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free - Display amount of free and used memory in the system
Free displays the total amount of free and used physical and swap memory in the system, as well as the buffers and caches used by the kernel.
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du - Estimate file space usage
Amount of disk space used by the specified files or folders
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df - Report file system disk space usage
Displays the amount of disk space available on the file system containing each file name argument. If no file name is given, the space available on all currently mounted file systems is shown.