one-liner with && and ||

frater · 02-09-2011, 02:26 PM

I recently noticed some of my one-liners with && and || didn't work as I expected so I googled for "bash && || pitfalls" and found this page http://mywiki.wooledge.org/BashPitfalls (#22)

It's probably my fault, but I still don't understand why it wouldn't work as I want it.....

I don't even have the code anymore that wasn't working properly because I rewrote it using if ; then ; command ; fi

Could someone explain me a bit better what I probably did wrong....

I know my question is a bit vague and if I need to rephrase it, no problem.

everToulouse · 02-09-2011, 03:31 PM

Hi,

if expr then...
considers the expression "tested", when
test expr && cmd || cmd
considers only the last command's success or failure.

frater · 02-09-2011, 05:52 PM

I don't think it's the answer to my question....
Let me ask it in a different way....

If I want to do something like this:

Code:

echo 'yy' >/tmp/file1
if grep -q 'xx' /tmp/file1 ; then
  if ! grep -q 'yy' /tmp/file1 ; then
    rm /tmp/file1
  fi
fi
ls -l /tmp/file1

I prefer to write it as a one-liner and I would expect this line to do the same

Code:

echo 'yy' >/tmp/file1
grep -q 'xx' /tmp/file1 && grep -q 'yy' /tmp/file1 || rm /tmp/file1
ls -l /tmp/file1

Somehow this isn't the case and I don't get why not....
Even if there is no 'xx' in the file, the file could still get deleted.

Isn't there a better explanation than the one on that site and yours (it may be my fault that I don't get it)

This one does work the same

Code:

echo 'yy' >/tmp/file1
if grep -q 'xx' /tmp/file1 ; then
  grep -q 'yy' /tmp/file1 || rm /tmp/file1
fi
ls -l /tmp/file1

sys64738 · 02-09-2011, 06:11 PM

Hi
I'm not sure if I can help you.
What should be the condition when the file is to be deleted?
No "yy" and no "xx" in the file?
Or something else?

grail · 02-09-2011, 06:23 PM

If you read the link you posted the information is all there (I know as have recently used it several times myself

), but I will try to explain further.

When you are using the 'if' construct it is very specific in the fact that the 'then' clause or the 'else' clause ONLY run when the expression is true or false in the 'if'.

However, when using the single line entry (&& and ||), you face the issue that ANY false expression will jump to || clause and ANY true expression will jump to &&.
I find the final example to be helpful:

Code:

$ true && { echo true; false; } || echo false
true
false
$ false && { echo true; false; } || echo false
false
$ false || { echo false; true; } && echo true
false
true
$ true || { echo false; true; } && echo true
true

Hope this helps