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I want to be able to 'su' to a specific user, allowing me to run any command without a password being entered.

For example:

If my login were user1 and the user I want to 'su' to is user2:

I would use the command:

su - user2

but then it prompts me with

Password:

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Ask the other user for the password. At least the other user knows what's been done under his/her id. –  Global nomad Feb 17 '13 at 13:17
1  
This is nothing to do with another physical user. Both ID's are mine. I know the password as I created the account. I just don't want to have to type the password every time. –  zio Feb 17 '13 at 13:24
    
Would it be ok to ssh to at user or do you need to inherit one shell in particular and need su to work? –  bmike Feb 17 '13 at 15:32
1  
@zio Great use case. Does open -na Skype not work for you? –  bmike Feb 17 '13 at 23:59
1  
You could also try copying the application bundle and changing CFBundleIdentifier. –  Lri Feb 18 '13 at 4:50

2 Answers 2

up vote 14 down vote accepted

sudo can do just that for you :)

It needs a bit of configuration though, but once done you would only do this:

sudo -u user2 -s

And you would be logged in as user2 without entering a password.

Configuration

To configure sudo, you must edit its configuration file via: visudo. Note: this command will open the configuration using the vi text editor, if you are unconfortable with that, you need to set another editor (using export EDITOR=<command>) before executing the following line. Another command line editor sometimes regarded as easier is nano, so you would do export EDITOR=/usr/bin/nano. You usually need super user privilege for visudo:

sudo visudo

This file is structured in different section, the aliases, then defaults and finally at the end you have the rules. This is where you need to add the new line. So you navigate at the end of the file and add this:

user1    ALL=(user2) NOPASSWD: /bin/bash

You can replace also /bin/bash by ALL and then you could launch any command as user2 without a password: sudo -u user2 <command>.

Update

I have just seen your comment regarding Skype. You could consider adding Skype directly to the sudo's configuration file. I assume you have Skype installed in your Applications folder:

user1    ALL=(user2) NOPASSWD: /Applications/Skype.app/Contents/MacOS/Skype

Then you would call from the terminal:

sudo -u user2 /Applications/Skype.app/Contents/MacOS/Skype
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Thank you. Exactly what I needed. –  zio Feb 18 '13 at 11:55
    
This is far less complicated than the ssh keys idea, so use this unless you need the ssh keys for remote access as well. –  bmike May 28 at 16:04

I would set up public/private ssh keys for the second account and store the key in the first account.

Then you could run a command like:

 ssh user@localhost -n /Applications/Skype.app/Contents/MacOS/Skype &

You'd still have the issues where Skype gets confused since two instances are running on one user account and files read/written by that program might conflict. It also might work well enough for your needs and you'd not need an iPod touch to run your second Skype instance.

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This is a good secure solution for the general case of password-free login to any account on any host, but I'd say it's probably overkill when both accounts are on the same host and belong to the same user. –  scottishwildcat Feb 18 '13 at 9:54
    
@scottishwildcat It's far more secure than the alternative of scripting the password and feeding it in clear text or using a variable and storing the password in the keychain and using a tool like expect to script the interaction. I just use sudo su - blah and type my password. I think the other answer covers sudo well enough to keep this as a comment. –  bmike Feb 18 '13 at 14:02
    
Oh, I certainly wasn't suggesting your answer should be removed… I didn't even down-vote, it's a perfectly good answer. –  scottishwildcat Feb 18 '13 at 17:47
    
We appear to be in total agreement - thanks for the addition - feel free to edit it into the answer if you can improve on it. –  bmike Feb 18 '13 at 18:46

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