Can I test if a parameter expansion has worked

Can I test if a parameter expansion has worked

[zzapper]

Hi

Q1) Can I test if a parameter expansion has 'fired'?

other than by testing if input == output

zzapper

Re: Can I test if a parameter expansion has worked

[Bart Schaefer]

On Wed, Sep 14, 2022 at 9:52 AM zzapper <zsh@rayninfo.co.uk> wrote:
>
> Q1) Can I test if a parameter expansion has 'fired'?

I'm not sure what that means.  Does 'fired' mean expanded to a
non-empty string, as in, the parameter has a value?  And, test when,
relative to the placement of the parameter reference?  And, what do
you want to do based on the result of the test?

The most obvious answer would be that ${param:?} will cause a fatal
error if the parameter is either unset or empty, but that doesn't
provide any avenue for recovery unless you use an "always" block
around it.

Re: Can I test if a parameter expansion has worked

[Alex Satrapa]

On 14 Sep 2022, at 20:03, zzapper <zsh@rayninfo.co.uk> wrote:
> 
> Q1) Can I test if a parameter expansion has ‘fired’?

Are you able to describe the scenario where parameter expansion is not behaving as you expected?

What is ZSH doing that is different to what you expected?

Re: Can I test if a parameter expansion has worked

[zzapper]

On 14/09/2022 11:03, zzapper wrote:
> Hi
>
> Q1) Can I test if a parameter expansion has 'fired'?
>
> other than by testing if input == output
>
Bit embarrassed I didn't provide an example :(

so here it is

# adding a prefix '_s' to an image name

f=stuff.jpg;

print ${f/%(#m).[pjg][npi]e#[gf]/_s$MATCH}

stuff_s.jpg  # good this is what i want

# now pass 'bad' input

f=stuff.txt;

print ${f/%(#m).[pjg][npi]e#[gf]/_s$MATCH}
stuff.txt  #  expansion has obviously left the input unchanged but can I 
test for that???

I suppose I should be testing the filename before passing it to the 
expansion

if [[ $f =~ '.jpg|gif|png|jpeg' ]] ;then echo 'ok' ;else echo 'nok' ;fi

Re: Can I test if a parameter expansion has worked

[Peter Stephenson]

> On 15/09/2022 08:34 zzapper <zsh@rayninfo.co.uk> wrote:
> # adding a prefix '_s' to an image name
> 
> f=stuff.jpg;
> 
> print ${f/%(#m).[pjg][npi]e#[gf]/_s$MATCH}
> 
> stuff_s.jpg  # good this is what i want
> 
> # now pass 'bad' input
> 
> f=stuff.txt;
> 
> print ${f/%(#m).[pjg][npi]e#[gf]/_s$MATCH}
> stuff.txt  #  expansion has obviously left the input unchanged but can I 
> test for that???

No, there's no way of telling if some expansion actually changed an
expression without actually testing the final result.  The differences
between the cases (changed / not changed) are buried deep within the
shell and don't leave an obvious trace above --- and in any case given
how complicated zsh expressions can get finding a useful binary way of
testing what the whole expression did would be incredibly hairy.

Just test the resulting expression.

pws

Re: Can I test if a parameter expansion has worked

[zzapper]

On 15/09/2022 09:24, Peter Stephenson wrote:
>> On 15/09/2022 08:34 zzapper <zsh@rayninfo.co.uk> wrote:
>> # adding a prefix '_s' to an image name
>>
>> f=stuff.jpg;
>>
>> print ${f/%(#m).[pjg][npi]e#[gf]/_s$MATCH}
>>
>> stuff_s.jpg  # good this is what i want
>>
>> # now pass 'bad' input
>>
>> f=stuff.txt;
>>
>> print ${f/%(#m).[pjg][npi]e#[gf]/_s$MATCH}
>> stuff.txt  #  expansion has obviously left the input unchanged but can I
>> test for that???
> No, there's no way of telling if some expansion actually changed an
> expression without actually testing the final result.  The differences
> between the cases (changed / not changed) are buried deep within the
> shell and don't leave an obvious trace above --- and in any case given
> how complicated zsh expressions can get finding a useful binary way of
> testing what the whole expression did would be incredibly hairy.
>
> Just test the resulting expression.
>
> pws
>

I now realise that it would horribly subjective to decide whether or not 
an expansion had succeeded in any case :)

f=pws

echo ${f/%(#m)pws/thanks $MATCH}


zzapper

Re: Can I test if a parameter expansion has worked

[Bart Schaefer]

On Thu, Sep 15, 2022 at 12:34 AM zzapper <zsh@rayninfo.co.uk> wrote:
>
> print ${f/%(#m).[pjg][npi]e#[gf]/_s$MATCH}
> stuff.txt  #  expansion has obviously left the input unchanged but can I
> test for that???

