Replace after searched string with a variable (containing slashes) in sed/awk

Tools

rxvt-unicode (urxvt) v9.31
GNU bash, version 5.2.26(1)-release (x86_64-pc-linux-gnu)
GNU Awk 5.3.0
sed (GNU sed) 4.9
tr (GNU coreutils) 9.4

Attempted behaviour

Grab the first 9 colours from ~/.colour_list which has the colours printed with a newline between each one
Remove the leading # from each colour
Place a / every 2nd character; FF/FF/FF/
Append each colour with a transparency value; FF/FF/FF/2F
Prepend each colour with the string rgba:; rgba: FF/FF/FF/2F
Place a , every 16 characters (except for the last character) to create a comma separated list; rgba: FF/FF/FF/2F, rgba: 2F/AA/66/2F ...
Place this entire comma separated list into a variable bc
Place the variable bc after the exact string match bar_color = (including spaces) in the file test.conf

Input

.colour_list;

Process (script)

tst.sh;

#!/bin/bash

value=9
transparency=FF

bc=$(head -n${value} ~/.colour_list | 
    tr -d '#' | 
    sed 's/.\{2\}/&\//g;s/.$//' | 
    awk '{print "rgba:" $0 transparency}' transparency=$transparency | 
    sed 's/.\{16\}/&,/g')

sed -ri "s|^(bar_colour             \s*=\s*).*|\1$bc|" ~/test.conf

echo $bc

Output

Output:

bash output;

sed: -e expression #1, char 52: unterminated `s' command
rgba:00/00/00/FF rgba:00/00/01/FF rgba:00/00/02/FF rgba:00/00/04/FF rgba:00/00/05/FF rgba:00/00/06/FF rgba:00/00/07/FF rgba:00/00/08/FF rgba:00/00/09/FF

~/test.conf not changed;

...
bar_colour              = test
...

Expected output:

bash output;

rgba:00/00/00/FF, rgba:00/00/01/FF, rgba:00/00/02/FF, rgba:00/00/04/FF, rgba:00/00/05/FF, rgba:00/00/06/FF, rgba:00/00/07/FF, rgba:00/00/08/FF, rgba:00/00/09/FF

~/test.conf changed;

...
bar_colour              = rgba:00/00/00/FF, rgba:00/00/01/FF, rgba:00/00/02/FF, rgba:00/00/04/FF, rgba:00/00/05/FF, rgba:00/00/06/FF, rgba:00/00/07/FF, rgba:00/00/08/FF, rgba:00/00/09/FF
...

Issues

I have two issues.

The $bc assignment (see sed 's/.\{2\}/&\//g;s/.$//') removes all commas, where I would only like to remove the final comma (i.e. the final character) of the entire string. Please note, I do require the $bc assignment (see here).
Upon finding the string, colour = , I want to place the value of $bc after it but I instead get an error (see above). Please note that I would like to retain the gap between colour and =, which is a mixture of tabs and spaces.

Methods tried

I have tried previous answers (1, 2, 3, 4) but none of them seem to work and invariably return as above.

I am guessing for the awk error at least, this is because the variable I am trying to replace with contains slashes, but I am not sure why sed is throwing an error. I require the slashes as the expected format is as per XQueryColor specifications.

I have placed the script into ShellCheck. Apart from suggesting $bc be wrapped in double quotes at the final line (which is not necessary for the script's functioning), it reports No issues detected!.

Changed script to the following:

value=9
transparency=FF

bc=$(head -n${value} ~/.colour_list | 
    tr -d '#' | 
    sed 's@.\{2\}@&\/@g;s@.$@@' | 
    awk '{print "rgba:" $0 transparency}' transparency=$transparency | 
    sed 's@.\{16\}@&,@g')

sed -ri "s|^(bar_colour             \s*=\s*).*|\1$bc|" ~/test.conf

echo $bc

This also provided the same errors as per the Output section, unfortunately.

Solution

$ cat test.conf
...
bar_colour             = foo
bar_colour              = test
bar_colour               = bar
...

$ cat tst.sh
#!/usr/bin/env bash

tmp=$(mktemp) || exit
trap 'rm -f "$tmp"; exit' EXIT

awk '
    NR == FNR {
        sub(/^#/,"")
        gsub(/../,"&/"); sub("/$","")
        bc = bc sprintf("%srgba: %s/2F", (NR>1 ? ", " : ""), $0)
        if ( NR == 9 ) {
            nextfile
        }
        next
    }
    match($0,/^bar_colour              = /) {
        $0 = substr($0,1,RLENGTH) bc
    }
    { print }
' .colour_list test.conf > "$tmp" &&
mv "$tmp" test.conf

$ ./tst.sh

$ cat test.conf
...
bar_colour             = foo
bar_colour              = rgba: 00/00/00/2F, rgba: 00/00/01/2F, rgba: 00/00/02/2F, rgba: 00/00/04/2F, rgba: 00/00/05/2F, rgba: 00/00/06/2F, rgba: 00/00/07/2F, rgba: 00/00/08/2F, rgba: 00/00/09/2F
bar_colour               = bar
...

