Output file doesn’t match Write-Host

Caveat:

• Write-Host is meant for to-display output, not for outputting data – it bypasses PowerShell’s success output stream (PowerShell’s stdout equivalent), so that output from Write-Host cannot (directly^[1]) be captured in a variable, nor redirected to file – see the bottom half of this answer for more information.

• Use Write-Output or – preferably – PowerShell’s implicit output behavior to output data, suitable for further programmatic processing.

In addition to this fundamental difference, Write-Host and Write-Output also differ in how they handle arguments:

# What Write-Host prints to the display is a *single string* that is 
# the space-separated list of the (stringification of) its arguments.
PS> Write-Host file1, '>>', file2
file1 >> file2  # printed to *display* only

# Write-Output outputs each argument - whatever its data type - *separately*
# to the success output stream.
# In the case of *string* arguments, each string renders *on its own line*.
PS> Write-Output file1, '>>', file2
file1
>>
file2

Using implicit output, the equivalent of the above Write-Output command is:

# Send an array of 3 strings to the success stream.
PS> 'file1', '>>', 'file2'
file1
>>
file2

If you redirect the Write-Output command or its implicit equivalent to a file (with > / Out-File or Set-Content^[2]), you’ll get the same 3-line representation.

Additionally, Write-Host performs simple .ToString() stringification on complex objects, which often results in unhelpful output; by contrast, Write-Output / implicit output uses PowerShell’s rich formatting system:

# Write-Host: Unhelpful representation; entries are enumerated
#             and .ToString() is called on each.
PS> Write-Host @{ foo = 1; bar = 2 }
System.Collections.DictionaryEntry System.Collections.DictionaryEntry

# Write-Output / implicit output: rich formatting
PS> @{ foo = 1 }

Name                           Value
----                           -----
foo                            1
bar                            2

Note: If you use the Out-Host cmdlet and pipe a command to it (e.g.
@{ foo = 1; bar = 2 } | Out-Host) you do get the usual, rich output formatting that you get via Write-Output / by default – while still printing to the display only.

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If you do want to output a single line as data, use a quoted string; to reference variables and embed subexpressions in a string, use an expandable string (string interpolation), "...” (see about_Quoting_Rules), or use string concatenation (+)

$arg1 = 'file1'; $arg2 = 'file2'

# Expandable string
PS> "$arg1 >> $arg2"
file1 >> file2

# String concatenation
PS> $arg1 + ' >> ' + $arg2
file1 >> file2

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^{[1] In PowerShell v5 or higher, you can capture/redirect Write-Host output, via the information output stream, number 6; e.g.: $writeHostOutput = & { Write-Host hi } 6>&1. However, note that the information output stream is primarily designed to work with the PSv5+ Write-Information cmdlet and the common -InformationAction parameter.}

^{[2] With strings only, > / Out-File behave the same as Set-Content, except that in Windows PowerShell a different default character applies (not in PowerShell Core). For more information about the differences, see this answer.}