, what is the converse of set -o posix?

            – Sean Allred
            2 hours ago













          • @SeanAllred Since sh does not need to be bash and indeed, sh is often not bash, you should rather make sure that your scripts are executed by bash. A simple way to do this is to make the script executable and add #!/bin/bash as the first line in the script (adapt that to the correct path to the bash executable on your system). Then run it without specifying an explicit interpreter.

            – Kusalananda
            2 hours ago













          • Yes I can ensure that my tools use bash, via shebang or otherwise, but I'm concerned that those using the script I'm making will not invoke it correctly. Is there no converse to option posix? (Working under the assumption now that there's no means by which to accomplish my goal – without temp files – with only POSIX features.)

            – Sean Allred
            2 hours ago













          • @SeanAllred If you are sharing your script with others, it extra important to have a #!/bin/bash line! The sh shell on many Linux machines is dash, and that shell would not understand process substitutions however hard you tried. On my machine (OpenBSD), sh is pdksh, and that also can't be made to run process substitutions. What you are asking is the equivalent to how to make someone invoke a Python script with the correct Python interpreter even when they run it with a Ruby interpreter. If the user picks the wrong interpreter, then the documentation was not good enough.

            – Kusalananda
            2 hours ago













          • Well of course there will be a shebang line :-) but fair enough – I guess there's a limit to how much you can protect others from themselves. Thanks for your help!

            – Sean Allred
            2 hours ago













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          1 Answer
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          1 Answer
          1






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          7














          Those are the errors you get when trying to perform a process substitution in bash when the shell is running in POSIX mode. The bash shell does not support process substitutions in POSIX mode.



          bash will run in POSIX mode when either





          1. set -o posix has been used, or

          2. the shell is being invoked as sh.


          My hunch is that you have a script, test.sh, that you are running with sh test.sh or that has a #!/bin/sh hashbang line, and that your sh happens to be bash. Another possibility is that the script does not have #!-line at all, and it is being invoked by bash-as-sh in some other way.



          Instead, see to that your test.sh script is being invoked by bash.



          Example:



          $ cat script.sh
          
echo hello
          





          $ cat test.sh
          
var=$(bash <( cat script.sh ))
          
printf 'var="%s"n' "$var"
          





          $ bash -o posix test.sh
          
test.sh: command substitution: line 2: syntax error near unexpected token `('
          
test.sh: command substitution: line 2: `bash <( cat script.sh ))'
          
var=""
          





          $ bash test.sh
          
var="hello"





          share|improve this answer


























          • Yep, I was invoking as sh – I didn't realize POSIX didn't support these features. Thanks! As a short follow-up, is there a way to ensure bash-as-sh (if invoked incorrectly) will not run in POSIX mode? In other words, what is the converse of set -o posix?

            – Sean Allred
            2 hours ago













          • @SeanAllred Since sh does not need to be bash and indeed, sh is often not bash, you should rather make sure that your scripts are executed by bash. A simple way to do this is to make the script executable and add #!/bin/bash as the first line in the script (adapt that to the correct path to the bash executable on your system). Then run it without specifying an explicit interpreter.

            – Kusalananda
            2 hours ago













          • Yes I can ensure that my tools use bash, via shebang or otherwise, but I'm concerned that those using the script I'm making will not invoke it correctly. Is there no converse to option posix? (Working under the assumption now that there's no means by which to accomplish my goal – without temp files – with only POSIX features.)

            – Sean Allred
            2 hours ago













          • @SeanAllred If you are sharing your script with others, it extra important to have a #!/bin/bash line! The sh shell on many Linux machines is dash, and that shell would not understand process substitutions however hard you tried. On my machine (OpenBSD), sh is pdksh, and that also can't be made to run process substitutions. What you are asking is the equivalent to how to make someone invoke a Python script with the correct Python interpreter even when they run it with a Ruby interpreter. If the user picks the wrong interpreter, then the documentation was not good enough.

