`xsel`’s default behaviour depends on whether its input or output is connected to a terminal, so redirecting to and from files typically does the right thing. As mentioned by [codeforester][1], the solution in your case is to run

 xsel -b > file

You can make your intent explicit by adding `-o` (when outputting the contents of the clipboard) or `-i` (when inputting to the clipboard). Without these options, if `xsel`’s context is indeterminate (*i.e.* neither standard input or standard output are connected to a terminal), it behaves in `-o` mode: `xsel -b < /dev/null > file` works as you’d expect, but `xsel -b < file > /dev/null` doesn’t.

You can also use `xclip` to copy the clipboard’s contents to a file:

 xclip -sel c -o > file

`xclip` can additionally request specific versions of the clipboard’s contents (known as targets), depending on the selection’s owner; for example, if you copied text from a web browser, you could retrieve it as HTML using

 xclip -sel -c -o -t text/html > file

The special `TARGETS` target will list the available targets:

 xclip -sel -c -o -t TARGETS


 [1]: https://unix.stackexchange.com/users/201820/codeforester