$(< file ) and $( cat file )


 
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# 8  
Old 02-03-2018
It does not work that way. $(<file) is a special case, which is a faster alternative to $(cat file). If you are using anything other than just a file, it becomes something else. $( ... ) is just command sustitution, so if we leave out that, it becomes:
Code:
< /dev/urandom tr -dc '[:alnum:],@#:!?+-' | head -c10

This is the same as tr -dc '[:alnum:],@#:!?+-' < /dev/urandom | head -c10
Code:
cat /dev/urandom -- tr -dc '[:alnum:],@#:!?+-' | head -c10

This is strange, it effectively just means cat /dev/urandom | head -c10
Code:
cat /dev/urandom | tr -dc '[:alnum:],@#:!?+-' | head -c10

This is the same as the first one with UUOC
Code:
< /dev/urandom | tr -dc '[:alnum:],@#:!?+-'  | head -c10

This is equivalent to :< /dev/urandom | tr -dc '[:alnum:],@#:!?+-' | head -c10, which is that same as :
: | tr -dc '[:alnum:],@#:!?+-' | head -c10
This User Gave Thanks to Scrutinizer For This Post:
# 9  
Old 02-03-2018
Quote:
Originally Posted by Scrutinizer
. . .
Code:
cat /dev/urandom -- tr -dc '[:alnum:],@#:!?+-' | head -c10

This is strange,
. . .
It is. In fact, tr -dc etc. are interpreted as input files
Code:
cat: tr: No such file or directory
cat: -dc: No such file or directory
cat: '[:alnum:],@#:!?+-': No such file or directory

and generate error messages which are suppressed in above case as /dev/urandom never ends and the pipe is chopped off by the head -c10 from the other side.
This User Gave Thanks to RudiC For This Post:
# 10  
Old 02-03-2018
Yeah, I agree with you regarding the UUOC one.
The test with -- was just bulk try just to see if it would get the -- as an end of options and see how it would handle the rest of the command line.

I ran it on an AIX machine which didn't return an error message, but a fooled output instead, with some strange control character (the kind of output that sometimes may mess up your PuTTY screen ...)

As Yoda and Scruti noticed, I think the confusing point was that, $(< /dev/urandom tr -dc '[:alnum:],@#:!?+-' | head -c10 )
is not interpreted as the special case $(<filename) but as the $( cmd )

This brings me to the question :
Does the special case $(<filename) support only and strictly 1 file ?

Last edited by ctsgnb; 02-03-2018 at 05:37 AM..
# 11  
Old 02-03-2018
Quote:
Originally Posted by ctsgnb
This brings me to the question :
Does the special case $(<filename) support only and strictly 1 file ?
Passing more than one file to the redirection wont be interpreted that way, the second file will just be understood as a command name.
Code:
$(< file1 file2)

Using more than one redirection lead to undefined behavior.
Code:
$(< file1 <file2)

- ksh ignores all redirections but the first one, i.e. output file1. It doesn't check file2 for readability/existence.

- bash silently ignores the whole command, i.e. output nothing, however, it returns an error if file1 or file2 isn't readable (or doesn't exists)
This User Gave Thanks to jlliagre For This Post:
# 12  
Old 02-07-2018
Obviously $( < filename ) copies the stream with a shell-internal, just like the external command $( cat < filename ) does.
For a concatenation of file1 and file2 you have to use the external command $( cat file1 file2 )
or a string concatenation like this:
Code:
s=$( < /etc/passwd )$( < /etc/group )
echo "$s"

# 13  
Old 02-07-2018
Quote:
Originally Posted by jlliagre
Code:
$(< file1 file2)

Using more than one redirection lead to undefined behavior.
Perhaps this is just an aside, but wouldn't redirecting from a named pipe work? That is, if in $(< file1) file1 would be a named pipe being filled by cat file1 file2 ... ?

And second, wouldn't $(< $(cat file1 file2) ) also work? Right now i am travelling with this damn work-laptop and have no U*X-system at hand, so i can only speculate instead of trying....

bakunin
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