$(< file ) and $( cat file )

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# 1  
Old 02-02-2018
$(< file ) and $( cat file )

What is the difference between $(< filename ) and $( cat filename ) ? Smilie
Or should i ask ... are they equivalent ?

Last edited by ctsgnb; 02-02-2018 at 02:46 PM..
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# 2  
Old 02-02-2018
I can't give you a quantitative answer, but cite man bash:
The command substitution $(cat file) can be replaced by the equivalent but faster $(< file).
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# 3  
Old 02-02-2018
$(< filename ) - shell opens the file. $( cat filename ) - cat opens the file.

Create a simple script:-
#!/bin/ksh -xv

var1=$( cat file )
var2=$( < file )

and run strace to understand the difference:-
strace ./script.ksh 2> strace.out

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# 4  
Old 02-02-2018
Run strace with -f (follow child processes) to see the full overhead.
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# 5  
Old 02-02-2018
-f was important to get the real picture!
# 6  
Old 02-02-2018
They are only equivalent in bash, ksh93 and zsh. $(< file) is faster than $(cat file), but the latter is POSIX compliant whereas the former is not.

Last edited by Scrutinizer; 02-03-2018 at 01:44 AM..
# 7  
Old 02-02-2018
Just for fun playing with subsequent args ... consider giving a try to:

# for i in $(seq 1 10) ; do echo $(< /dev/urandom tr -dc '[:alnum:],@#:!?+-' | head -c10) ; done
# for i in $(seq 1 10) ; do echo $(cat /dev/urandom -- tr -dc '[:alnum:],@#:!?+-' | head -c10) ; done
# for i in $(seq 1 10) ; do echo $(cat /dev/urandom | tr -dc '[:alnum:],@#:!?+-' | head -c10) ; done
# for i in $(seq 1 10) ; do echo $(< /dev/urandom | tr -dc '[:alnum:],@#:!?+-'  | head -c10) ; done

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