How to read a variable from file, modify and safe it to an other variale

What I want:

There is a file /scripts/backup/config.cfg which contains variables. In my specific case the important ones are:

BACKUPLOCATION=""
ROOTLOCATION="/backup"

Then there is a script /scripts/backup/performBackup.sh
For a specific reason I want a part of the script do the following operations:

1. read the value of the variable ROOTLOCATION
2. add a (“/” and) timestamp (Date&Time)
3. safe the new created value to BACKUPLOCATION (by replacing its current value)

Example

If this is the previous state of the config.cfg:

BACKUPLOCATION="dummy"
ROOTLOCATION="/backup"

After the script ran it should be:

BACKUPLOCATION="/backup/2020-05-02-23-00"
ROOTLOCATION="/backup/"

What I tried

First of all the config file gets “loaded” using

source /scripts/backup/config.cfg

I then tried to use the sed command but the quotes are messing with me. Here is one try (which didn’t work):

sed -i 's/BACKUPLOCATION=.*/BACKUPLOCATION="'$ROOTLOCATION/$(date +%Y-%m-%d-%H-%M)'"/' /scripts/backup/config.cfg

Answer

Try this:

source /scripts/backup/config.cfg
sed -i 's|BACKUPLOCATION=.*|BACKUPLOCATION="'"$ROOTLOCATION/$(date +%Y-%m-%d-%H-%M)"'"|' /scripts/backup/config.cfg

The problem with your sed is that you use / as delimiter, which is present in $ROOTLOCATION after expansion, therefore sed fails. I used |, which is usually is not present in filenames. If you ever create a file with |, that sed will fail too! So, “know your data” 🙂