Ubuntu – How to find text and replace that line if exists with terminal otherwise just append line to end

bashcommand linesedswap

I want to put in sudo gedit /etc/sysctl.conf the one line vm.swappiness=10 which I sometimes change.

By default this line doesnt exist so I use echo "vm.swappiness=10" | sudo tee -a /etc/sysctl.conf.

If I would always be putting the same exact line vm.swappiness=10, then in case I want to replace I could use sudo sed -i 's/vm.swappiness=10/vm.swappiness=1/g' /etc/sysctl.conf But since there could be vm.swappiness=12 or something else, I want–with just a single command–to find if, in /etc/sysctl.conf, there exists line starting vm.swappiness=. Then if it does exist I want to remove the whole line (then by appending && echo "vm.swappiness=1" | sudo tee -a /etc/sysctl.conf to that command, it would also subsequently add the new configuration line that I want to the end.

But again since there could be a lot of different parameters in one line, it wouldn't be good to delete it all, but would be better to change only the number (to the immediate right of vm.swappiness=).

What you think? Would it be better to search for vm.swappiness=x(x(x)) with 1 to 3 numbers (of course, 100 also exists…), replace if it's there (by putting it into a variable and using a command like `sudo sed -i 's/$oldline/$newline/g'), and if not then just append vm.swappiness=10?

Best Answer

You can use

sed 's/vm.swappiness=[0-9]*/vm.swappiness=1/g' /etc/sysctl.conf

If you don't mind how many digits your number has.

If you want a maximum of 3 digits, you need extended (modern) regular expressions rather than basic regular expressions (BRE's). You then need to provide the -E parameter

sed -E 's/vm.swappiness=[0-9]{1,3}/vm.swappiness=1/g' /etc/sysctl.conf
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