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

[Pierre Gaston]

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"