Negative length parameter expansion

Negative length parameter expansion

[Thorsten Kampe]

Hi,

Bash got negative length parameter expansion (like in `${STRING:11:-
17}`) in version 4.2 and Zsh in 4.3.12.

How would I do negative length parameter expansion in Bash or Zsh if 
my version doesn't support it?

Thorsten

Re: Negative length parameter expansion

[ZyX]

12.04.2015, 10:47, "Thorsten Kampe" <thorsten@thorstenkampe.de>:
> Hi,
>
> Bash got negative length parameter expansion (like in `${STRING:11:-
> 17}`) in version 4.2 and Zsh in 4.3.12.
>
> How would I do negative length parameter expansion in Bash or Zsh if
> my version doesn't support it?
>
> Thorsten

`$#STRING` is a length of the string. `-17` is `$(( ${#STRING}-17+1 ))`. (`$(( ))` may be not necessary, most likely they will not be necessary in zsh, don’t know about bash). `${#PARAMETER}` form is supported by any POSIX shell (but not `$#PARAMETER` without figure braces).

Re: Negative length parameter expansion

[Thorsten Kampe]

* ZyX (Sun, 12 Apr 2015 12:33:52 +0300)
> 
> 12.04.2015, 10:47, "Thorsten Kampe" <thorsten@thorstenkampe.de>:
> > Hi,
> >
> > Bash got negative length parameter expansion (like in `${STRING:11:-
> > 17}`) in version 4.2 and Zsh in 4.3.12.
> >
> > How would I do negative length parameter expansion in Bash or Zsh if
> > my version doesn't support it?
> 
> `$#STRING` is a length of the string. `-17` is `$(( ${#STRING}-17+1 ))`. (`$(( ))` may be not necessary, most likely they will not be necessary in zsh, don’t know about bash). `${#PARAMETER}` form is supported by any POSIX shell (but not `$#PARAMETER` without figure braces).

Simple and elegant solution, thanks.

Just for correctness: your solution has an off-by-one error (actually 
an "off by offset" error). Offset starts at 0. You're adding 1 but 
you would have to subtract 0 (or in general: offset).

Following the example from
<http://wiki.bash-hackers.org/syntax/pe#negative_length_value>

```
MYSTRING="Be liberal in what you accept, and conservative in what you 
send"
offset=11
length=17

printf "${MYSTRING:$offset:-$length}\n"
in what you accept, and conservative

length=$((${#MYSTRING} - offset - length))
printf "${MYSTRING:$offset:$length}\n"
in what you accept, and conservative
```

Thorsten

Re: Negative length parameter expansion

[ZyX]

12.04.2015, 14:45, "Thorsten Kampe" <thorsten@thorstenkampe.de>:
> * ZyX (Sun, 12 Apr 2015 12:33:52 +0300)
>>  12.04.2015, 10:47, "Thorsten Kampe" <thorsten@thorstenkampe.de>:
>>>  Hi,
>>>
>>>  Bash got negative length parameter expansion (like in `${STRING:11:-
>>>  17}`) in version 4.2 and Zsh in 4.3.12.
>>>
>>>  How would I do negative length parameter expansion in Bash or Zsh if
>>>  my version doesn't support it?
>>  `$#STRING` is a length of the string. `-17` is `$(( ${#STRING}-17+1 ))`. (`$(( ))` may be not necessary, most likely they will not be necessary in zsh, don’t know about bash). `${#PARAMETER}` form is supported by any POSIX shell (but not `$#PARAMETER` without figure braces).
>
> Simple and elegant solution, thanks.
>
> Just for correctness: your solution has an off-by-one error (actually
> an "off by offset" error). Offset starts at 0. You're adding 1 but
> you would have to subtract 0 (or in general: offset).

Sorry, I was always using ${VAR[start,end]} form. My variant is correct for ${VAR[start,end]} (where `start` starts from one, not from zero), not for ${VAR:offset:length} which I never used (and thus for some reason was under impression that it was the other form of expressing the same thing).

Thanks for the correction.

>
> Following the example from
> <http://wiki.bash-hackers.org/syntax/pe#negative_length_value>
>
> ```
> MYSTRING="Be liberal in what you accept, and conservative in what you
> send"
> offset=11
> length=17
>
> printf "${MYSTRING:$offset:-$length}\n"
> in what you accept, and conservative
>
> length=$((${#MYSTRING} - offset - length))
> printf "${MYSTRING:$offset:$length}\n"
> in what you accept, and conservative
> ```
>
> Thorsten