Most people in here will probably know how to pipe output from one command to another:
command 1 | command2However, what if
command2 doesn't allow reading from standard input, and only supports filenames? How can you do this without writing to a file?
It turns out that you can do this in bash and ksh by using the
<(command) syntax. For example, the above command can be written:
command2 <(command1)This will execute
command1 in a subshell, and at the same time, call
command2 with a
file descriptor looking something like
/dev/fd/63. When
command2 reads from that, it'll get the output of
command1.
At first this doesn't seem too useful, but this means that you can do nifty things like this:
diff -u <(sort filea.txt) <(sort fileb.txt)Which will sort filea.txt and fileb.txt, and then diff the outputs - all without writing a single file.
Note that if the subshells require user input, this isn't going to work, so you can't use this to capture user input and pass it to a script which would otherwise require a filename. However, as long as this isn't the case, everything should work smoothly.
[edit: Oh, and I should mention that,
unlike piping, you can execute several commands in the subshell. For example:
rev <(echo wheeness; sleep 2; echo blarg)This also demonstrates how both the subshell and the main process run simultaneously; the output is "sseneehw", followed by a delay of 2 seconds, followed by "gralb".]
[edit 2: See
this comment for an example of how to run multiple commands via piping!]
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sophie
