RHEL 6 su command question

What is the difference between

su tcn

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and

su - tcn

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I looked up the man page and it states:

Change the effective user id and group id to that of USER.

-, -l, --login

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Doesn't "su tcn" change the user id.  Would this command not change the group id without the dash ?
Los Angeles1Asked:
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woolmilkporcCommented:
Sorry, seems I've been typing too fast!

I should have written

Only the effective IDs are changed with "su" alone, the login profile of the target user is only executed with "su -".
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woolmilkporcCommented:
There is a difference between userid/groupid and "effective" userid/groupid.

The effective IDs are only changed with "su -", and also the login profile of the target user is only executed with "su -".

wmp
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upanwarCommented:
With su you just become that user but home directory will not be changed and it will not read his .bash_profile and .bashrc so env variable set by him will not be available but

With su - it is like you have logged in with that user. home directory will be changed. Path will be set according that user. It will read that users .bash_profile and .bashrc, all custome env variable will be available.
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