How to use sed to replace a config file's variable?

You can try this sed:

sed -i.bak 's/^\(VAR5=\).*/\1VALUE10/' file

It gives:

VAR1=VALUE1
VAR2=VALUE2
VAR3=VALUE3
VAR4=VALUE4
VAR5=VALUE10
VAR6=VALUE6

Even though the answer has been added to the question. I spent some time on how it works, I would like add some facts and my version of the answer,

sed -i 's,^\(THISISMYVARIABLE[ ]*=\).*,\1'THISISMYVALUE',g' config.cfg

Explanation:

As a basic of sed 's/find_this/replace_with/', we are saying sed to search and replace. Also remember there are multiple other delimiters that we can use instead of /. Here , is used.
Here we find the line that matches ^\(THISISMYVARIABLE[ ]*=\).* . This means we are grouping the match THISISMYVARIABLE[ ]*= . ([ ]* to cover if there are any spaces after the key)
In replace section \1 is a back-reference. We are referencing the first group in the regular expression that we used for match.

You can say:

sed '/^VAR5=/s/=.*/=VALUE10/' filename

To make in the change to the file in-place, use the -i option:

sed -i '/^VAR5=/s/=.*/=VALUE10/' filename

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