PowerShell: Setting an environment variable for a single command only

Question

Generally, it would be better to pass info to the script via a parameter rather than a global (environment) variable. But if that is what you need to do you can do it this way:

$env:FOO = 'BAR'; ./myscript

The environment variable $env:FOO can be deleted later like so:

Remove-Item Env:\FOO

I got motivated enough about this problem that I went ahead and wrote a script for it: with-env.ps1

Usage:

with-env.ps1 FOO=foo BAR=bar your command here

# Supports dot-env files as well
with-env.ps1 .\.env OTHER_ENV=env command here

On the other hand, if you install Gow you can use env.exe which might be a little more robust than the quick script I wrote above.

Usage:

env.exe FOO=foo BAR=bar your command here

# To use it with dot-env files
env.exe $(cat .env | grep.exe -v '^#') SOME_OTHER_ENV=val your command

2 easy ways to do it in a single line:

$env:FOO='BAR'; .\myscript; $env:FOO=''
$env:FOO='BAR'; .\myscript; Remove-Item Env:\FOO

Just summarized information from other answers (thank you folks) which don't contain pure one-liners for some reason.

To accomplish the equivalent of the Unix syntax, you not only have to set the environment variable, but you have to reset it to its former value after executing the command. I've accomplished this for common commands I use by adding functions similar to the following to my PowerShell profile.

function cmd_special()
{
  $orig_master = $env:app_master
  $env:app_master = 'http://host.example.com'
  mycmd $args
  $env:app_master = $orig_master
}

So mycmd is some executable that operates differently depending on the value of the environment variable app_master. By defining cmd_special, I can now execute cmd_special from the command line (including other parameters) with the app_master environment variable set... and it gets reset (or even unset) after execution of the command.

Presumably, you could also do this ad-hoc for a single invocation.

& { $orig_master = $env:appmaster; $env:app_master = 'http://host.example.com'; mycmd $args; $env:app_master = $orig_master }

It really should be easier than this, but apparently this isn't a use-case that's readily supported by PowerShell. Maybe a future version (or third-party function) will facilitate this use-case. It would be nice if PowerShell had a cmdlet that would do this, e.g.:

with-env app_master='http://host.example.com' mycmd

Perhaps a PowerShell guru can suggest how one might write such a cmdlet.

You could do this by running the script as a Job:

Start-Job -InitializationScript { $env:FOO = 'BAR' } -FilePath .\myscript.ps1 |
    Receive-Job -Wait -AutoRemoveJob

You could also pass arguments to the script, using the ArgumentList parameter of Start-Job:

$jobArgs = @{
    InitializationScript = { $env:FOO = 'BAR' } 
    FilePath             = '.\myscript.ps1'
    ArgumentList         = 'arg1', 'arg2' 
}
Start-Job @jobArgs | Receive-Job -Wait -AutoRemoveJob

Advantages and disadvantages

• You don't have to reset the environment variable after the script finishes (which would require try / finally to do it correctly even in the presence of exceptions).
• The environment variable will be really local to the launched script. It won't affect other, possibly launched in parallel, jobs.
• The script will run in its own, somewhat isolated environment. This means that the launched script can't set variables of the main script, it will have to use Write-Output to communicate back to the main script. This could be an advantage or a disadvantage, depending on the use case.

Making a 'subshell' by invoking powershell with a script block allows you to scope the changes to the environment:

pwsh -Command { $env:MYVAR="myvalue"; .\path\to.exe }