Hi, guys. I have one question:

How can I search for a line with certain string in it and then insert a string into this line?
For example:

There is a file called shadow, the contents of it are below:

**************************
...
yuanz:VIRADxMsadfDF/Q:0:0:50:7:::
sherrys:ASdfe#$DfdadfFAd:0:0:50:7:::
brownj:ADdfTRMnD$%3dfa:0:0:50:7:::
...
**************************

A variable called $user, another variable called $string. I want to search the line which has $user in it, and add $string at the beginning of the second column. If $user is brownj, and $string is STRING_, then the line will be:

brownj:STRING_ADdfTRMnD$%3dfa:0:0:50:7:::

I use the following code, but it doesn't work

sed 's/^$user:/$user:$string/' shadow

Can anybody help me with this?

Thank you very much for your time in advance

-Keyang