How to replace specific contents in a file?

Shell Programming and Scripting
1

From the existing file, I need to replace specific contents possibly with var every time when the user changes the var.
e.g the contents in the file file.txt is 'My name is $n and I am $y years old' and every time user changed the var outside the file, the contents of the file should be created with the specific words updated.

2

Sounds like something sed could do.
Please show sample input file, and your desired outcome.

3

It's basically a file called db.txt:

dbname: database1
dbusername: samplename1
dbpwd: pwd1

The actual file is more complicated than this, so if someone else input database2, samplename2 and pwd2 from stdin, the db.txt file should change its contents to

dbname: database2
dbusername: samplename2
dbpwd: pwd2
4 
$ cat db.sh

echo "dbname: $1"
echo "dbusername: $2"
echo "dbpwd: $3"

$ db.sh database2 samplename2 pwd2
5

Here is what I do for things like this firstly setup your template file with tags surrounded by "%%" eg:

dbname: %%DBNAME%%
dbusername:  %%DB_USER_NAME%%
dbpwd:  %%DB_PWD%%

Now set and export environment variables with the values you want and then call expand.sh:

DBNAME="testing1"; export DBNAME
DB_USER_NAME="Test user"; export DB_USER_NAME
DB_PWD="/home/testing1"; export DB_PWD
expand.sh mytemplate > template.expanded

expand.sh looks like this:

awk -F% ' /%%/ {
 while(match($0, "%%[A-Za-z0-9_]+%%")) {
    tmp = substr($0, 1, RSTART-1)
    var = substr($0, RSTART+2, RLENGTH-4);
    printf "%s%s", tmp, ENVIRON[var];
    $0=substr($0,RSTART+RLENGTH);
 }
 print; next } 1' $@