Add pv.md.

pages/common/pv.md

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+# pv
+
+> Monitor the progress of data through a pipe.
+
+- Print the contents of the file and display a progress bar:
+
+`pv {{file}}`
+
+- Measure the speed and amount of data flow between pipes (`-s` is optional):
+
+`command1 | pv -s {{expected_amount_of_data_for_eta}} | command2`
+
+- Filter a file, see both progress and amount of output data:
+
+`pv -cN in {{big_text_file}} | grep {{pattern}} | pv -cN out > {{filtered_file}}`
+
+- Attach to an already running process and see its file reading progress:
+
+`pv -d {{PID}}`
+
+- Read an erroneous file, skip errors as `dd conv=sync,noerror` would:
+
+`pv -EE {{path_to_faulty_media}} > image.img`
+
+- Stop reading after reading specified amount of data, rate limit to 1K/s:
+
+`pv -L 1K -S {{maximum_file_size_to_be_read}}`