GNU bug report logs - #16618
How to show the diff command when there is a difference when comparing two files?

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Peng Yu <pengyu.ut <at> gmail.com>
To: bug-diffutils <at> gnu.org
Subject: How to show the diff command when there is a difference when
 comparing two files?
Date: Sat, 1 Feb 2014 13:27:55 -0600

Hi,

The following shows that diff -r print the command e.g. "diff -r
/var/folders/ft/9trh7jt57kqc_hyq0wm5rny40000gx/T/tmp.wcrIdLELps/b.txt
/var/folders/ft/9trh7jt57kqc_hyq0wm5rny40000gx/T/tmp.AJieJ9Zk1O/b.txt"
if there is an error. I'm wondering if I just compare two files, it is
possible to print the diff command if there is a difference between
the two files? Thanks.

~/linux/test/gnu/diffutils/diff/-r$ cat main.sh
#!/usr/bin/env bash

dir1=$(mktemp -d)
dir2=$(mktemp -d)
echo 'Hello World' > "$dir1/a.txt"
echo 'Hello World' > "$dir2/a.txt"

echo 'Hello World' > "$dir1/b.txt"
echo 'Hello World!' > "$dir2/b.txt"

#echo 'Hello World' > "$dir1/c.txt"
echo 'Hello World' | gzip -n > "$dir1/c.txt.gz"
diff -r "$dir1" "$dir2"

~/linux/test/gnu/diffutils/diff/-r$ ./main.sh
diff -r /var/folders/ft/9trh7jt57kqc_hyq0wm5rny40000gx/T/tmp.wcrIdLELps/b.txt
/var/folders/ft/9trh7jt57kqc_hyq0wm5rny40000gx/T/tmp.AJieJ9Zk1O/b.txt
1c1
< Hello World
---
> Hello World!
Only in /var/folders/ft/9trh7jt57kqc_hyq0wm5rny40000gx/T/tmp.wcrIdLELps:
c.txt.gz

-- 
Regards,
Peng

Message #10 received at 16618-done <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Peng Yu <pengyu.ut <at> gmail.com>
Cc: 16618-done <at> debbugs.gnu.org
Subject: Re: How to show the diff command when there is a difference when
 comparing two files?
Date: Sat, 29 Mar 2014 22:22:40 -0700

> I'm wondering if I just compare two files, it is
> possible to print the diff command if there is a difference between
> the two files?

Not in diff itself, but it's easy to do from a shell script.  E.g.,

command="diff a b"
$command >output || {
  echo "$command"
  cat output
}

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