Question

How come I can do this in bash:

$ diff -u <(echo -e 'line1\nline2') <(echo -e 'line1\nline3') --- /dev/fd/63  2009-03-30 09:49:07.527272646 +0100 +++ /dev/fd/62  2009-03-30 09:49:07.527272646 +0100 @@ -1,2 +1,2 @@  line1 -line2 +line3 

i.e. I can use named pipes / process substituion to get the diff of a small chunk of text. However when I try to do it with wdiff, the diff for words, not just lines, I get no useful output

wdiff <(echo -e 'line1\nline2') <(echo -e 'line1\nline3') [--]{++} 

UPDATE: looks like there’s an existing ubuntu bug report for this: https://bugs.launchpad.net/ubuntu/+source/wdiff/+bug/160912

Answer 1

An strace reveals that wdiff stats the files (probably to find out their size). Since named pipes report a size of 0 it probably assumes that both files are empty and therefore equal:

 $ strace -efile wdiff -1 <(echo -e 'line1\nline2') <(echo -e 'line1\nline3') execve('/usr/bin/wdiff', ['wdiff', '-1', '/dev/fd/63', '/dev/fd/62'], [/* 44 vars */]) = 0 [snip uninteresting stuff] stat64('/dev/fd/63', {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0 open('/dev/fd/63', O_RDONLY)            = 3 open('/tmp/wdiff.MzPXmH', O_RDWR|O_CREAT|O_EXCL, 0600) = 4 stat64('/dev/fd/62', {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0 open('/dev/fd/62', O_RDONLY)            = 4 open('/tmp/wdiff.5nma9j', O_RDWR|O_CREAT|O_EXCL, 0600) = 5 --- SIGCHLD (Child exited) @ 0 (0) --- unlink('/tmp/wdiff.MzPXmH')             = 0 unlink('/tmp/wdiff.5nma9j')             = 0 {++}Process 27699 detached 

Edit: also note that bash may use /dev/fd-style filenames instead of named pipes if the kernel supports it (most recent ones do, the example above shows this), but the effect is pretty much the same.