Update cheatsheets

sgitopnm

@@ -0,0 +1,21 @@
+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# sgitopnm
+
+> Convert an SGI file to a PNM file.
+> More information: <https://netpbm.sourceforge.net/doc/sgitopnm.html>.
+
+- Convert an SGI image to a PNM file:
+
+`sgitopnm {{path/to/input.sgi}} > {{path/to/output.pnm}}`
+
+- Display information about the SGI file:
+
+`sgitopnm -verbose {{path/to/input.sgi}} > {{path/to/output.pnm}}`
+
+- Extract channel n of the SGI file:
+
+`sgitopnm -channel {{n}} {{path/to/input.sgi}} > {{path/to/output.pnm}}`