diff

[Yedidyah Bar David]

On Tue, Dec 14, 2021 at 5:45 PM Mark E. Fuller <mark.e.fuller at gmail.com> wrote:
>
>
> It's such a small thing, but I really enjoyed being privy to this
> question and discussion today.
>
> On 14/12/2021 17:21, Daniel Shahaf wrote:
> > אורי wrote on Tue, 14 Dec 2021 07:44 +00:00:
> >> Actually I prefer the "<(..)" method, because sometimes I want to compare 2
> >> commands:
> >>
> >> diff <(pip freeze | sort) <(cat requirements.txt | sort)
> >>
> >
> > Consider s/diff/comm -12/.
> >
> >> Or even use cat and echo to add a specific line to one of the outputs. If I
> >> want the result to be completely null. For example to run in crontab and
> >> send me mail whether it's not null.
> >
> > Careful here.  By default, cron jobs run not under your login shell but
> > under /bin/sh, and it's possible the latter doesn't support «<(…)» even
> > if the former does.
>
> Out of curiosity I asked the internet:
> https://unix.stackexchange.com/questions/159513/what-are-the-shells-control-and-redirection-operators
>
> Looks like < as a redirect is POSIX-compliant, but as soon as it's
> <(...) it's not a redirect anymore but instead a process substitution
> which is implemented in BASH, ZSH, etc.:
> https://www.gnu.org/software/bash/manual/bash.html#Process-Substitution
> https://zsh.sourceforge.io/Intro/intro_7.html
>
> Nice to learn a new trick - I probably would have just redirected to a
> temp file instead of the compact solution discussed today.

If we are heading in this direction, may I remind of another, likely
POSIX-compliant
(didn't check), more "traditional" way, that still does not require
(much) I/O, at least on
any reasonably-modern *nix (perhaps e.g. not including cygwin etc. -
again, didn't check):

$ mkfifo pipe1
$ mkfifo pipe2
$ command1 > pipe1 &
$ command2 > pipe2 &
- These are now blocked, waiting in the background until someone reads
from the pipes
$ diff pipe1 pipe2
- After 'diff' finishes reading, command1 and command2 will finish too.
- On a job-controlling shell, you'll get some messages about this.
$ rm pipe1 pipe2

Best regards,
-- 
Didi