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Process Model

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

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

Shell Constructs That Start Processes

Pipelines

• shopt -s lastpipe
• set -o pipefail

Functions Can Be Transparently Put in Pipelines

Implicit 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.

Process Optimizations - noforklast

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

• xargs and xargs -P
• find -exec
• make -j
  • doesn't do anything smart with output
• ninja
  • buffers output too