[SOLVED] Overwrite a single line in a text file

Issue

I am looking for a safe and reliable way to overwrite ONE line in a text file. I don't care if it's using sed, grep, perl whatever. It just needs to be portable and reliable. Specifically what I am trying to do is replace the value of a variable I have saved in a text file at runtime. Let's say I have a file named variables.txt which contains a line that reads userName=stephen. Let's say my program wants to change the userName to frank. Here's what I've come up with using sed:

sed -i '' 's/userName.*/userName=frank/' variables.txt

The concern I have with this is that I've read that on some versions of sed using the '-i' switch without specifying a backup file could cause the command to fail or risk possible file corruption. Not an option. What do you guys think?

Edit

For those asking where I read about command failure and file corruption. The manpage for my version of sed recommends against providing an empty value for the -i switch as well as take a look at the comments on this page here:

It seems that some versions of sed require the argument after -i and others do not. With GNU sed version 4.1.x, it seems that the -i does not require an argument and specifying an empty argument after it actually fails.

It sounds like the unanimous recommendation is to provide a backup file and then delete it after the command completes. However I'm still concerned about this solution since my version of sed doesn't even support the --version switch. My primary concern here is that the solution is both reliable and portable.

Solution

t=`tempfile`
sed -e 's/userName.*/userName=frank/' variables.txt >$tempfile
cp $tempfile >variables.txt
rm $tempfile

you can also use mv but that won't preserve file rights

if tempfile is not available then use some other method ($$.bak) to create the filename.

Answered By - Karoly Horvath

Answer Checked By - David Goodson (WPSolving Volunteer)