            – Kusalananda
            2 hours ago













          • Well of course there will be a shebang line :-) but fair enough – I guess there's a limit to how much you can protect others from themselves. Thanks for your help!

            – Sean Allred
            2 hours ago


















          7














          Those are the errors you get when trying to perform a process substitution in bash when the shell is running in POSIX mode. The bash shell does not support process substitutions in POSIX mode.



          bash will run in POSIX mode when either





          1. set -o posix has been used, or

          2. the shell is being invoked as sh.


          My hunch is that you have a script, test.sh, that you are running with sh test.sh or that has a #!/bin/sh hashbang line, and that your sh happens to be bash. Another possibility is that the script does not have #!-line at all, and it is being invoked by bash-as-sh in some other way.



          Instead, see to that your test.sh script is being invoked by bash.



          Example:



          $ cat script.sh
          
echo hello
          





          $ cat test.sh
          
var=$(bash <( cat script.sh ))
          
printf 'var="%s"n' "$var"
          





          $ bash -o posix test.sh
          
test.sh: command substitution: line 2: syntax error near unexpected token `('
          
test.sh: command substitution: line 2: `bash <( cat script.sh ))'
          
var=""
          





          $ bash test.sh
          
var="hello"





          share|improve this answer


























          • Yep, I was invoking as sh – I didn't realize POSIX didn't support these features. Thanks! As a short follow-up, is there a way to ensure bash-as-sh (if invoked incorrectly) will not run in POSIX mode? In other words, what is the converse of set -o posix?

            – Sean Allred
            2 hours ago













          • @SeanAllred Since sh does not need to be bash and indeed, sh is often not bash, you should rather make sure that your scripts are executed by bash. A simple way to do this is to make the script executable and add #!/bin/bash as the first line in the script (adapt that to the correct path to the bash executable on your system). Then run it without specifying an explicit interpreter.

            – Kusalananda
            2 hours ago













          • Yes I can ensure that my tools use bash, via shebang or otherwise, but I'm concerned that those using the script I'm making will not invoke it correctly. Is there no converse to option posix? (Working under the assumption now that there's no means by which to accomplish my goal – without temp files – with only POSIX features.)

            – Sean Allred
            2 hours ago













          • @SeanAllred If you are sharing your script with others, it extra important to have a #!/bin/bash line! The sh shell on many Linux machines is dash, and that shell would not understand process substitutions however hard you tried. On my machine (OpenBSD), sh is pdksh, and that also can't be made to run process substitutions. What you are asking is the equivalent to how to make someone invoke a Python script with the correct Python interpreter even when they run it with a Ruby interpreter. If the user picks the wrong interpreter, then the documentation was not good enough.

            – Kusalananda
            2 hours ago













          • Well of course there will be a shebang line :-) but fair enough – I guess there's a limit to how much you can protect others from themselves. Thanks for your help!

            – Sean Allred
            2 hours ago
















          7












          7








          7







          Those are the errors you get when trying to perform a process substitution in bash when the shell is running in POSIX mode. The bash shell does not support process substitutions in POSIX mode.



          bash will run in POSIX mode when either





          1. set -o posix has been used, or

          2. the shell is being invoked as sh.


          My hunch is that you have a script, test.sh, that you are running with sh test.sh or that has a #!/bin/sh hashbang line, and that your sh happens to be bash. Another possibility is that the script does not have #!-line at all, and it is being invoked by bash-as-sh in some other way.



          Instead, see to that your test.sh script is being invoked by bash.



          Example:



          $ cat script.sh
          
echo hello
          





          $ cat test.sh
          
var=$(bash <( cat script.sh ))
          
printf 'var="%s"n' "$var"
          





          $ bash -o posix test.sh
          
test.sh: command substitution: line 2: syntax error near unexpected token `('
          
test.sh: command substitution: line 2: `bash <( cat script.sh ))'
          
var=""
          





          $ bash test.sh
          
var="hello"





          share|improve this answer















          Those are the errors you get when trying to perform a process substitution in bash when the shell is running in POSIX mode. The bash shell does not support process substitutions in POSIX mode.



          bash will run in POSIX mode when either





          1. set -o posix has been used, or

          2. the shell is being invoked as sh.


