hello
I would like to change group of set of files but only group1 to group2 - files with other group than group1 should remain untouched. Is it possible in any easy way?

Yes. The chown command can do what you want (man chown and look at the "--from" and "-R" options).

As an example, pretend you have a directory that contains all the files you need to change named /home/projects/projB (which includes subdirectories) and that you want to change files owned by "groupa" to "alphag". This command should work:
Do not run this on your important files until you have tested it first.

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or you can first find the files of specified group and then re-own with another if you want:
to change the owner the complete command would be:
good luck

man find

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Keep in mind that lithos's approach will work, but has some potential problems.

1. If you have a LOT of matching files, then piping find's output to xargs could create a "line too long" error for the chown. This could be worked around by using find's "-exec" option.

2. The form of the chown command: "chown user:group <file>" will overwrite the owner of the file. If the original owner needs to be retained, omit the "owner" part (for example: "chown :group <file>")

Not criticizing--just as an fyi.

Because, as lithos mentioned, you can use "find /path/to/dir -group groupname" as a test to see what files will be modified--as opposed to creating a testing environment to be certain my command will do what you need.

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I'd have done something like this:

I don't have enough experience with xargs to know when to use it or not.

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Thanks a lot!
I investigated only options of chgrp thinking that it is more appropriate. It was my mistake.