How to check if command line tools is installed

10.15 Catalina Update:

See Yosemite Update.

10.14 Mojave Update:

See Yosemite Update.

10.13 High Sierra Update:

See Yosemite Update.

10.12 Sierra Update:

See Yosemite Update.

10.11 El Capitan Update:

See Yosemite Update.

10.10 Yosemite Update:

Just enter in gcc or make on the command line! OSX will know that you do not have the command line tools and prompt you to install them!

To check if they exist, xcode-select -p will print the directory. Alternatively, the return value will be 2 if they do NOT exist, and 0 if they do. To just print the return value (thanks @Andy):

xcode-select -p 1>/dev/null;echo $?

10.9 Mavericks Update:

Use pkgutil --pkg-info=com.apple.pkg.CLTools_Executables

10.8 Update:

Option 1: Rob Napier suggested to use pkgutil --pkg-info=com.apple.pkg.DeveloperToolsCLI, which is probably cleaner.

Option 2: Check inside /var/db/receipts/com.apple.pkg.DeveloperToolsCLI.plist for a reference to com.apple.pkg.DeveloperToolsCLI and it will list the version 4.5.0.

[Mar 12 17:04] [jnovack@yourmom ~]$ defaults read /var/db/receipts/com.apple.pkg.DeveloperToolsCLI.plist
{
    InstallDate = "2012-12-26 22:45:54 +0000";
    InstallPrefixPath = "/";
    InstallProcessName = Xcode;
    PackageFileName = "DeveloperToolsCLI.pkg";
    PackageGroups =     (
        "com.apple.FindSystemFiles.pkg-group",
        "com.apple.DevToolsBoth.pkg-group",
        "com.apple.DevToolsNonRelocatableShared.pkg-group"
    );
    PackageIdentifier = "com.apple.pkg.DeveloperToolsCLI";
    PackageVersion = "4.5.0.0.1.1249367152";
    PathACLs =     {
        Library = "!#acl 1\\ngroup:ABCDEFAB-CDEF-ABCD-EFAB-CDEF0000000C:everyone:12:deny:delete\\n";
        System = "!#acl 1\\ngroup:ABCDEFAB-CDEF-ABCD-EFAB-CDEF0000000C:everyone:12:deny:delete\\n";
    };
}

Yosemite

Below are a few extra steps on a fresh Mac that some people might need. This adds a little to @jnovack's excellent answer.

Update: A few other notes when setting this up:

Make sure your admin user has a password. A blank password won't work when trying to enable a root user.

System Preferences > Users and Groups > (select user) > Change password

Then to enable root, run dsenableroot in a terminal:

$ dsenableroot
username = mac_admin_user
user password:
root password:
verify root password:

dsenableroot:: ***Successfully enabled root user.

Type in the admin user's password, then the new enabled root password twice.

Next type:

sudo gcc

or

sudo make

It will respond with something like the following:

WARNING: Improper use of the sudo command could lead to data loss
or the deletion of important system files. Please double-check your
typing when using sudo. Type "man sudo" for more information.

To proceed, enter your password, or type Ctrl-C to abort.

Password:


You have not agreed to the Xcode license agreements. You must agree to 
both license agreements below in order to use Xcode.

Press enter when it prompts to show you the license agreement.

Hit the Enter key to view the license agreements at 
'/Applications/Xcode.app/Contents/Resources/English.lproj/License.rtf'

IMPORTANT: BY USING THIS SOFTWARE, YOU ARE AGREEING TO BE BOUND BY THE 
FOLLOWING APPLE TERMS:

//...

Press q to exit the license agreement view.

By typing 'agree' you are agreeing to the terms of the software license 
agreements. Type 'print' to print them or anything else to cancel, 
[agree, print, cancel]

Type agree. And then it will end with:

clang: error: no input files 

Which basically means that you didn't give make or gcc any input files.

Here is what the check looked like:

$ xcode-select -p
/Applications/Xcode.app/Contents/Developer

Mavericks

With Mavericks, it is a little different now.

When the tools were NOT found, this is what the command pkgutil command returned:

$ pkgutil --pkg-info=com.apple.pkg.CLTools_Executables
No receipt for 'com.apple.pkg.CLTools_Executables' found at '/'.

To install the command line tools, this works nicely from the Terminal, with a nice gui and everything.

$ xcode-select --install

http://macops.ca/installing-command-line-tools-automatically-on-mavericks/

When they were found, this is what the pkgutil command returned:

$ pkgutil --pkg-info=com.apple.pkg.CLTools_Executables
package-id: com.apple.pkg.CLTools_Executables
version: 5.0.1.0.1.1382131676
volume: /
location: /
install-time: 1384149984
groups: com.apple.FindSystemFiles.pkg-group com.apple.DevToolsBoth.pkg-group com.apple.DevToolsNonRelocatableShared.pkg-group 

This command returned the same before and after the install.

$ pkgutil --pkg-info=com.apple.pkg.DeveloperToolsCLI
No receipt for 'com.apple.pkg.DeveloperToolsCLI' found at '/'.

Also I had the component for the CLT selected and installed in xcode's downloads section before, but it seems like it didn't make it to the terminal...

Hope that helps.

To check if command line tools are installed run:

xcode-select --version

// if installed you will see the below with the version found in your system
// xcode-select version 1234.

If command line tools are not installed run:

xcode-select --install