In the general case, no, as PWS has already addressed.

In this specific case, tho, if you "unset MATCH" before doing the
substitution, you can test $+MATCH after.

Re: Can I test if a parameter expansion has worked

[david rayner]

On 15/09/2022 17:48, Bart Schaefer wrote:
> On Thu, Sep 15, 2022 at 12:34 AM zzapper <zsh@rayninfo.co.uk> wrote:
>> print ${f/%(#m).[pjg][npi]e#[gf]/_s$MATCH}
>> stuff.txt  #  expansion has obviously left the input unchanged but can I
>> test for that???
> In the general case, no, as PWS has already addressed.
>
> In this specific case, tho, if you "unset MATCH" before doing the
> substitution, you can test $+MATCH after.
>

Bart

Generically useful thanks (sorry late reply)

f=fred.jpg

print ${f/%(#m).[pjg][npi]e#[gf]/_s$MATCH}

echo $+MATCH

1

zzapper

Re: Can I test if a parameter expansion has worked

[Alex Satrapa]

[-- Attachment #1: Type: text/plain, Size: 755 bytes --]

On 15 Sep 2022, at 17:34, zzapper <zsh@rayninfo.co.uk> wrote:
> 
> I suppose I should be testing the filename before passing it to the expansion
> 
> if [[ $f =~ '.jpg|gif|png|jpeg' ]] ;then echo 'ok' ;else echo 'nok' ;fi

That’s how I’d approach it.

Also make sure to check that the new file name doesn’t exist. A common workaround to trying to name a file the same as an existing file is to add a numerical suffix. Then you need to check if there’s already a file/some files with numerical suffixes and figure out what the next suffix needs to be. One example I saw was to start with N=1, then keep incrementing that counter until file_s_${N} doesn’t exist.

Anyway, if you need help overthinking stuff I’ll be around.

Alex


[-- Attachment #2: Type: text/html, Size: 2758 bytes --]

Re: Can I test if a parameter expansion has worked

[Stephane Chazelas]

2022-09-15 08:34:34 +0100, zzapper:
[...]
> f=stuff.jpg;
> 
> print ${f/%(#m).[pjg][npi]e#[gf]/_s$MATCH}
> 
> stuff_s.jpg  # good this is what i want
> 
> # now pass 'bad' input
> 
> f=stuff.txt;
> 
> print ${f/%(#m).[pjg][npi]e#[gf]/_s$MATCH}
> stuff.txt  #  expansion has obviously left the input unchanged but can I
> test for that???
[...]

You could always do:

unset flag
print -r -- ${f/%(#m).[pjg][npi]e#[gf]/_s$MATCH${flag=}}
if (( $+flag )) echo it did substitute

Note that "print $var" without the -- is a command injection
vulnerability and without -r generally not what you want.

-- 
Stephane

Re: Can I test if a parameter expansion has worked

[Stephane Chazelas]

2022-09-15 08:34:34 +0100, zzapper:
[...]
> if [[ $f =~ '.jpg|gif|png|jpeg' ]] ;then echo 'ok' ;else echo 'nok' ;fi
[...]

ITYM

if [[ $f =~ '\.(jpe?g|gif|png)$' ]]

Or (better as regexps can often not be used with non-text while
zsh globs can):

if [[ $f = *.(jp(e|)g|gif|png) ]]

-- 
Stephane

Re: Can I test if a parameter expansion has worked

[Stephane Chazelas]

2022-09-20 14:25:51 +0100, Stephane Chazelas:
[...]
> ITYM
> 
> if [[ $f =~ '\.(jpe?g|gif|png)$' ]]
[...]

Which, btw, if the rematchpcre option is on must be changed to:

if [[ $f =~ '\.(jpe?g|gif|png)\z' ]]

As otherwise, that would match on f=$'foo.gif\n' for instance.

With rematchpcre, see also:

$ f=$'St\xe9phane.jpg'
$ [[ $f =~ '\.(jpe?g|gif|png)\z' ]]
zsh: pcre_exec() error [-10]

Not fixed with:

$ LC_ALL=C [ "$f" '=~' '\.(jpe?g|gif|png)\z' ]
zsh: pcre_exec() error [-10]

(bug?)

-- 
Stephane