How we got there...

Stepping through your "Attempted behaviour" list one at a time:

Grab the first 9 colours from ~/.colour_list which has the colours printed with a newline between each one

awk '
    { print }
    NR == 9 { exit }
' .colour_list
#000000
#000001
#000002
#000004
#000005
#000006
#000007
#000008
#000009

Remove the leading # from each colour

awk '
    {
        sub(/^#/,"")
        print
    }
    NR == 9 { exit }
' .colour_list
000000
000001
000002
000004
000005
000006
000007
000008
000009

Place a / every 2nd character; FF/FF/FF/

awk '
    {
        sub(/^#/,"")
        gsub(/../,"&/"); sub("/$","")
        print
    }
    NR == 9 { exit }
' .colour_list
00/00/00
00/00/01
00/00/02
00/00/04
00/00/05
00/00/06
00/00/07
00/00/08
00/00/09

Append each colour with a transparency value; FF/FF/FF/2F

awk '
    {
        sub(/^#/,"")
        gsub(/../,"&/"); sub("/$","")
        print $0 "/2F"
    }
    NR == 9 { exit }
' .colour_list
00/00/00/2F
00/00/01/2F
00/00/02/2F
00/00/04/2F
00/00/05/2F
00/00/06/2F
00/00/07/2F
00/00/08/2F
00/00/09/2F

Prepend each colour with the string rgba:; rgba: FF/FF/FF/2F

awk '
    {
        sub(/^#/,"")
        gsub(/../,"&/"); sub("/$","")
        print "rgba:", $0 "/2F"
    }
    NR == 9 { exit }
' .colour_list
rgba: 00/00/00/2F
rgba: 00/00/01/2F
rgba: 00/00/02/2F
rgba: 00/00/04/2F
rgba: 00/00/05/2F
rgba: 00/00/06/2F
rgba: 00/00/07/2F
rgba: 00/00/08/2F
rgba: 00/00/09/2F

Place a , every 16 characters (except for the last character) to create a comma separated list; rgba: FF/FF/FF/2F, rgba: 2F/AA/66/2F ...

awk '
    {
        sub(/^#/,"")
        gsub(/../,"&/"); sub("/$","")
        printf "%srgba: %s/2F", (NR>1 ? ", " : ""), $0
    }
    NR == 9 { print ""; exit }
' .colour_list
rgba: 00/00/00/2F, rgba: 00/00/01/2F, rgba: 00/00/02/2F, rgba: 00/00/04/2F, rgba: 00/00/05/2F, rgba: 00/00/06/2F, rgba: 00/00/07/2F, rgba: 00/00/08/2F, rgba: 00/00/09/2F

Place this entire comma separated list into a variable bc

bc=$( awk '
    {
        sub(/^#/,"")
        gsub(/../,"&/"); sub("/$","")
        printf "%srgba: %s/2F", (NR>1 ? ", " : ""), $0
    }
    NR == 9 { print ""; exit }
' .colour_list )

Place the variable bc after the exact string match bar_color = (including spaces) in the file test.conf

$ cat tst.sh
#!/usr/bin/env bash

bc=$( awk '
    {
        sub(/^#/,"")
        gsub(/../,"&/"); sub("/$","")
        printf "%srgba: %s/2F", (NR>1 ? ", " : ""), $0
    }
    NR == 9 { print ""; exit }
' .colour_list )
printf 'bar_colour              = %s\n' "$bc" > test.conf

cat test.conf

$ ./tst.sh
bar_colour              = rgba: 00/00/00/2F, rgba: 00/00/01/2F, rgba: 00/00/02/2F, rgba: 00/00/04/2F, rgba: 00/00/05/2F, rgba: 00/00/06/2F, rgba: 00/00/07/2F, rgba: 00/00/08/2F, rgba: 00/00/09/2F

It's not clear why you want a shell variable bc or to format the output for test.conf outside of awk instead of just:

awk '
    {
        sub(/^#/,"")
        gsub(/../,"&/"); sub("/$","")
        bc = bc sprintf("%srgba: %s/2F", (NR>1 ? ", " : ""), $0)
    }
    NR == 9 {
        printf "bar_colour              = %s\n", bc
        exit
    }
' .colour_list > test.conf

$ cat test.conf
bar_colour              = rgba: 00/00/00/2F, rgba: 00/00/01/2F, rgba: 00/00/02/2F, rgba: 00/00/04/2F, rgba: 00/00/05/2F, rgba: 00/00/06/2F, rgba: 00/00/07/2F, rgba: 00/00/08/2F, rgba: 00/00/09/2F