truncate: add page (#2196)

pages/linux/truncate.md

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+# truncate
+
+> Shrink or extend the size of a file to the specified size.
+
+- Set a size of 10 GB to an exsting file, or create a new file with the specified size:
+
+`truncate -s {{10G}} {{filename}}`
+
+- Extend the file size by 50M, fill with holes (which reads as zero bytes):
+
+`truncate -s +{{50M}} {{filename}}`
+
+- Shrink the file by 2GiB, by removing data from the end of file:
+
+`truncate -s -{{2G}} {{filename}`