Introduction to the env command

From UnixCasts.com (formerly Learning the UNIX command line on OS X)

Hey there!  

It's time for another installment of UNIX command line tips.  In the previous newsletter, we talked about UNIX aliases.  This week, we're going continue with that theme a bit more regarding our shell's environment.  I hope you enjoy...
 

Using env to print and set your environment variables

From time to time, I need to know the value of a specific UNIX environment variable. Of course, that’s easy enough to do if you know the exact spelling of the variable, as in the case of the current working directory:

☺ echo $PWD
/Users/chip/code/chip.github.com

However, what if I’m unsure of the spelling, or I simply want to see all of the variables that are set for my current environment? That’s easy enough to find as well, using the env command (output below is limited for readability):

☺ env 
ARCHFLAGS=-arch x86_64
Apple_PubSub_Socket_Render=/tmp/launch-gANhjR/Render
Apple_Ubiquity_Message=/tmp/launch-yn5YIm/Apple_Ubiquity_Message
CLICOLOR=1
COMMAND_MODE=unix2003
DISPLAY=/tmp/launch-okscQD/org.macosforge.xquartz:0
EDITOR=/usr/bin/vim -f
.
.
.

That’s pretty effective and since the environment variables are sorted, it makes it easy to find a specific setting.

But, env, can do more! It can also set the environment for you for specific utilities. Let’s look at some example Ruby code to demonstrate this example:

☺ cat env.rb

#!/usr/bin/env ruby

puts ENV['PWD']

Here you can see that this little script simply prints the contents of the PWD environment variable to STDOUT.

For sanity, let’s check the current setting of that variable just so we can see if it matches our expectations:

☺ echo $PWD
/Users/chip/code/chip.github.com

That was what I expected. So, let’s say I want this script to use another setting for that environment variable. I’ll try to set it first before calling the script:

☺ env PWD=/tmp ./env.rb
/Users/chip/code/chip.github.com

But, wait! That didn’t do anything. That’s because the env command inherits the current environment by default, which as we saw before had a different value than the one we passed: /tmp.

The way to get around this is to pass the -i flag to env, like so:

☺ env -i PWD=/tmp ./env.rb
/tmp

Now it uses the variable as expected. Keep in mind that it didn’t set this variable permanently, but instead just for the execution of the utility we passed it, which in this case was the env.rb script.

To prove it, let’s take a look at the PWD variable one more time:

☺ echo $PWD
/Users/chip/code/chip.github.com

Based on the information above, if you ever need to review your current environment, or set it for a single run of a utility, it’s easy enough to do using the env command.