To check the running Windows installation, open Command Prompt and run compact.exe /CompactOS:query. To check an offline Windows installation, such as one viewed from Windows PE, include its Windows directory: compact.exe /CompactOS:query /WinDir:E:Windows. Replace that path with the actual location of the offline installation.
Check Compact OS on the running Windows installation
- Open Command Prompt.
- Run
compact.exe /CompactOS:query. - Read the result to see whether the system is in Compact OS state.
Microsoft documents /CompactOS as an option that sets or queries the system’s compression state. Use query for a status check: Microsoft’s compact command reference lists always and never as state-changing options, not alternatives for checking status.
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Query an offline Windows installation
If the Windows installation is not currently running—for example, you are inspecting it from Windows PE—specify the path to that installation’s Windows directory:
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Replace E:Windows with the correct directory. Microsoft documents /WinDir for querying an offline operating system and provides an example in its Compact OS documentation.
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Measure compression for a selected set of files
The compact command can report compression information for a directory or files. To include subdirectories, use /S; to include hidden and system files, use /A. For example, to report recursively for a chosen directory while including hidden and system files, run:
compact.exe /S /A "C:pathtofolder"
Replace the example path with the directory you want to examine. The reported figures describe the files within that selected scope and the options you used. They are not an official, complete Compact OS savings total for the entire Windows installation. The /CompactOS:query status check answers a different question: whether the system is using Compact OS.
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Interpret a disk-space figure cautiously
Microsoft’s documentation does not establish a universal number of gigabytes saved by Compact OS or a standard method for measuring savings across a whole installation. A file-compression report can help describe a particular tree, but its totals depend on which paths and files are included. Hidden and system-file inclusion, in particular, changes the report’s scope.
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What the status result does—and does not—tell you
Compact OS keeps operating system files compressed. Microsoft notes that performance depends on the relative performance of the device’s processor and storage, so a Compact OS status result alone does not predict the effect on a particular PC. The query is a state check, not a performance test or a measurement of total disk space recovered.
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