Setting variables in environment.plist

• Unload the launch agent:

  launchctl unload ~/Library/LaunchAgents/environment.plist
  

• Modify the plist/env settings

• Load the launch agent again:

  launchctl load ~/Library/LaunchAgents/environment.plist
  

You have to quit and relaunch Terminal to get the new value of the variable in the shell.

With the example of Kuldeep Yadav's comment the usr.aws.plist in ~/Library/LaunchAgent/ would look like this then:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
        <key>Label</key>
        <string>usr.aws</string>
        <key>ProgramArguments</key>
        <array>
                <string>/bin/launchctl</string>
                <string>setenv</string>
                <string>AWS_SECRET_ACCESS_KEY</string>
                <string>loremipsum</string>
                <string>/bin/launchctl</string>
                <string>setenv</string>
                <string>AWS_ACCESS_KEY_ID</string>
                <string>loremipsum2</string>
                <string>/bin/launchctl</string>
                <string>setenv</string>
                <string>AWS_ACCESS_KEY</string>
                <string>loremipsum3</string>
                <string>/bin/launchctl</string>
                <string>setenv</string>
                <string>AWS_SECRET_KEY</string>
                <string>loremipsum4</string>
        </array>
        <key>RunAtLoad</key>
        <true/>
</dict>
</plist>

This is only tested (& working) with bash & zsh.