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How to determine if result of process substitution is a file path


Problem understanding 'hash' command in a .sh fileSome puzzles of how to close file descriptors on sh or bashHow do I test if a file does not exist using ZSH?Preform operation in bash only if a variable is less than a second variableHow do I pass a file path to a function instead of the contents of the file?read a file from Server path - bashConditional process substitutionstderr redirection by reading a fileHow does (echo 'text' ; cat file.txt ) > new file.txt actually work?How to use an escaped file-name string saved within a bash variable in a new shell command?






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1















If I do this:



echo <(cat)


I get:



/dev/fd/63


so say at the command line I have:



myapp -f <(cat)


when I run it I get this error:




You need to pass a file after the -f flag. The resolved file path was:
'/dev/fd/63'. This path did not appear to exist on the filesystem.




How can I determine if the result of the process substitution is an actual file (for validation purposes)? Here is my bash code which generated the error:



 if [[ -L "$file_path" ]]; then
 file_path="$(readlink "$file_path")";
 fi

 if [[ ! -f "$file_path" ]]; then
 echo "You need to pass a file after the -f flag. The resolved file path was: '$file_path'. This path did not appear to exist on the filesystem".;
 return 1;
 fi


if I get rid of my validation, code, I get this:




Could not open the following file for reading: /dev/fd/63 EBADF: bad
file descriptor, open '/dev/fd/63'




The node.js code I am using to read from the path is:



const fd = fs.openSync(file_path, 'r');
fs.read(fd, ...);





bash shell test read fd







share|improve this question





















  • 2





    [ -f file ] tests if file is a regular file, and the file created by a process substitution is a pipe, not a regular file. Also, on Linux, the target of a /dev/fd/n -> /proc/self/fd/n "symlink" is some informative string like type:[inum], not a path for pipes, sockets, etc.

    – mosvy
    8 hours ago












  • so much for everything is a file lulz..anyway I just want to be able to read from /dev/fd/x, is there some way to do that?

    – Alexander Mills
    7 hours ago






  • 1





    maybe you should post a complete example; node -e 'fs=require("fs"); fs.openSync(process.argv[1], "r")' <(cat /dev/null) works for me.

    – mosvy
    7 hours ago






  • 1





    But simply cat <(cat) will not work from console, because the cat inside the process substitution may run in a different process group than the foreground one, and cannot read from the tty.

    – mosvy
    7 hours ago






  • 1





    I don't have a MacOS system. But someone else may have, so post a complete reproducible testcase. FWIW, on BSD, /dev/fd/N are character special files (they aren't regular files, either). And on FreeBSD you should mount fdescfs if you want to access filedescs > 2 via /dev/fd/N.

    – mosvy
    7 hours ago


















1















If I do this:



echo <(cat)


I get:



/dev/fd/63


so say at the command line I have:



myapp -f <(cat)


when I run it I get this error:




You need to pass a file after the -f flag. The resolved file path was:
'/dev/fd/63'. This path did not appear to exist on the filesystem.




How can I determine if the result of the process substitution is an actual file (for validation purposes)? Here is my bash code which generated the error:



 if [[ -L "$file_path" ]]; then
 file_path="$(readlink "$file_path")";
 fi

 if [[ ! -f "$file_path" ]]; then
 echo "You need to pass a file after the -f flag. The resolved file path was: '$file_path'. This path did not appear to exist on the filesystem".;
 return 1;
 fi


if I get rid of my validation, code, I get this:




Could not open the following file for reading: /dev/fd/63 EBADF: bad
file descriptor, open '/dev/fd/63'




The node.js code I am using to read from the path is:



const fd = fs.openSync(file_path, 'r');
fs.read(fd, ...);





bash shell test read fd







share|improve this question





















  • 2





    [ -f file ] tests if file is a regular file, and the file created by a process substitution is a pipe, not a regular file. Also, on Linux, the target of a /dev/fd/n -> /proc/self/fd/n "symlink" is some informative string like type:[inum], not a path for pipes, sockets, etc.

    – mosvy
    8 hours ago












  • so much for everything is a file lulz..anyway I just want to be able to read from /dev/fd/x, is there some way to do that?

    – Alexander Mills
    7 hours ago






  • 1





    maybe you should post a complete example; node -e 'fs=require("fs"); fs.openSync(process.argv[1], "r")' <(cat /dev/null) works for me.

    – mosvy
    7 hours ago






  • 1





    But simply cat <(cat) will not work from console, because the cat inside the process substitution may run in a different process group than the foreground one, and cannot read from the tty.

