For people familiar with unix philosophy, "Everything is a file" is common phrase. Rarely do we get a case to use it in practice. This is one of those cases.

Problem

At work a pipeline which had to be run by End of Day, got stuck. After checking the logs, it looked like it was stuck waiting for a input prompt. The Prompts was suppose to be bypassed by setting environment variable. But recent changes had introduced a flaw.

The patch would take some time and this was a lengthy pipeline. So, we had to manually resume it. But the prompt was not directly available, as this was a process which was deeply nested within pipeline logic.

Solution in theory

"Everything is a file" in unix. This includes directories, devices and even process. We leverage this fact to provide input to the process.

Process are directories under /proc/ directory. Pid is unique number assigned to the process. These are used to name the process directory. Ex: /proc/1234/ is directory representing process with pid 1234.

In each process directory, files opened by the process are stored in fd/ directory. File descriptors are numbers assigned to files unique to that process. These are used to name the files. Ex: /proc/1234/fd/0 is file associated with file descriptors 0 of process with pid 1234.

In normal cases file descriptor one is standard input or the file which is the prompt of the process. This was true in our case.

Note: There are to many interesting thing here to cover but lets concentrate on the solution.

Implemented Solution

To demonstrate, let us use cat program.

1. Run cat in a terminal. It wait for input.
2. Run echo "Hello World" >> "/proc/$(pidof -s cat)/fd/1" to input to cat program.