P-1.4 Files, Processes and What the Operating System Does

Work through these

• A process: one running program, with its own memory and its own view of the machine

  Starting a program creates a process, which the operating system gives its own memory and its own handles on files. Two runs of the same program share nothing by default, which is what makes them safe to run side by side.

• The file system, absolute paths and relative paths

  A relative path is resolved against the directory the process happens to be in, so the same string names different files depending on where it was run. A surprising share of first deployment failures are exactly this.

• Standard input, standard output and standard error

  Every process starts with three streams already open, which is why programs can be chained together without knowing about each other. Sending diagnostics to the error stream keeps them out of the data another program is reading.

  The Linux command line for beginners · Tutorial

• Exit codes: how a program says whether it worked

  A process ends with a small number, zero meaning success, and every script and build system in existence is reading it. A program that fails while reporting zero will be believed.

• Environment values and why configuration lives in them

  The operating system hands each process a set of named values it can read, which is how settings reach a program without being written into it. It is also how secrets reach a program without being committed alongside the code.