    – mosvy
    7 hours ago






  • 1





    I don't have a MacOS system. But someone else may have, so post a complete reproducible testcase. FWIW, on BSD, /dev/fd/N are character special files (they aren't regular files, either). And on FreeBSD you should mount fdescfs if you want to access filedescs > 2 via /dev/fd/N.

    – mosvy
    7 hours ago














1












1








1








If I do this:



echo <(cat)


I get:



/dev/fd/63


so say at the command line I have:



myapp -f <(cat)


when I run it I get this error:




You need to pass a file after the -f flag. The resolved file path was:
'/dev/fd/63'. This path did not appear to exist on the filesystem.




How can I determine if the result of the process substitution is an actual file (for validation purposes)? Here is my bash code which generated the error:



 if [[ -L "$file_path" ]]; then
 file_path="$(readlink "$file_path")";
 fi

 if [[ ! -f "$file_path" ]]; then
 echo "You need to pass a file after the -f flag. The resolved file path was: '$file_path'. This path did not appear to exist on the filesystem".;
 return 1;
 fi


if I get rid of my validation, code, I get this:




Could not open the following file for reading: /dev/fd/63 EBADF: bad
file descriptor, open '/dev/fd/63'




The node.js code I am using to read from the path is:



const fd = fs.openSync(file_path, 'r');
fs.read(fd, ...);





bash shell test read fd







share|improve this question
















If I do this:



echo <(cat)


I get:



/dev/fd/63


so say at the command line I have:



myapp -f <(cat)


when I run it I get this error:




You need to pass a file after the -f flag. The resolved file path was:
'/dev/fd/63'. This path did not appear to exist on the filesystem.




How can I determine if the result of the process substitution is an actual file (for validation purposes)? Here is my bash code which generated the error:



 if [[ -L "$file_path" ]]; then
 file_path="$(readlink "$file_path")";
 fi

 if [[ ! -f "$file_path" ]]; then
 echo "You need to pass a file after the -f flag. The resolved file path was: '$file_path'. This path did not appear to exist on the filesystem".;
 return 1;
 fi


if I get rid of my validation, code, I get this:




Could not open the following file for reading: /dev/fd/63 EBADF: bad
file descriptor, open '/dev/fd/63'




The node.js code I am using to read from the path is:



const fd = fs.openSync(file_path, 'r');
fs.read(fd, ...);





bash shell test read fd




bash shell test read fd






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited 8 hours ago







Alexander Mills

















asked 8 hours ago









Alexander MillsAlexander Mills

2,4642 gold badges21 silver badges67 bronze badges




2,4642 gold badges21 silver badges67 bronze badges










  • 2





    [ -f file ] tests if file is a regular file, and the file created by a process substitution is a pipe, not a regular file. Also, on Linux, the target of a /dev/fd/n -> /proc/self/fd/n "symlink" is some informative string like type:[inum], not a path for pipes, sockets, etc.

    – mosvy
    8 hours ago












  • so much for everything is a file lulz..anyway I just want to be able to read from /dev/fd/x, is there some way to do that?

    – Alexander Mills
    7 hours ago






  • 1





    maybe you should post a complete example; node -e 'fs=require("fs"); fs.openSync(process.argv[1], "r")' <(cat /dev/null) works for me.

    – mosvy
    7 hours ago






  • 1





    But simply cat <(cat) will not work from console, because the cat inside the process substitution may run in a different process group than the foreground one, and cannot read from the tty.

    – mosvy
    7 hours ago






  • 1





    I don't have a MacOS system. But someone else may have, so post a complete reproducible testcase. FWIW, on BSD, /dev/fd/N are character special files (they aren't regular files, either). And on FreeBSD you should mount fdescfs if you want to access filedescs > 2 via /dev/fd/N.

    – mosvy
    7 hours ago













  • 2





    [ -f file ] tests if file is a regular file, and the file created by a process substitution is a pipe, not a regular file. Also, on Linux, the target of a /dev/fd/n -> /proc/self/fd/n "symlink" is some informative string like type:[inum], not a path for pipes, sockets, etc.

    – mosvy
    8 hours ago












  • so much for everything is a file lulz..anyway I just want to be able to read from /dev/fd/x, is there some way to do that?

    – Alexander Mills
    7 hours ago






  • 1





    maybe you should post a complete example; node -e 'fs=require("fs"); fs.openSync(process.argv[1], "r")' <(cat /dev/null) works for me.

