Redirecting STDIN

\< reads from its right argument and writes to its left argument.

To write a file into STDIN we should read /tmp/a_file and write into STDIN i.e 0</tmp/a_file

Note: Internal file descriptor defaults to 0 (STDIN) for \<

$ echo "b" > /tmp/list.txt
$ echo "a" >> /tmp/list.txt
$ echo "c" >> /tmp/list.txt
$ sort < /tmp/list.txt
a
b
c