Create tr.md

tr is a small utility for translating characters in text documents

pages/common/tr.md

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+# tr
+
+> translate characters - run replacements based on single characters
+
+- replace all occurrences of a character in a file, and print the result
+
+`tr {{find_character}} {{replace_character}} < {{filename}}`
+
+ Translate the contents of the file to upper-case, prints result.
+
+`tr "[:lower:]" "[:upper:]" < {{filename}}`
+
+Strip out non-printable characters from the file, prints result.
+
+`tr -cd "[:print:]" < {{filename}}`