          My hunch is that you have a script, test.sh, that you are running with sh test.sh or that has a #!/bin/sh hashbang line, and that your sh happens to be bash. Another possibility is that the script does not have #!-line at all, and it is being invoked by bash-as-sh in some other way.



          Instead, see to that your test.sh script is being invoked by bash.



          Example:



          $ cat script.sh
          
echo hello
          





          $ cat test.sh
          
var=$(bash <( cat script.sh ))
          
printf 'var="%s"n' "$var"
          





          $ bash -o posix test.sh
          
test.sh: command substitution: line 2: syntax error near unexpected token `('
          
test.sh: command substitution: line 2: `bash <( cat script.sh ))'
          
var=""
          





          $ bash test.sh
          
var="hello"






          share|improve this answer














          share|improve this answer



          share|improve this answer








          edited 2 hours ago

























          answered 2 hours ago









          KusalanandaKusalananda

          132k17250411




          132k17250411













          • Yep, I was invoking as sh – I didn't realize POSIX didn't support these features. Thanks! As a short follow-up, is there a way to ensure bash-as-sh (if invoked incorrectly) will not run in POSIX mode? In other words, what is the converse of set -o posix?

            – Sean Allred
            2 hours ago













          • @SeanAllred Since sh does not need to be bash and indeed, sh is often not bash, you should rather make sure that your scripts are executed by bash. A simple way to do this is to make the script executable and add #!/bin/bash as the first line in the script (adapt that to the correct path to the bash executable on your system). Then run it without specifying an explicit interpreter.

            – Kusalananda
            2 hours ago













          • Yes I can ensure that my tools use bash, via shebang or otherwise, but I'm concerned that those using the script I'm making will not invoke it correctly. Is there no converse to option posix? (Working under the assumption now that there's no means by which to accomplish my goal – without temp files – with only POSIX features.)

            – Sean Allred
            2 hours ago













          • @SeanAllred If you are sharing your script with others, it extra important to have a #!/bin/bash line! The sh shell on many Linux machines is dash, and that shell would not understand process substitutions however hard you tried. On my machine (OpenBSD), sh is pdksh, and that also can't be made to run process substitutions. What you are asking is the equivalent to how to make someone invoke a Python script with the correct Python interpreter even when they run it with a Ruby interpreter. If the user picks the wrong interpreter, then the documentation was not good enough.

            – Kusalananda
            2 hours ago













          • Well of course there will be a shebang line :-) but fair enough – I guess there's a limit to how much you can protect others from themselves. Thanks for your help!

            – Sean Allred
            2 hours ago





















          • Yep, I was invoking as sh – I didn't realize POSIX didn't support these features. Thanks! As a short follow-up, is there a way to ensure bash-as-sh (if invoked incorrectly) will not run in POSIX mode? In other words, what is the converse of set -o posix?

            – Sean Allred
            2 hours ago













          • @SeanAllred Since sh does not need to be bash and indeed, sh is often not bash, you should rather make sure that your scripts are executed by bash. A simple way to do this is to make the script executable and add #!/bin/bash as the first line in the script (adapt that to the correct path to the bash executable on your system). Then run it without specifying an explicit interpreter.

            – Kusalananda
            2 hours ago













          • Yes I can ensure that my tools use bash, via shebang or otherwise, but I'm concerned that those using the script I'm making will not invoke it correctly. Is there no converse to option posix? (Working under the assumption now that there's no means by which to accomplish my goal – without temp files – with only POSIX features.)

