Source only part of a script from another script?

In bash/ksh and similar, you can use <() to avoid the subshell that | creates:

insourcable.sh:

echo hallo welt
echo hello world
echo ahoj světe

Suppose you only want to in-source line number two:

. <( sed -n '2 p' < insourcable.sh )

(prints "hello world")

First you need to rigorously determine what command will produce the specific part you want to source. For a trivial example, given the file

var1=value1
var2=value2

you could set only var1 using head -n1 filename. This could be a pipeline of arbitrary complexity, if you wanted.

Then run:

source <( pipeline_of_arbitrary_complexity some_filename )

Works only in bash. To do it in POSIX, I think you'd need to make a temp file.

dotfn(){ . /dev/fd/0
} <<IN
        $(printf '\n%s(){ "$@" '"'\n" "$2"
          sed   -e"/$2.*{/,\$!d;s/^/    /"\
                -e"/^. *} *$/q;s/'/'"'\\&&/g'  <"$1"
          printf "\n';}")
IN

obviously it's not fool proof, but it scans a file named in its first argument for the first series of input that begins with its second arg followed at some point on the same line with a { and pulls in all following lines until it encounters one which only matches blank space and a single }.

while sed does so it escapes all hard-quotes in input, and printf wraps the input in two hard-quotes and also prepends it with the string "$@". it sticks all of this in a function named for your second argument. here's an example:

printf '
some_fn(){
        echo some stuff
        cat </dev/null
}

another_fn(){
        for f in file*; do printf '\''%s\n'\'' "$f"; done
}
' >fn_file

i put those two functions in fn_file, and then...

dotfn fn_file another_fn

...nothing happened. but...

another_fn printf %s\\n

another_fn(){
    for f in file*; do printf '%s\n' "$f"; done
}

i enclosed the function within one named for itself, and i can inspect it with printf or whatever, or i can redefine it:

another_fn eval; another_fn

file
file1.tsv
file1.txt
file2.tsv
file2.txt
filea.xyz
fileb.xyz

I replaced the . /dev/fd/0 in dotfn() with a simple cat and went after some_fn():

dotfn fn_file some_fn

some_fn(){ "$@" '
    some_fn(){
        echo some stuff
        cat </dev/null
    }

';}