Using sed command to - bash

Question

I'm trying to be able to read a file, replace a variable or instead add it at the bottom if it hasn't been already specified.
I've had a bit of help to get this far:
#!/bin/bash
function replaceappend() {
if ! sed "/$2/{s//$3/;h};"'${x;/./{x;q0};x;q1}' $1
then
echo "$3" >> $1
fi
}
replaceappend "/etc/ssh/sshd_config" "Port 22" "Port 123"
However, after running this script the contents of the original file seem to be untouched, and remain at Port 22 with nothing appended.
In this example it's using a speicfied exit code on sed using the q flag to see if it finds it or not, and then appending it if not.
Can anyone see why this code may not work?

Answer 1

I think this is what you're trying to do:
$ cat tst.sh
function replaceappend() {
awk -v old="$2" -v new="$3" '
sub(old,new) { replaced=1 }
{ print }
END { if (!replaced) print new }
' "$1" > /usr/tmp/tmp$$ &&
mv /usr/tmp/tmp$$ "$1"
}
replaceappend file "Port 22" "Port 123"
replaceappend file "Port 22" "Port 123"
$ cat file
line 1
Port 22
line 3
Port 71
line 5
$ ./tst.sh
$ cat file
line 1
Port 123
line 3
Port 71
line 5
Port 123

Answer 2

This might work for you (GNU sed & bash):
function replaceappend() {
sed '/'"$2"'/{s//'"$3"'/;h};$!b;x;/./{x;b};x;a'"$3" $1
}
The function uses the original sed statement but instead of returning an exit code at the end of the file it checks the hold space (which has been used as a flag to mark if a substitution has taken place) and either does nothing when a substitution has already taken place or appends the required string to the end of the file when the substitution has failed.