Unable to define line number in sed by variable - bash

Question

I'm trying to use an array to define the lines to replace using sed; I can delete the lines using a variable for the line number but I can't get sed to use the variable to define the line number to write to. The problem seems to reside in the insert line. How do you pass the value of an array as a line number to sed?
#!/bin/bash
lineNum=$(sed -n '/max_allowed_packet/=' /etc/mysql/my.cnf)
IFS= #There's a space as the delimiter#
ary=($lineNum)
#for key in "${!ary[#]}";
# do
# sed -i '$ary[$key]'d /etc/mysql/my.cnf;
# #The folllowing line errors#
# sed -i "'$ary[$key]'imax_allowed_packet = 32M" /etc/mysql/my.cnf;
# #The above line errors#
#done
#for val in "${ary[#]}";
# do
# sed -i "${val}d" /etc/mysql/my.cnf;
# sed -i "${val}imax_allowed_packet = 32M" /etc/mysql/my.cnf;
# done
for val in "${ary[#]}";
do
sed -i "${val}s/.*/imax_allowed_packet = 32M" /etc/mysql/my.cnf";
done
For the first stanza of script I get the following output:
Error: sed: -e expression #1, char 1: unknown command: `''
For the second Stanza I get the following output:
sed: -e expression #1, char 3: unknown command:
'
sed: -e expression #1, char 3: unknown command:
'
For the third Stanza I get the following output:
./test.sh: line 22: unexpected EOF while looking for matching `"'
./test.sh: line 24: syntax error: unexpected end of file
Edit, rewriting the sed commands as sed -i "${ary[$key]}" generates the following error output: sed: -e expression #1, char 3: unknown command: `

Answer 1

I think you're over-complicating the issue. Your script can be reduced to this:
sed 's/\(max_allowed_packet\).*/\1 = 32M/' /etc/mysql.cnf
This performs a substitution on every occurrence of max_allowed_packet, setting the rest of the line to = 32M. Add the -i switch to overwrite the file when you're happy with the result.
Problems with your attempt
Shell parameters are not expanded within single quotes, so you would need to use double quotes, e.g. sed -i "${ary[$key]}d". You can use set -x to see what is happening here - at the moment, you will see the literal string $ary[$key], rather than the array value.
If I understand your intention correctly (you want to substitute the entire line), there's no need to call sed twice:
for val in "${ary[#]}"; do
sed -i.bak "${val}s/.*/imax_allowed_packet = 32M" /etc/mysql/my.cnf
done
I have chosen to loop through the values of the array, instead of the keys, in order to simplify things a little. When using the -i option, it is always a good idea to specify a backup file, as I have done.