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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

edited 9 hours ago

Alexander Mills

asked 9 hours ago

Alexander MillsAlexander Mills

2,46422 gold badges2121 silver badges6767 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
  8 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
  
  

  
  
  
  

 | 
show 2 more comments

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

edited 9 hours ago

Alexander Mills

asked 9 hours ago

Alexander MillsAlexander Mills

2,46422 gold badges2121 silver badges6767 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
  8 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
  
  

  
  
  
  

 | 
show 2 more comments

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

edited 9 hours ago

Alexander Mills

asked 9 hours ago

Alexander MillsAlexander Mills

2,46422 gold badges2121 silver badges6767 bronze badges

echo <(cat)

/dev/fd/63

myapp -f <(cat)

 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

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

share|improve this question

edited 9 hours ago

Alexander Mills

asked 9 hours ago

Alexander MillsAlexander Mills

2,46422 gold badges2121 silver badges6767 bronze badges

share|improve this question

edited 9 hours ago

Alexander Mills

asked 9 hours ago

Alexander MillsAlexander Mills

2,46422 gold badges2121 silver badges6767 bronze badges

asked 9 hours ago

Alexander MillsAlexander Mills

2,46422 gold badges2121 silver badges6767 bronze badges

asked 9 hours ago

Alexander MillsAlexander Mills

2,46422 gold badges2121 silver badges6767 bronze badges

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

answered 7 hours ago

l0b0l0b0

30.2k2323 gold badges128128 silver badges264264 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  this definitely helps, thanks
  
  – Alexander Mills
  7 hours ago
  

  
  
  
  

add a comment | 

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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

answered 7 hours ago

l0b0l0b0

30.2k2323 gold badges128128 silver badges264264 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  this definitely helps, thanks
  
  – Alexander Mills
  7 hours ago
  

  
  
  
  

add a comment | 

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

answered 7 hours ago

l0b0l0b0

30.2k2323 gold badges128128 silver badges264264 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  this definitely helps, thanks
  
  – Alexander Mills
  7 hours ago
  

  
  
  
  

add a comment | 

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

answered 7 hours ago

l0b0l0b0

30.2k2323 gold badges128128 silver badges264264 bronze badges

share|improve this answer

answered 7 hours ago

l0b0l0b0

30.2k2323 gold badges128128 silver badges264264 bronze badges

answered 7 hours ago

l0b0l0b0

30.2k2323 gold badges128128 silver badges264264 bronze badges

answered 7 hours ago

l0b0l0b0

30.2k2323 gold badges128128 silver badges264264 bronze badges