system command output different in guile than on command line

system command output different in guile than on command line

[vapnik spaknik]

Hi,
    I'm trying to write some code to get the size of a diff of two files.
If I run the following pipeline in my zsh shell:

> diff -ua /tmp/file1 /tmp/file2 | wc -c

it prints 215
However when I run the following in guile:

guile> (system "diff -ua /tmp/file1 /tmp/file2 | wc -c")

it prints 243.
I've tried for other pairs of files, and the difference between the command line results and the guile results always seems to be 28.
This is not too much of a problem in itself because I can just subtract 28 in my guile code, but it worries me; why the discrepancy? Are there other differences between guile system calls and the command line that I need to be aware of?
Another strange thing is that if I use the pipeline procedure to do the same thing, then I get an exit code of 1 from the wc command, but it still returns the same result:

(define (diffsize f1 f2)
  (receive (from to pids)
      (pipeline (list (list "/usr/bin/diff" "-ua" f1 f2)
		      (list "/usr/bin/wc" "-c")))
    (let ((rval (cons f2 (string->number
			  (string-delete #\newline
					 (read-delimited " " from)))))
	  (xvals (map (compose status:exit-val cdr waitpid) pids)))
      (close to)
      (close from)
      (format #t "Exit values: ~a\n" xvals)
      rval)))

guile> (diffsize "/tmp/file1" "/tmp/file2")
Exit values: (0 1)
("/tmp/file2" . 243)

Why?

Re: system command output different in guile than on command line

[tomas]

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On Tue, Aug 04, 2020 at 09:34:22PM +0000, vapnik spaknik wrote:
> Hi,
>     I'm trying to write some code to get the size of a diff of two files.
> If I run the following pipeline in my zsh shell:
> 
> > diff -ua /tmp/file1 /tmp/file2 | wc -c
> 
> it prints 215
> However when I run the following in guile:
> 
> guile> (system "diff -ua /tmp/file1 /tmp/file2 | wc -c")

Have you actually tried to compare both results?

There are a couple of things which could make a difference. The
one which first comes to mind would be different environments
(PATH, language settings in LANG, LC_).

One thing you might want to try (to keep most of your experimental
setup) would be:

  diff -ua /tmp/file1 /tmp/file2 > /tmp/delta1

and then

  guile> (system "diff -ua /tmp/file1 /tmp/file2 > /tmp/delta2")

...and then diff the diffs.

Cheers
-- t

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Re: system command output different in guile than on command line

[vapnik spaknik via General Guile related discussions]

 

On Tuesday, August 4, 2020, 10:34:22 PM GMT+1, vapnik spaknik <vapniks@yahoo.com> wrote:


>Hi,
>    I'm trying to write some code to get the size of a diff of two files.
>If I run the following pipeline in my zsh shell:
>
>> diff -ua /tmp/file1 /tmp/file2 | wc -c
>
>it prints 215
>However when I run the following in guile:
>
>guile> (system "diff -ua /tmp/file1 /tmp/file2 | wc -c")
>
>it prints 243.

OK... I was being stupid. The shell command that I actually ran was: diff -ua file1 file2 | wc -c
i.e. without the directories in the paths since I was running from within /tmp
This meant the command I ran in the shell was slightly different than the one ran in guile, and hence the difference in diffs.
However, I'm still perplexed by the non-zero error code returned in the pipeline:

>Another strange thing is that if I use the pipeline procedure to do the same thing, then I get an exit code of 1 >from the wc command, but it still returns the same result:

>(define (diffsize f1 f2)
>  (receive (from to pids)
>      (pipeline (list (list "/usr/bin/diff" "-ua" f1 f2)
>              (list "/usr/bin/wc" "-c")))
>    (let ((rval (cons f2 (string->number
>              (string-delete #\newline
>                    (read-delimited " " from)))))
>      (xvals (map (compose status:exit-val cdr waitpid) pids)))
>      (close to)
>      (close from)
>      (format #t "Exit values: ~a\n" xvals)
>      rval)))
>
>guile> (diffsize "/tmp/file1" "/tmp/file2")
>Exit values: (0 1)
>("/tmp/file2" . 243)
>
>Why?
  

Re: system command output different in guile than on command line

[tomas]

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On Wed, Aug 05, 2020 at 11:27:30AM +0000, vapnik spaknik via General Guile related discussions wrote:

[...]

> OK... I was being stupid.

We all are, most of us more often than not ;-)

>Another strange thing is that if I use the pipeline procedure to do the same thing, then I get an exit code of 1 >from the wc command, but it still returns the same result:

This is by design. Quote the "diff" man page:

  "Exit status is 0 if inputs are the same, 1 if different,
   2 if trouble."

That's probably so that you can do things in the shell like

  diff a b > /dev/null && echo "same" || echo "diff"

Cheers
-- t

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