On Sat, 28 Nov 2009 14:51:58 -0500, Steven wrote: > I am very interested in this question on multiple levels. > > Let's start at the first level: > > foo=$(< fn bar) > > apparently is functionally equivalent to > > foo=$( bar < fn ) True. File fn is opened for input on file descriptor 0 (stdin) in both examples. > which really is quite different from > > foo=$(cat fn | bar) True. This is something different due to introducing a pipeline, which does something else than redirecting input. > which actually runs at least two child processes. True. > I do *not* see anything in the bash man page, either in the description for > $(< ) which says that it is functionally equivalent to using cat, "cat file | command" connects standard output to standard input by using a pipeline. That's equivalent to "command 0< file" which redirects the contents of a file to standard input, file descriptor 0. The 0 can be omitted as it is the default. cat /proc/filesystems cat /proc/filesystems | cat cat < /proc/filesystems < /proc/filesystems cat all yield the same output and hence can be executed as name=$(...) or name=`...` to assign the output to a variable. > but does not > explain anything about pipes being illegal in this context. Not illegal, but plain wrong. A pipe is connected to output, the "<file" gives you input. Where you inserted the pipe, you don't have any output (on stdout). It was like 0< /proc/loadavg | cat which opens /proc/loadavg for reading (giving you input on stdin, fd 0) and '|' waiting for output on stdout. The input from /proc/loadavg is lost, since nothing reads it. The pipe doesn't see any output from anything, hence its input to "cat" is the same. Final output is empty. > I also do not see > anything of relevance to this topic in the bash man page on REDIRECTION. First chapter, as I said: | The following redirection operators may precede or appear | anywhere within a simple command or may follow a command. Redirections | are processed in the order they appear, from left to right. > At > best, the man page is deficient in that it *should* say that the syntax > defaults to qq=$(0< filename) and that other values besides zero are supported. > See further below in section "Redirecting Input" where it explains the general syntax "[n]<word". -- fedora-list mailing list fedora-list@xxxxxxxxxx To unsubscribe: https://www.redhat.com/mailman/listinfo/fedora-list Guidelines: http://fedoraproject.org/wiki/Communicate/MailingListGuidelines
