Replace an unknown string within a line using Bash - bash

Question

I want to be able to modify db001 with a string I pass into the command via CLI. At any given time db001 could be a different value so I can't just look for that value.
./myscript modify_db <new value>
myfile.txt
./myscript modify_db mynewdbname002
Before: database_node=db001.mydomain.local
After: database_node=mynewdbname002.mydomain.local
./myscript modify_db db003
Before: database_node=mynewdbname002.mydomain.local
After: database_node=db003.mydomain.local

Answer 1

You can use this sed command inside your script:
sed "s/^\(database_node=\)[^.]*/\1$1/" file
Example:
s='database_node=db001.mydomain.local'
repl() {
sed "s/^\(database_node=\)[^.]*/\1$1/" <<< "$s";
}
and call it as:
repl mynewdbname002
database_node=mynewdbname002.mydomain.local
repl db003
database_node=db003.mydomain.local

Answer 2

You could have a script like, just like below taking an input argument having the replacement value,
#!/bin/bash
perl -lpe "s/database_node=(\w+)/database_node=$1/g" file
and just do
./script.sh newdbname
Use the -i flag for in-place replacement and -i.bak for in-place replacement with a backup of your original file
perl -lpe -i.bak "s/database_node=(\w+)/database_node=$1/g" file
(or) with a simple bash function
function replaceFile() {
perl -lpe -i.bak "s/database_node=(\w+)/database_node=$1/g" file
}

Answer 3

I would avoid trying to produce the new state from the previous state and rather just use a template :
function modify_db() {
echo "database_node=$1.mydomain.local"
}
I use echo here for illustration but you should obviously do whatever you want to do with the "database_node=$1.mydomain.local".
Supposing it should modify the only line starting with database_node from a file db_conf after having printed the old value :
function modify_db() {
echo "Before: $(grep '^database_node=' db_conf)"
sed -i "s/^database_node=.*\.mydomain\.local/database_node=$1.mydomain.local/" db_conf
echo "After: $(grep '^database_node=' db_conf)"
}