What are the PowerShell equivalents of Bash's && and || operators?

Solution 1:

Update: && and || have finally come to PowerShell (Core), namely in v7; see this answer.

Many years after the question was first asked, let me summarize the state of affairs as of PowerShell v5.1:

• Bash's / cmd's && and || control operators have NO PowerShell equivalents, and since you cannot define custom operators in PowerShell, there are no good workarounds:

  • Use separate commands (on separate lines or separated with ;), and explicitly test the success status of each command via automatic variable $?, such as:
    command1 -arg1 -arg2; if ($?) { command2 -arg1 } # equivalent of &&
command1 -arg1 -arg2; if (-not $?) { command2 -arg1 } # equivalent of ||

  • See below for why PowerShell's -and and -or are generally not a solution.

• There was talk about adding them a while back, but it seemingly never made the top of the list.

  • Now that PowerShell has gone open-source, an issue has been opened on GitHub.
  • The tokens && and || are currently reserved for future use in PowerShell, so there's hope that the same syntax as in Bash can be implemented.
    (As of PSv5.1, attempting something like 1 && 1 yields error message The token '&&' is not a valid statement separator in this version.)

Why PowerShell's -and and -or are no substitute for && and ||:

Bash's control operators && (short-circuiting logical AND) and || (short-circuiting logical OR) implicitly check the success status of commands by their exit codes, without interfering with their output streams; e.g.:

ls / nosuchfile && echo 'ok'

Whatever ls outputs -- both stdout output (the files in /) and stderr output (the error message from attempting to access non-existent file nosuchfile) -- is passed through, but && checks the (invisible) exit code of the ls command to decide if the echo command - the RHS of the && control operator - should be executed.

ls reports exit code 1 in this case, signaling failure -- because file nosuchfile doesn't exist -- so && decides that ls failed and, by applying short-circuiting, decides that the echo command need not be executed.
Note that it is exit code 0 that signals success in the world of cmd.exe and bash, whereas any nonzero exit code indicates failure.

In other words: Bash's && and || operate completely independently of the commands' output and only act on the success status of the commands.

PowerShell's -and and -or, by contrast, act only on the commands' standard (success) output, consume it and then output only the Boolean result of the operation; e.g.:

(Get-ChildItem \, nosuchfile) -and 'ok'

The above:

• uses and consumes the success (standard) output -- the listing of \ -- and interprets it as a Boolean; a non-empty input collection is considered $true in a Boolean context, so if there's at least one entry, the expression evaluates to $true.

  • However, the error information resulting from nonexistent file nosuchfile is passed through, because errors are sent to a separate stream.

• Given that Get-ChildItem \, nosuchfile returns non-empty success output, the LHS evaluated to $true, so -and also evaluates the RHS, 'ok', but, again, consumes its output and interprets it as a Boolean, which, as a nonempty string, also evaluates to $true.

• Thus, the overall result of the -and expression is $true, which is (the only success) output.

The net effect is:

• The success output from both sides of the -and expression is consumed during evaluation and therefore effectively hidden.

• The expression's only (success) output is its Boolean result, which is $true in this case (which renders as True in the terminal).

Solution 2:

What Bash must be doing is implicitly casting the exit code of the commands to a Boolean when passed to the logical operators. PowerShell doesn't do this - but a function can be made to wrap the command and create the same behavior:

> function Get-ExitBoolean($cmd) { & $cmd | Out-Null; $? }

($? is a bool containing the success of the last exit code)

Given two batch files:

#pass.cmd
exit

and

#fail.cmd
exit /b 200

...the behavior can be tested:

> if (Get-ExitBoolean .\pass.cmd) { write pass } else { write fail }
pass
> if (Get-ExitBoolean .\fail.cmd) { write pass } else { write fail }
fail

The logical operators should be evaluated the same way as in Bash. First, set an alias:

> Set-Alias geb Get-ExitBoolean

Test:

> (geb .\pass.cmd) -and (geb .\fail.cmd)
False
> (geb .\fail.cmd) -and (geb .\pass.cmd)
False
> (geb .\pass.cmd) -and (geb .\pass.cmd)
True
> (geb .\pass.cmd) -or (geb .\fail.cmd)
True