Usually, we only need to pass one argument:
echo abc | cat
echo abc | cat some_file -
echo abc | cat - some_file
Is there a way to pass two arguments? Something like
{echo abc , echo xyz} | cat
cat `echo abc` `echo xyz`
I could just store both results in a file first
echo abc > file1
echo xyz > file2
cat file1 file2
But then I might accidentally overwrite a file, which is not ok. This is going into a non-interactive script. Basically, I need a way to pass the results of two arbitrary commands to cat
without writing to a file.
UPDATE: Sorry, the example masks the problem. While { echo abc ; echo xyz ; } | cat
does seem to work, the output is due to the echo
s, not the cat
.
A better example would be { cut -f2 -d, file1; cut -f1 -d, file2; } | paste -d,
which does not work as expected.
With
file1:
a,b
c,d
file2:
1,2
3,4
Expected output is:
b,1
d,3
RESOLVED:
Use process substitution: cat <(command1) <(command2)
Alternatively, make named pipes using mkfifo
:
mkfifo temp1
mkfifo temp2
command1 > temp1 &
command2 > temp2 &
cat temp1 temp2
Less elegant and more verbose, but works fine, as long as you make sure temp1 and temp2 don't exist before hand.
Answer
I think you want 'Process Substitution' http://tldp.org/LDP/abs/html/process-sub.html. It also works on zsh, though it has more options and the syntax may be different.
It creates a pseudo file (/dev/fd/something
) for each substitution. It's pretty useful. The command can only read as a stream, meaning it can not go back and forth with fseek. It needs to read it as a stream of bytes, like a pipe.
Your examples work as process substitution:
cat <(echo abc) <(echo xyz)
paste -d, <(cut -f2 -d, file1) <(cut -f1 -d, file2)
Think of it as a tool to use a process and have it pretend to be a file without any temp files.
(from my earlier answer )
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