The `<` program

< redirects a given file descriptor to a file, then executes a program.

Interface

     < [ -r | -w | -u | -a | -c | -x ] [ -n | -b ] fd file prog...

< redirects the file descriptor number fd to file, then execs into prog....

One and only one of the -r, -w, -u, -a, -c, or -x options must be given; the -n and -b options may be added in any case.

-r : open file for reading.
-w : open file for writing, truncating it if it already exists.
-u : open file for reading and writing.
-a : open file for appending, creating it if it doesn't exist.
-c : open file for appending. Do not create it if it doesn't exist.
-x : open file for writing, creating it, failing if it already exists.
-n : open file in non-blocking mode.
-b : change mode of file after opening it: to non-blocking mode if the -n option was not given, to blocking mode if it was.

< -r n file prog... is roughly equivalent to sh -c 'exec prog... n<file'
< -w n file prog... is roughly equivalent to sh -c 'exec prog... n>file'
< -u n file prog... is roughly equivalent to sh -c 'exec prog... n<>file'
< -a n file prog... is roughly equivalent to sh -c 'exec prog... n>>file'
< -c n file prog... has no portable shell equivalent. Some shells provide the noclobber option for a similar feature.
< -x n file prog... has no portable shell equivalent.

The -n and -b options are especially useful with named pipes.

Opening a fifo for reading, blocking if there is no writer: < -r n fifo prog...
Opening a fifo for reading, with instant success even if there is no writer, and blocking at the first attempt to read from it: < -r -nb n fifo prog...
Opening a fifo for writing, blocking if there is no reader: < -w n fifo prog...
Opening a fifo for writing, with instant success even if there is no reader: < -w -nb n fifo prog.... Warning: the first attempt to write to the fifo will raise a SIGPIPE if there is still no reader at that time. The named pipe semantics normally do not allow a fifo to be open for writing without a reading end, and you should know what you are doing if you're using < this way.