Re: [Help-bash] A string handling question please.

[Pierre Gaston]

On Tue, Jul 24, 2018 at 9:19 AM, Pierre Gaston <address@hidden>
wrote:

>
>
> On Tue, Jul 24, 2018 at 3:10 AM, George R Goffe <address@hidden> wrote:
>
>> Hi,
>>
>> I'm writing a function that will build a string based on the args passed
>> to it and ultimately be executed as a bash command or commands linked via
>> pipes. I'm a little confused and need some help please.
>>
>> For example:
>>
>> invoke doit() with args a b c d e: doit $a $b $c $d $e
>>
>> generate command" xx=$(history | $a | $b | $c -z | $d | $e) and then
>> execute $xx.
>>
>> The history command produces nothing. Bash or maybe me is confused about
>> what's happening vs what I want to happen.
>>
>> Any hints/tips/suggestions/clues are welcome.
>>
>> Regards,
>>
>> George...
>>
>
> I'll make some assumptions as to what you want exactly, but maybe this
> will help:
>
> First, just to be sure the function must be defined in your current shell,
> so in a file you are sourcing, your .bashrc etc.. (see
> http://mywiki.wooledge.org/BashFAQ/060)
>
> Then I assume the variable contains commands that are correctly quoted, eg:
>
> a='grep -v "foo\$"'
> b='sed  "s/^/b:"'
>
> a quick and simple way to check this is that if you  type
>
>   echo "$a"
>
> it should print a command that you can copy paste as is.
> (I suggest reading http://mywiki.wooledge.org/Quotes  )
>
> Now, just expanding the commands will not be enough they will not be
> executed, you will need to evaluate them:
>
> doit () { eval xx="\$(history  |  "$1" | "$2")" ;}
>
> Note:  the quotes are important here as well
>
> if you want something that takes a variable number of arguments:
> doit ()
> {
>     local pipe arg;
>     for arg in "$@";
>     do
>         pipe="$pipe | $arg";
>     done;
>     eval xx="\$(history ${pipe+"$pipe"})"
> }
>
> a='sed "s/^/filtered:/"'
> b="grep doit"
> doit "$a" "$b"
> echo "$xx"
>

I forgot to mention http://mywiki.wooledge.org/BashFAQ/048 about eval