[SOLVED] why won't if [ ! cmp -s A B ] work?

centguy · 05-16-2021, 02:37 AM

If I am hard pressed I might do this:

Code:

echo "Mary had a little lamb" > A
echo "Mary had not a little lamb" > B
 
echo Test 1
if cmp -s A A; then
  echo > /dev/null
else
  echo diff
fi
 
echo Test 2
if cmp -s A B; then
  echo > /dev/null
else
  echo diff
fi

Output :

Code:

Test 1
Test 2
diff

I need something that produce nothing (yet respect bash if-then-else) and the best I can think of is
echo > /dev/null

echo will not be good enough since it will produce an empty line that upsets other script.

MadeInGermany · 05-16-2021, 04:43 AM

There were better suggestions already.

Code:

if cmp -s A A; then
  :
else
  echo diff
fi

The nop command can even take arguments

Code:

  : no diff
  : echo "no diff"

In your initial post you had a negation of the exit status

Code:

if ! cmp -s A A; then
  echo diff
fi

Another possibility

Code:

cmp -s A A || {
  echo diff
}

pan64 · 05-16-2021, 04:53 AM

if you wish you can use a variable:

Code:

if cmp -s A B; then
  VAR=ok
else
  VAR=diff
fi

and later you can do whatever you want based on that variable.
Do not attempt to echo anything and use (process) that later, that is just not reliable. Use exit code and variables.

centguy · 05-16-2021, 08:49 PM

My problem is with Post #12 by shruggy:

Code:

if cmp -s A B
then :
else echo diff
fi

Now MadeInGermany says:

Code:

if cmp -s A A; then
  :
else
  echo diff
fi

There is missing ; between the two.

For that I feel very uncomfortable.

Any official source to look it up?

centguy · 05-16-2021, 08:59 PM

Please ignore my previous post.
I think I am just not used to

if A
then

vs

if A; then

Because of that I did not understand

if A
then :

Now that MadeInGermany has typeset it in a way that I am most familiar, I now undertand the magic of

:

which is
do nothing.

Cheers!

centguy · 05-16-2021, 09:34 PM

Quote:

The nop command can even take arguments

points to the right direction.

A simple google search "bash nop" gives me the much needed info:

Quote:

https://stackoverflow.com/questions/...tional-in-bash

Personally, i could not agree more with this comment.

Quote:

Also, personally, I prefer to write code that is more "human friendly". It's often possible to restructure code to be more concise or even efficient. But when that's not critical, I prefer my code to be more readable for whichever developer comes along afterwards. (I also like commenting my code.) – osullic Jan 6 at 10:08

just because I have not learned : and I am forced to negate my logic that is silly. I will rather learn things properly.
Thanks for all the tips from all posts!

pan64 · 05-17-2021, 01:04 AM

just a minor comment:

Quote:

Originally Posted by centguy

A simple google search "bash nop" gives me the much needed info: https://stackoverflow.com/questions/...tional-in-bash

As it was described on that link: : is not a noop, but a valid command which has a regular exit code too (which is always 0 - true). Actually it accepts command line arguments and for example

Code:

: $(ls /tmp > /tmp/aaa)

will execute that ls command (so it is definitely not a noop).
true is a more human readable synonym of the same command. Additionally we also have a /bin/true binary executable.
true has a counterpart, the command false, which works exactly the same way, the only difference is that it returns always a non-zero exit code (=false).

we used to construct for example an endless loop using it:

Code:

while true;
do
    # something
done

MadeInGermany · 05-17-2021, 03:59 AM

Code:

while true
do
    : # something
done

The : or true command can solve some portability issues.

Code:

: $((i+=1))

works in all Posix-compliant shells, in contrast to the not yet standardized

Code:

((i+=1))

Of course you can as well do

Code:

i=$((i+1))

Code:

: > filename

creates/empties "filename". While

Code:

> filename

in zsh works like

Code:

cat > filename

boughtonp · 05-17-2021, 08:48 AM

Quote:

Originally Posted by centguy

There is missing ; between the two.

There isn't a missing semi-colon - a command can be terminated by a semi-colon or a newline (or an ampersand, which runs it in async in background).

In the examples above, the need for a colon after "then" is because the then must be followed by a command, for example, this is a syntax error: "if something; then; something; fi"

However, replacing all three semi-colons with newlines is valid - the "then" must be followed by a command, but can be separated from it by whitespace (i.e. the newline isn't a terminator in that context).

I'm not sure how well I've explained that, and unfortunately there isn't really a good explanation in the Bash manual either, but this page is relevant: https://www.gnu.org/software/bash/manual/html_node/Lists.html