Vincent Lefevre wrote:
> The zsh parameter expansion syntax is very cryptic (and the
> above command is far from being the worst).
>
>> so can you please explain that syntax or provide a URL
>> where it is explained? (Or, just a list with names and/or
>> examples should be sufficient, actually.)
>>
>> The M, f, quotes, colons, and hashes?
>
> Using the "units" example, we start with:
>
> print -r "$(aptitude show units)"
>
> The double quotes are necessary, as the zshexpn(1) man page
> says: for command substitution:
>
> "If the substitution is not enclosed in double quotes, the
> output is broken into words using the IFS parameter."
>
> But here, we want to keep the end of lines.
>
> Then one wants to get the "Version:" line, and one can do
> this via an array. The "(f)" will turn the aptitude output
> to an array with splitting at the end of lines (\n).
> This can be seen with
>
> for i in ${(f)"$(aptitude show units)"}; print -r "[$i]"
>
> Without the "(f)", one just gets one element.
>
> The ${name:#pattern} syntax on an array does filtering of
> elements matching the pattern. By default, matching array
> elements are removed. Here, we want the opposite: to keep
> the matching array elements. This is done with the "(M)"
> flag. Here, the pattern to use is "Version:*", which matches
> lines that start with "Version:", the * character matching
> any sequence of characters, like in filename generation
> (a.k.a. globbing). So,
>
> print -r ${(M)${(f)"$(aptitude show units)"}:#Version:*}
>
> gives on my machine:
>
> Version: 2.21-1
>
> Then, we want to remove the "Version: " prefix. This is done
> with the ${name#pattern} syntax. One could use the "Version:
> " pattern, but "* " is shorter and safe since there should
> be a single space. Hence
>
> print -r ${${(M)${(f)"$(aptitude show units)"}:#Version:*}#* }
>
> Note: instead of nested substitutions, one can use
> intermediate variables, which can make the code easier
> to understand.
Thank you, very useful :)
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