Exploring Expect

Example—unbuffer

Most non-interactive programs behave differently depending on whether their output goes to the terminal or is redirected. In particular, output to a terminal normally appears as soon as a full line is produced. In contrast, output that is redirected to a file or a process is buffered by much larger amounts in the name of efficiency. This difference in buffering is automatically chosen by the UNIX stdio system.

Unfortunately, this means that some simple UNIX commands do not work as nicely as you might expect. For example, suppose a slow source is sending output to a fifo called /tmp/fifo and you want to read it using od and then pipe it into a pager such as more. The obvious shell command to do this is:

od -c /tmp/fifo | more

Alas, the stdio system compiled into od sees that its output is a pipe so the output is automatically buffered. Even if od receives a complete line, od does not send anything down the pipe until the buffer has been filled.

There is no way to fix od short of modifying and recompiling it. However, by using Expect, it is possible to make od think that its output is destined for a terminal. Since Expect connects processes to a pty, this is sufficient to satisfy the stdio system, and it changes to line-buffered I/O.

A script to do this is simple. All it has to do is spawn the process and wait for it to finish. Here is a script which does this, called unbuffer:

#/usr/local/bin/expect — # Name: unbuffer # Description: unbuffer stdout of a program eval ...