The redirfd program
redirfd redirects a given file descriptor to a file, then executes a program.
Interface
redirfd [ -r | -w | -u | -a | -x ] [ -n ] [ -b ] fd file prog...
redirfd -f [ -r | -w | -u | -a | -x ] [ -n ] [ -b ] [ -e | -E ] var file prog...
redirfd redirects the file descriptor number fd to file, then execs into prog....
When run with the -f option, the first argument is instead interpreted as an environment variable. redirfd opens file, stores the obtained file descriptor into environment variable var, then execs into prog....
Options
One and only one of the -r, -w, -u, -a, 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.
- -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.
- -f : interpret the first argument as an environment variable, and stores the new file descriptor into that variable before execing.
- -e : no autoimport. This is the default.
- -E : autoimport. This is only useful with the -f option. Instead of executing prog..., execute importas -uSi var prog.... This substitutes var into the command line instead of putting it into the environment.
Notes
- redirfd -r n file prog... is roughly equivalent to sh -c 'exec prog... n<file'
- redirfd -w n file prog... is roughly equivalent to sh -c 'exec prog... n>file'
- redirfd -u n file prog... is roughly equivalent to sh -c 'exec prog... n<>file'
- redirfd -a n file prog... is roughly equivalent to sh -c 'exec prog... n>>file'
- redirfd -x n file prog... has no portable shell equivalent.
Special fifo handling
The -n and -b options are especially useful with named pipes.
- Opening a fifo for reading, blocking if there is no writer: redirfd -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: redirfd -r -nb n fifo prog...
- Opening a fifo for writing, blocking if there is no reader: redirfd -w n fifo prog...
- Opening a fifo for writing, with instant success even if there is no reader: redirfd -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 redirfd this way.