            – Sean Allred
            2 hours ago













          • @SeanAllred If you are sharing your script with others, it extra important to have a #!/bin/bash line! The sh shell on many Linux machines is dash, and that shell would not understand process substitutions however hard you tried. On my machine (OpenBSD), sh is pdksh, and that also can't be made to run process substitutions. What you are asking is the equivalent to how to make someone invoke a Python script with the correct Python interpreter even when they run it with a Ruby interpreter. If the user picks the wrong interpreter, then the documentation was not good enough.

            – Kusalananda
            2 hours ago













          • Well of course there will be a shebang line :-) but fair enough – I guess there's a limit to how much you can protect others from themselves. Thanks for your help!

            – Sean Allred
            2 hours ago



















          Yep, I was invoking as sh – I didn't realize POSIX didn't support these features. Thanks! As a short follow-up, is there a way to ensure bash-as-sh (if invoked incorrectly) will not run in POSIX mode? In other words, what is the converse of set -o posix?

          – Sean Allred
          2 hours ago







          Yep, I was invoking as sh – I didn't realize POSIX didn't support these features. Thanks! As a short follow-up, is there a way to ensure bash-as-sh (if invoked incorrectly) will not run in POSIX mode? In other words, what is the converse of set -o posix?

          – Sean Allred
          2 hours ago















          @SeanAllred Since sh does not need to be bash and indeed, sh is often not bash, you should rather make sure that your scripts are executed by bash. A simple way to do this is to make the script executable and add #!/bin/bash as the first line in the script (adapt that to the correct path to the bash executable on your system). Then run it without specifying an explicit interpreter.

          – Kusalananda
          2 hours ago







          @SeanAllred Since sh does not need to be bash and indeed, sh is often not bash, you should rather make sure that your scripts are executed by bash. A simple way to do this is to make the script executable and add #!/bin/bash as the first line in the script (adapt that to the correct path to the bash executable on your system). Then run it without specifying an explicit interpreter.

          – Kusalananda
          2 hours ago















          Yes I can ensure that my tools use bash, via shebang or otherwise, but I'm concerned that those using the script I'm making will not invoke it correctly. Is there no converse to option posix? (Working under the assumption now that there's no means by which to accomplish my goal – without temp files – with only POSIX features.)

          – Sean Allred
          2 hours ago







          Yes I can ensure that my tools use bash, via shebang or otherwise, but I'm concerned that those using the script I'm making will not invoke it correctly. Is there no converse to option posix? (Working under the assumption now that there's no means by which to accomplish my goal – without temp files – with only POSIX features.)

          – Sean Allred
          2 hours ago















          @SeanAllred If you are sharing your script with others, it extra important to have a #!/bin/bash line! The sh shell on many Linux machines is dash, and that shell would not understand process substitutions however hard you tried. On my machine (OpenBSD), sh is pdksh, and that also can't be made to run process substitutions. What you are asking is the equivalent to how to make someone invoke a Python script with the correct Python interpreter even when they run it with a Ruby interpreter. If the user picks the wrong interpreter, then the documentation was not good enough.

          – Kusalananda
          2 hours ago







          @SeanAllred If you are sharing your script with others, it extra important to have a #!/bin/bash line! The sh shell on many Linux machines is dash, and that shell would not understand process substitutions however hard you tried. On my machine (OpenBSD), sh is pdksh, and that also can't be made to run process substitutions. What you are asking is the equivalent to how to make someone invoke a Python script with the correct Python interpreter even when they run it with a Ruby interpreter. If the user picks the wrong interpreter, then the documentation was not good enough.

          – Kusalananda
          2 hours ago















          Well of course there will be a shebang line :-) but fair enough – I guess there's a limit to how much you can protect others from themselves. Thanks for your help!

          – Sean Allred
          2 hours ago







          Well of course there will be a shebang line :-) but fair enough – I guess there's a limit to how much you can protect others from themselves. Thanks for your help!

          – Sean Allred
          2 hours ago




















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