Question

I need to manage a configuration file in Linux using a shell script, that will read and write settings to and from the cfg file. I have following function to write a new value of a key in existing config file used in my script.

set_setting() {
    sed -i "s/$1=[^ ]*/$1=$2/" $3
}

Usage:

set_setting keyname NewValue /home/Config.cfg

Thus, this would replace the value of keyname in Config.cfg to NewValue.

I also have a function get_setting which uses source command to read the config file. It accepts similarly two parameters, keyname and the source config file, and returns the value of the supplied key.

Now the problem is that, I have config file as follows.

name=Abc
surname=Xyz

And when I call the set_setting function as set_setting name Def /home/Config.cfg, ideally it should change value of name from Abc to Def, but along with changing value of key name, it also changes the value of surname from Xyz to Def resulting into following in config file.

name=Def
surname=Def

I suspect this is due to having the term “name” common in both keys, as when I changed the key surname to surnames, it worked as expected and only value of name was changed. Also note that, the value for the key can be anything except the space.

I’m new to shell scripting and recently discovered sed command.

I’ve also refered to SO questions this and this. While both of them were quite helpful, I’m stuck with above mentioned situation.

Answer 1

You should anchor your match to the beginning of the line:

 sed -i "s/^$1=[^ ]*/$1=$2/" $3

Note the ^ in front of $1. This regex operator anchors the expression to the beginning of the line, effectively requiring the name being everything from the first character to the = sign. There’s also an end-of-line anchor called $ (which you do not need here).