    – mosvy
    7 hours ago






  • 1





    But simply cat <(cat) will not work from console, because the cat inside the process substitution may run in a different process group than the foreground one, and cannot read from the tty.

    – mosvy
    7 hours ago






  • 1





    I don't have a MacOS system. But someone else may have, so post a complete reproducible testcase. FWIW, on BSD, /dev/fd/N are character special files (they aren't regular files, either). And on FreeBSD you should mount fdescfs if you want to access filedescs > 2 via /dev/fd/N.

    – mosvy
    7 hours ago








2




2





[ -f file ] tests if file is a regular file, and the file created by a process substitution is a pipe, not a regular file. Also, on Linux, the target of a /dev/fd/n -> /proc/self/fd/n "symlink" is some informative string like type:[inum], not a path for pipes, sockets, etc.

– mosvy
8 hours ago






[ -f file ] tests if file is a regular file, and the file created by a process substitution is a pipe, not a regular file. Also, on Linux, the target of a /dev/fd/n -> /proc/self/fd/n "symlink" is some informative string like type:[inum], not a path for pipes, sockets, etc.

– mosvy
8 hours ago














so much for everything is a file lulz..anyway I just want to be able to read from /dev/fd/x, is there some way to do that?

– Alexander Mills
7 hours ago





so much for everything is a file lulz..anyway I just want to be able to read from /dev/fd/x, is there some way to do that?

– Alexander Mills
7 hours ago




1




1





maybe you should post a complete example; node -e 'fs=require("fs"); fs.openSync(process.argv[1], "r")' <(cat /dev/null) works for me.

– mosvy
7 hours ago





maybe you should post a complete example; node -e 'fs=require("fs"); fs.openSync(process.argv[1], "r")' <(cat /dev/null) works for me.

– mosvy
7 hours ago




1




1





But simply cat <(cat) will not work from console, because the cat inside the process substitution may run in a different process group than the foreground one, and cannot read from the tty.

– mosvy
7 hours ago





But simply cat <(cat) will not work from console, because the cat inside the process substitution may run in a different process group than the foreground one, and cannot read from the tty.

– mosvy
7 hours ago




1




1





I don't have a MacOS system. But someone else may have, so post a complete reproducible testcase. FWIW, on BSD, /dev/fd/N are character special files (they aren't regular files, either). And on FreeBSD you should mount fdescfs if you want to access filedescs > 2 via /dev/fd/N.

– mosvy
7 hours ago






I don't have a MacOS system. But someone else may have, so post a complete reproducible testcase. FWIW, on BSD, /dev/fd/N are character special files (they aren't regular files, either). And on FreeBSD you should mount fdescfs if you want to access filedescs > 2 via /dev/fd/N.

– mosvy
7 hours ago











1 Answer
1






active

oldest

votes


















3














To determine, in Bash, whether a string value is a path on your current system, use [[ -e "$path" ]]. This checks whether the path exists, and doesn't make any assumptions about the type of file it points to.






share|improve this answer

























  • this definitely helps, thanks

    – Alexander Mills
    7 hours ago













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1 Answer
1






active

oldest

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active

oldest

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active

oldest

votes









3














To determine, in Bash, whether a string value is a path on your current system, use [[ -e "$path" ]]. This checks whether the path exists, and doesn't make any assumptions about the type of file it points to.






share|improve this answer

























  • this definitely helps, thanks

    – Alexander Mills
    7 hours ago















3














To determine, in Bash, whether a string value is a path on your current system, use [[ -e "$path" ]]. This checks whether the path exists, and doesn't make any assumptions about the type of file it points to.






share|improve this answer

























  • this definitely helps, thanks

    – Alexander Mills
    7 hours ago













3












3








3







To determine, in Bash, whether a string value is a path on your current system, use [[ -e "$path" ]]. This checks whether the path exists, and doesn't make any assumptions about the type of file it points to.






share|improve this answer













To determine, in Bash, whether a string value is a path on your current system, use [[ -e "$path" ]]. This checks whether the path exists, and doesn't make any assumptions about the type of file it points to.







share|improve this answer












share|improve this answer



share|improve this answer










answered 7 hours ago









l0b0l0b0

30.2k23 gold badges128 silver badges264 bronze badges




30.2k23 gold badges128 silver badges264 bronze badges















  • this definitely helps, thanks

    – Alexander Mills
    7 hours ago

















  • this definitely helps, thanks

    – Alexander Mills
    7 hours ago
















this definitely helps, thanks

– Alexander Mills
7 hours ago





this definitely helps, thanks

– Alexander Mills
7 hours ago

















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