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The Unix Shell Process Model - When Are Processes Created?

OSH and YSH are both extensions of POSIX shell, and share its underlying "process model".

Each Unix process has its own memory, that is not shared with other processes. (It's created by fork(), which means that the memory is "copy-on-write".)

Understanding when a shell starts processes will make you a better shell programmer.

As a concrete example, here is some code that behaves differently in bash and zsh:

$ bash -c 'echo hi | read x; echo x=$x'
x=

$ zsh -c 'echo hi | read x; echo x=$x'
x=hi

If you understand why they are different, then that means you understand the process model!

(OSH behaves like zsh.)

Related: Interpreter State. These two docs are the missing documentation for shell!

Shell Constructs That Start Processes

Pipelines myproc | wc -l

• shopt -s lastpipe
• set -o pipefail

Note that functions Can Be Transparently Put in Pipelines:

Hidden subshell:

{ echo 1; echo 2; } | wc -l

A SubProgramThunk is started for the LHS of |.

Command Sub d=$(date)

d=$(date)   

Process Sub <(sort left.txt)

diff -u <(sort left.txt) <(sort right.txt)

Async - fork or sleep 2 &

Explicit Subshell - forkwait or ( echo hi )

Explicit Subshells are Rarely Needed.

• prefer pushd / popd, or cd { } in YSH.

FAQ: "Subshells By Surprise"

Sometimes subshells have no syntax.

Common issues:

shopt -s lastpipe

Mentioned in the intro:

$ bash -c 'echo hi | read x; echo x=$x'
x=

$ zsh -c 'echo hi | read x; echo x=$x'
x=hi

Other Pipelines

myproc (&p) | grep foo

Process Optimizations - noforklast

Why does a Unix shell start processes? How many processes are started?

Bugs / issues

• job control:
  • restoring process state after the shell runs
  • sh -i -c 'echo hi'
• traps
  • not run - issue #1853
• Bug with set -o pipefail
  • likewise we have to disable process optimizations for ! false and ! false | true

Oils/YSH specific:

• shopt -s verbose_errexit
• crash dump
  • because we don't get to test if it failed
• stats / tracing - counting exit codes

Process State

Redirects

Builtins

wait

fg

bg

trap

Appendix: Non-Shell Tools

These Unix tools start processes:

• xargs
  • xargs -P starts parallel processes (but doesn't buffer output)
• find -exec
• make
  • make -j starts parallel processes (but doesn't buffer output)
• ninja (buffers output)