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PowerShell Switch

Introduction to Windows PowerShell's Switch Command

Whenever I add a 'Switch' clause to my script, I think - 'good job'.  And I follow up by saying: 'Why don't I use the Switch construction more often?'  As with other scripting languages, PowerShell provides a variety of commands to perform branching logic.  For simple cases, with few options, the 'If' construction works well.  The difficult arises when 'If' becomes a victim of its own success and you have 5 or 6 options, for that situation it is more efficient to use PowerShell's 'Switch' command.  Incidentally, the equivalent of 'Switch' in VBScript is 'Select Case'.

Topics for PowerShell's Switch Command

• The Basic Structure of PowerShell's Switch Command
• Case Study - WMI Disk
• Learning more about PowerShell's Switch command

It is likely that your real-life task for Switch will be trickier than the following simple examples.  However, it is worth studying a range of basic examples to get a feel for the structure and the rhythm of the command.

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The Basic Structure of PowerShell's Switch Command

A curious feature of the construction is that the word 'Switch' introduces the input, and is then never seen again, all that you see thereafter is rows of patterns and matching {Statement Blocks}.  Also observe that there is an extra pair of {braces} surrounding the whole pattern section.

The layout below emphasises the branches, or the multiple 'Patterns' whose values get switched to their respective {Blocks}.

Switch (pipeline) {

Pattern 1 {Statement block}
Pattern 2 {Statement block}
Pattern n {Statement block}

}

For simple examples, you could write the Switch command all on one line:

Switch (3) { 1{ "Red" } 2{ "Yellow" } 3{ "Green" } }

Result Green.  PowerShell switches the input 3 for Green.

Learning Points

Note 1: Trace the overall structure of the Switch command:
Switch (Value, or Pipeline in parenthesis) {Actions in braces}

Note 2: The "Block" for each Switch is enclosed not only by {braces}, but also by speech marks { "Yellow" }.

Note 3: Remember to pair the initial { brace, with a final matching brace, even if the whole structure spans multiple lines.}

Case Study - WMI Disk

A difficulty occurred when we interrogated the computer's disks with WMI.  Specifically, the output reports DriveType as a number, whereas we want a meaningful name for the type of disk.  The extra step in this example was to research which drive type corresponded to which number, for example if $Drive.DeviceID returns a value of 3, that means the disk type is a "Hard drive".

Case Study 1 - Problem with output

We want an answer to the question: 'What does the DriveType number mean?'  It would be useful if the script gave us a name rather than a number.  Research reveals that there are at least 5 possible disk types, therefore multiple 'If' statements would be cumbersome, 'Switch' is more elegant.

Case Study 2 - Solution with 'Switch'

Learning Points

Note 1:  Before examining the solution in Case Study 2, take the time to understand the underlying WmiObject construction in Case Study 1.  In particular observe the format:
foreach (condition) {Block Command}.

Note 2: The case study solution builds on the first script by adding the Switch block.  To my way of thinking, there are two loops, the outer foreach and an inner Switch loop.  Check what I mean by matching the last two {lonely} braces with their opening counterparts.

Challenges

Challenge 1:  Just to get experience and control of the script try changing "Hard Drive" to "Fixed Disk"

Challenge 2:  Create a mapped network drive, then run the script again.  Launch Windows Explorer then click on the Tools menu, this is the easiest way to map a local drive letter to a UNC share.

Learning more about PowerShell's Switch command

When I wanted help about the switch command, at first PowerShell's help did not do what I wanted.  The secret was to add the about_ prefix.  Try:
help about_switch

What the about_switch help file reveals is that you could use wildcards in the matching clause.

Incidentally, PowerShell supports a whole family of about_ commands, for example you could try:
help about_foreach

Case Study 2 - Featuring Switch with -wildcard

Case Study 2 - Count the types of error message in the Application eventlog

Note 1:  Observe the -wildcard switch with err*, info* and warn*.  To test the effect, try removing the -wildcard parameter.

Summary of PowerShell's 'Switch' command

The 'If' family are easy to use for scripts that require branching logic.  However, when the number of options exceeds about 5, then 'Switch' is easier to code and easier to add more options.  By trying a few simple examples you will soon appreciate the types of bracket, and the structure of the pattern with its matching statement block.  I say again, 'Switch' is one of the most satisfying constructions to create, therefore never miss a chance to replace multiple 'If's with one 'Switch'.

See more Windows PowerShell examples

• PowerShell Home  • If Statement  • Conditional Operators  • Switch  • Loops  • Brackets

Please write in if you see errors of any kind.  Please report any factual mistakes, grammatical errors or broken links, I will be happy to not only to correct the fault, but also to give you credit.

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