| 1 | ---
|
| 2 | in_progress: yes
|
| 3 | ---
|
| 4 |
|
| 5 | Process Model
|
| 6 | =============
|
| 7 |
|
| 8 | Why does a Unix shell start processes? How many processes are started?
|
| 9 |
|
| 10 | Related: [Interpreter State](interpreter-state.html). These two docs are the
|
| 11 | missing documentation for shell!
|
| 12 |
|
| 13 | <div id="toc">
|
| 14 | </div>
|
| 15 |
|
| 16 | ## Shell Constructs That Start Processes
|
| 17 |
|
| 18 | ### Pipelines
|
| 19 |
|
| 20 | - `shopt -s lastpipe`
|
| 21 | - `set -o pipefail`
|
| 22 |
|
| 23 | #### Functions Can Be Transparently Put in Pipelines
|
| 24 |
|
| 25 | Implicit subshell:
|
| 26 |
|
| 27 | { echo 1; echo 2; } | wc -l
|
| 28 |
|
| 29 | A `SubProgramThunk` is started for the LHS of `|`.
|
| 30 |
|
| 31 | ### Command Sub `d=$(date)`
|
| 32 |
|
| 33 | d=$(date)
|
| 34 |
|
| 35 | ### Process Sub `<(sort left.txt)`
|
| 36 |
|
| 37 | diff -u <(sort left.txt) <(sort right.txt)
|
| 38 |
|
| 39 | ### Async - `fork` or `sleep 2 &`
|
| 40 |
|
| 41 | ### Explicit Subshell - `forkwait` or `( echo hi )`
|
| 42 |
|
| 43 | Explicit Subshells are Rarely Needed.
|
| 44 |
|
| 45 | - prefer `pushd` / `popd`, or `cd { }` in YSH.
|
| 46 |
|
| 47 | ## Process Optimizations - `noforklast`
|
| 48 |
|
| 49 | Bugs / issues
|
| 50 |
|
| 51 | - job control:
|
| 52 | - restoring process state after the shell runs
|
| 53 | - `sh -i -c 'echo hi'`
|
| 54 | - traps
|
| 55 | - not run - issue #1853
|
| 56 | - Bug with `set -o pipefail`
|
| 57 | - likewise we have to disable process optimizations for `! false` and
|
| 58 | `! false | true`
|
| 59 |
|
| 60 | Oils/YSH specific:
|
| 61 |
|
| 62 | - `shopt -s verbose_errexit`
|
| 63 | - crash dump
|
| 64 | - because we don't get to test if it failed
|
| 65 | - stats / tracing - counting exit codes
|
| 66 |
|
| 67 | ## Process State
|
| 68 |
|
| 69 | ### Redirects
|
| 70 |
|
| 71 |
|
| 72 | ## Builtins
|
| 73 |
|
| 74 | ### [wait]($help)
|
| 75 |
|
| 76 | ### [fg]($help)
|
| 77 |
|
| 78 | ### [bg]($help)
|
| 79 |
|
| 80 | ### [trap]($help)
|
| 81 |
|
| 82 |
|
| 83 | ## Appendix: Non-Shell Tools
|
| 84 |
|
| 85 | - `xargs` and `xargs -P`
|
| 86 | - `find -exec`
|
| 87 | - `make -j`
|
| 88 | - doesn't do anything smart with output
|
| 89 | - `ninja`
|
| 90 | - buffers output too
|