Replace specific variables in external bash-script

Question

I am new to coding and already found a few topics about this on stackoverflow, but couldn't make it work, as they seem overextended to me. I might need some guidance.

I need to change some variables in an external bash script 'comlink.conf'. But only specific ones. Others should be left like they are.

ready=0
test=1
new=2

echo 'ready='$ready > comlink.conf
sleep 10

ready=1

echo 'ready='$ready > comlink.conf

If I do it like that, 'test=1' and 'new=2' will be overwritten completely and are gone from the file. That should not happen.

What would be the most easiest way to do this?

when you use > in echo it overwrites the content in comlink.conf. If you want to add data you could use >>. — betaros
– betaros, Commented Aug 11, 2017 at 11:50
Great idea! But then 'ready' is 0 and in the next line it is 1. I need to update the specific variable, not add the same again with a different value, because another script will read and use the variables in that file. — daniel_e
– daniel_e, Commented Aug 11, 2017 at 11:55
You should look at a tool like sed for something like this — Eric Renouf
– Eric Renouf, Commented Aug 11, 2017 at 11:55

Eric Renouf · Accepted Answer · 2017-08-11 12:30:15Z

1

You can use sed with a different substitution like:

sed 's/ready=.*/ready=1/' comlink.conf > tmp
mv tmp comlink.conf

or if you are using GNU sed:

sed -i 's/ready=.*/ready=1/' comlink.conf

or BSD sed:

sed -i'' 's/ready=.*/ready=1/' comlink.conf

answered Aug 11, 2017 at 12:30

Eric Renouf

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4 Comments

daniel_e Over a year ago

That works perfectly. Thank you!!! But why is it only working with a temp file? Is it not possible to edit an external file directly?

Eric Renouf Over a year ago

@daniel_e the versions of sed with -i support will modify it in-place. If you try to use redirection though, you'll end up truncating the file before reading it, so you need a temp file (or something that works like it)

daniel_e Over a year ago

Perfect. Thanks for the info, Eric. Now I learned something. :-)

Eric Renouf Over a year ago

@daniel_e to be honest, the -i options do a temp file solution too instead of directly in place (which can matter if you have a full disk for example). If you need real in place editing you can use a tool like ed

betaros · Accepted Answer · 2017-08-11 11:57:59Z

1

If I understand you right, you want to replace a line in a file. You shouldn't use echo for that.

Instead I would suggest using sed:

sed '/.*ready*/s/.*/ready=1/' comlink.conf

answered Aug 11, 2017 at 11:57

betaros

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