wiki.bash-hackers.org/scripting/processtree.md

Bash and the process tree


The process tree

The processes in UNIX(r) are - unlike other systems - organized as a tree. Every process has a parent process that started, or is responsible, for it. Every process has its own context memory (Not the memory where the process stores its data, rather, the memory where data is stored that doesn't directly belong to the process, but is needed to run the process) i.e. The environment.

Every process has its own environment space.

The environment stores, among other things, data that's useful to us, the environment variables. These are strings in common NAME=VALUE form, but they are not related to shell variables. A variable named LANG, for example, is used by every program that looks it up in its environment to determinate the current locale.

Attention: A variable that is set, like with MYVAR=Hello, is not automatically part of the environment. You need to put it into the environment with the bash builtin command export:

export MYVAR

Common system variables like PATH or HOME are usually part of the environment (as set by login scripts or programs).

Executing programs

All the diagrams of the process tree use names like "xterm" or "bash", but that's just to make it easier to understand what's going on, it doesn't mean those processes are actually executed.

Let's take a short look at what happens when you "execute a program" from the Bash prompt, a program like "ls":

$ ls

Bash will now perform two steps:

• It will make a copy of itself
• The copy will replace itself with the "ls" program

The copy of Bash will inherit the environment from the "main Bash" process: All environment variables will also be copied to the new process. This step is called forking.

For a short moment, you have a process tree that might look like this...

xterm ----- bash ----- bash(copy)

...and after the "second Bash" (the copy) replaces itself with the ls program (the copy execs it), it might look like

xterm ----- bash ----- ls

If everything was okay, the two steps resulted in one program being run. The copy of the environment from the first step (forking) becomes the environment for the final running program (in this case, ls).

What is so important about it? In our example, what the program ls does inside its own environment, it can't affect the environment of its parent process (in this case, bash). The environment was copied when ls was executed. Nothing is "copied back" to the parent environment when ls terminates.

Bash playing with pipes

Pipes are a very powerful tool. You can connect the output of one process to the input of another process. We won't delve into piping at this point, we just want to see how it looks in the process tree. Again, we execute some commands, this time, we'll run ls and grep:

$ ls | grep myfile

It results in a tree like this:

                   +-- ls
xterm ----- bash --|
                   +-- grep

Note once again, ls can't influence the grep environment, grep can't influence the ls environment, and neither grep nor ls can influence the bash environment.

How is that related to shell programming?!?

Well, imagine some Bash code that reads data from a pipe. For example, the internal command read, which reads data from stdin and puts it into a variable. We run it in a loop here to count input lines:

counter=0

cat /etc/passwd | while read; do ((counter++)); done
echo "Lines: $counter"

What? It's 0? Yes! The number of lines might not be 0, but the variable $counter still is 0. Why? Remember the diagram from above? Rewriting it a bit, we have:

                   +-- cat /etc/passwd
xterm ----- bash --|
                   +-- bash (while read; do ((counter++)); done)

See the relationship? The forked Bash process will count the lines like a charm. It will also set the variable counter as directed. But if everything ends, this extra process will be terminated - your "counter" variable is gone. You see a 0 because in the main shell it was 0, and wasn't changed by the child process!

So, how do we count the lines? Easy: Avoid the subshell. The details don't matter, the important thing is the shell that sets the counter must be the "main shell". For example:

counter=0

while read; do ((counter++)); done </etc/passwd
echo "Lines: $counter"

It's nearly self-explanatory. The while loop runs in the current shell, the counter is incremented in the current shell, everything vital happens in the current shell, also the read command sets the variable REPLY (the default if nothing is given), though we don't use it here.

Actions that create a subshell

Bash creates subshells or subprocesses on various actions it performs:

Executing commands

As shown above, Bash will create subprocesses everytime it executes commands. That's nothing new.

But if your command is a subprocess that sets variables you want to use in your main script, that won't work.

For exactly this purpose, there's the source command (also: the dot . command). Source doesn't execute the script, it imports the other script's code into the current shell:

source ./myvariables.sh
# equivalent to:
. ./myvariables.sh

Pipes

The last big section was about pipes, so no example here.

Explicit subshell

If you group commands by enclosing them in parentheses, these commands are run inside a subshell:

(echo PASSWD follows; cat /etc/passwd; echo GROUP follows; cat /etc/group) >output.txt

Command substitution

With command substitution you re-use the output of another command as text in your command line, for example to set a variable. The other command is run in a subshell:

number_of_users=$(cat /etc/passwd | wc -l)

Note that, in this example, a second subshell was created by using a pipe in the command substitution:

                                            +-- cat /etc/passwd
xterm ----- bash ----- bash (cmd. subst.) --|
                                            +-- wc -l