&& and || and && again confusing

lqmsh · 08-02-2022, 10:36 AM

I am trying to figure out my script to be simple

Code:

$ a=11
$ [ -z $a ] && echo empty || echo noempty && date

retruns

Code:

noempty
Tue Aug  2 09:54:17 PM +0630 2022

And

Code:

$ a=11
$ [ ! -z $a ] && echo empty || echo noempty && date

retruns

Code:

empty
Tue Aug  2 09:54:17 PM +0630 2022

Why does the second script with ! still return the output of

Code:

date

command?

Actually I am trying to change this script

Code:

#!/bin/dash

while true; do
	open=$(ls -a --group-directories-first --file-type | grep -v "^\.[a-zA-Z0-9]\|^\./" | dmenu -p "dmenuFM: ")
	if [ -d "$open" ]; then
		cd "$open"
	else
		if [ ! -z "$open" ]; then
			xdg-open "$open"
		fi
		break
	fi
done

to

Code:

#!/bin/dash

while true; do
	open=$(ls -a --group-directories-first --file-type | grep -v "^\.[a-zA-Z0-9]\|^\./" | dmenu -p "dmenuFM: ")
	[ -d "$open" ] && cd "$open" || xdg-open "$open" && break
done

But the way I am changing is not working as expected.

smallpond · 08-02-2022, 11:01 AM

In bash, && and || have equal precedence and are evaluated left-to-right.
Since a is not empty:

Code:

[ ! -z $a ] && echo empty || echo noempty && date

is the same as:

Code:

true && echo empty || echo noempty && date

So the first && is not short-circuited, so prints "empty", then we have:

Code:

true || echo noempty && date

the || is short-circuited, so does not print "noempty". Then we have:

Code:

true && date

which is not short-circuited, so prints date.

michaelk · 08-02-2022, 04:01 PM

Basically &&, || and ; chain commands together on a single line.

To elaborate the && and || work on the exit status of the command, && if the command completed successfully and || if it does not.

The "echo string" command should always return successfully and therefore the date command is always executed.

rnturn · 08-02-2022, 06:50 PM

Quote:

Originally Posted by lqmsh

I am trying to figure out my script to be simple

Code:

$ a=11
$ [ -z $a ] && echo empty || echo noempty && date

retruns
[CODE]

<snip>

Code:

#!/bin/dash

while true; do
	open=$(ls -a --group-directories-first --file-type | grep -v "^\.[a-zA-Z0-9]\|^\./" | dmenu -p "dmenuFM: ")
	if [ -d "$open" ]; then
		cd "$open"
	else
		if [ ! -z "$open" ]; then
			xdg-open "$open"
		fi
		break
	fi
done

Just my US$0.02 but, assuming that the original script is actually working, taking the clear "if-then-else" constructs and trying to turn them into a one-liner may not be making the script simpler. Shorter, sure, but not necessarily simpler. Clear-and-simple-to-understand beats clever any day. Will the one-liner make sense when the script is visited again in months or a year from now?

Just tossing this into the mix: K.I.S.S.

grail · 08-02-2022, 07:38 PM

Others may have provided the why?, but rnturn has provided the best answer IMHO

pan64 · 08-03-2022, 03:28 AM

The "usual" mistake here is: the if/then/else construct cannot be replaced by && || or something similar, that works differently (there are some cases when they produce the same result and there are some other cases when they produce different result).
As it was already mentioned, if the && || && construct is confusing, use if/then/elif/else. That is much more readable and more easily understandable.

NevemTeve · 08-03-2022, 05:25 AM

Or perhaps:

Code:

test -z "$a" && echo empty || { echo noempty; date; }

boughtonp · 08-03-2022, 09:51 AM

Sometimes a parenthized sub-shell can be used to deal with order of precedence issues:

Code:

( [ -n "$a" ] && echo empty ) || ( echo noempty && date )

But that wont work with the actual code involving cd, because the directory change is lost when the sub-shell ends.

Still the actual code can be simplified...

I had expected to see someone pointing out that "! -z" is a longer way to write "-n", though that check possibly isn't even needed - an empty value comes from pressing Esc in dmenu, right? So just move the assignment into the while check and rely on the dmenu exit status to abort the loop instead.

This allows there to be a single inverted test of "not a directory" which can bring that part down to a couple of lines, (the break removes the need for an explicit else clause).

Also, the grep is unnecessary; far easier to remove the -a option so the offending items never get output, then prefix the "..".

Also I haven't checked but it may be necessary to explicitly set an ls quoting style. (This script wont handle filenames with newlines in, but that's a limitation with dmenu too?)

Code:

#!/bin/dash

while selection=$( ( echo ".." ; ls -1 --g --file-type ) | dmenu -p "dmenuFM: " )
do
   if [ ! -d "$selection" ]; then echo xdg-open "$selection" ; break ; fi
   cd "$selection" || echo "error: failed to cd [$selection] in [$PWD]"
done

ntubski · 08-03-2022, 01:57 PM

Quote:

Originally Posted by boughtonp

Sometimes a parenthized sub-shell can be used to deal with order of precedence issues:

Code:

( [ -n "$a" ] && echo empty ) || ( echo noempty && date )

But that wont work with the actual code involving cd, because the directory change is lost when the sub-shell ends.

I think NevemTeve's suggestion of using curly braces fixes this without having trouble with cd (since curly braces don't make sub-shells).

boughtonp · 08-03-2022, 04:40 PM

Quote:

Originally Posted by ntubski

I think NevemTeve's suggestion of using curly braces fixes this without having trouble with cd (since curly braces don't make sub-shells).

Thanks, I'm blaming that brainfart on the heat. Looks like I also forgot to switch the order of the outputs.

GazL · 08-03-2022, 06:17 PM

Looking at what the code is doing, I think I'd have gone with a 'case' construct. Maybe, something along the lines of:

Code:

while [ -z "$open" ]
do
  if open=$( ls --group-directories-first -Fd * | dmenu -p select: -l 10 ); then
    case "$open" in
      "")  :  ;;
      */)  cd "$open" ; unset open  ;;
       *)  xdg-open "$open"  ;;
    esac
  else 
    break
  fi
done

chrism01 · 08-05-2022, 12:38 AM

Another vote for rnturn - my immediate reaction was the same

MadeInGermany · 08-05-2022, 07:26 AM

+1 for if-then-else

Code:

if [ -n "$a" ]; then echo empty; else echo noempty; date; fi

Another suggestion for the loop:

Code:

while
  open=$(ls -a --group-directories-first --file-type | grep -v "^\.[a-zA-Z0-9/]" | dmenu -p "dmenuFM: ")
  [ -d "$open" ]
do
  cd "$open"
done
if [ -n "$open" ]
then
  xdg-open "$open"
fi