xsel: Add page (#882)

pages/linux/xsel.md

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+# xsel
+
+> Selection manipulation tool.
+> For example, xsel can copy STDIN to clipboard, or print clipboard to STDOUT.
+
+- Clipboard selection:
+
+`-b or --clipboard`
+
+- Primary selection:
+
+`-p or --primary`
+
+- Secndary selection:
+
+`-s or --secondary`
+
+- Copy output of a command into the clipboard:
+
+`echo 123 | xclip -bi`
+
+- Copy contents of a file into the clipboard:
+
+`cat {{file}} | xsel -bo`
+
+- Print the clipboard to STDOUT:
+
+`xsel -bo`
+
+- Print the clipboard into a file:
+
+`xsel -bo > {{file}}`
+
+- Clear the clipboard:
+
+`xsel -bc`