Question

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.

Answer 1

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.