Adding a directory to $PATH in CentOS?

It's not a good idea to edit /etc/profile for things like this, because you'll lose all your changes whenever CentOS publishes an update for this file. This is exactly what /etc/profile.d is for:

echo 'pathmunge /usr/lib/ruby-enterprise/bin' > /etc/profile.d/ree.sh
chmod +x /etc/profile.d/ree.sh

Log back in and enjoy your (safely) updated $PATH:

echo $PATH
/usr/lib/ruby-enterprise/bin:/usr/kerberos/sbin:/usr/kerberos/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

which ruby
/usr/lib/ruby-enterprise/bin/ruby

Instead of logging back in, you could reload the profile:

. /etc/profile

This will update the $PATH variable.

After following fmonk's advice I checked out /etc/bashrc, where I noticed it said that "Environment stuff goes in /etc/profile." I proceeded to look in /etc/profile, I saw this:

pathmunge () {
    if ! echo $PATH | /bin/egrep -q "(^|:)$1($|:)" ; then
       if [ "$2" = "after" ] ; then
          PATH=$PATH:$1
       else
          PATH=$1:$PATH
       fi
    fi
}

[...]

# Path manipulation
if [ "$EUID" = "0" ]; then
    pathmunge /sbin
    pathmunge /usr/sbin
    pathmunge /usr/local/sbin
fi

To solve my problem, I simply added pathmunge /usr/lib/ruby-enterprise/bin underneath the if statement. This solved my issue.

SORRY misinterpretted the question the following asnwer is for a USER's profile leaving it in case it helps someone

modify .bash_profile

nano ~/.bash_profile

then somewhere in the file add/modify your paths seperated by :

 PATH=$PATH:$HOME/bin:/your/path
 export PATH

then reload your profile

source ~/.bash_profile

or logout and login again

if you check PATH it should include your newly added paths

echo $PATH

"An interactive login shell is started after a successful login, using /bin/login, by reading the /etc/passwd file. This shell invocation normally reads /etc/profile and its private equivalent ~/.bash_profile upon startup.

An interactive non-login shell is normally started at the command-line using a shell program (e.g., [prompt]$/bin/bash) or by the /bin/su command. An interactive non-login shell is also started with a terminal program such as xterm or konsole from within a graphical environment. This type of shell invocation normally copies the parent environment and then reads the user's ~/.bashrc file for additional startup configuration instructions." http://www.linuxfromscratch.org/blfs/view/6.3/postlfs/profile.html

Therefore I would not put environment variables in bashrc, because it is not only against common convention, but you will also miss your bashrc varialbles when invoking a terminal from a graphical Desktop environment.

On Redhat in the /etc/profile I found this comment:

"System wide aliases and functions should go in /etc/bashrc. Personal environment variables and startup programs should go into ~/.bash_profile. Personal aliases and functions should go into ~/.bashrc."

So if you want to set environment variables on a user basis, do it in the user's .bash_profile file.

Heading over to the .bash_profile I read:

"Personal environment variables and startup programs.

Personal aliases and functions should go in ~/.bashrc. System wide environment variables and startup programs are in /etc/profile. System wide aliases and functions are in /etc/bashrc."

Conclusion
If you want only root to see programs residing, for instance in /sbin I would add that path to root's .bash_profile file. But if you want every user to see what root specific programs are installed on your box I would put /sbin into /etc/.profile. Now every user can use tab completion to look for root specific programs and elevate rights if necessary.

Special Case: SSH
When ssh is started with a commandline, an interactive login shell is started. But in this case /etc/profile is not read. When I defined environment variables in the .bash_profile file of each user it worked with ssh.

You can set environment variables in a .rc file; for bash shells (I believe the most common, and default in CentOS) each user has a file called .bashrc in his home directory.

Add the command PATH=/usr/lib/ruby-enterprise/bin:$PATH to this file to set it for any one particular user.

To set it for all users (as you mention), change it in /etc/bashrc (the default .bashrc in each user's home directory should source this file, but you should doublecheck that).

Why does everyone forget about /etc/environment. Setting environment variables for all users is what this file is for. Just add the name value pair of the environmental variable on one line. Don't use variables.

/etc/environment

PATH=:/usr/lib/ruby-enterprise/bin:/usr/bin:/usr/local/bin:

This will add the directories /usr/lib/ruby-enterprise/bin /usr/bin and /usr/local/bin to your PATH variable, for all users, once you logout or restart.

/usr/bin and /usr/local/bin were already on my path before, but were removed for some reason after I edited this file. So add these two directories to be safe.