Bash Compatibility & Scripting¶

hellish runs real scripts — not a toy REPL. Status legend: ✅ shipped · 🚧 in progress · 📋 planned

The proof: it built a Linux distribution¶

The strongest compatibility claim a shell can make isn't a feature list — it's running real-world scripts. hellish was used as the build-script interpreter for a complete Linux From Scratch build: it drove ./configure, autotools, gcc/glibc/binutils, and ~80 packages, producing a disk image that boots (uname -r → 6.6.32-dlesieur, network up, SysVinit, clean shutdown).

Most "write your own shell" projects die on the first ./configure. hellish built an OS. That's the bar this page documents against.

Feature matrix vs. bash¶

Area	Feature	Status
Control flow	`if`/`for`/`while`/`until`/`case`, functions, `&&`/`\\|\\|`/`;`/`!`	✅
Pipelines/redir	pipes, `>`/`>>`/`<`, `2>&1`, fd dup, subshells `( )`, groups `{ }`	✅
Conditionals	`[[ … ]]` (string/int/file/unary, `&&`/`\\|\\|`/`!`/`( )`, `==`), `[ ]`/`test`	✅
Substitution	`$(…)` / backticks, arithmetic `$(( … ))` (`++ -- += ?: ,` , bases)	✅
Heredocs	`<<`, `<<-`, quoted & unquoted, `${VAR}` bodies, consecutive, in-function	✅
Process subst.	`<(…)`, `>(…)`, `exec > >(tee …)`	✅
Parameter exp.	`${v:-w}` `${v:+w}` `${#v}` `${v#p}` `${v%p}`, substring, positional	✅
Brace expansion	`{a,b,c}`, `{1..n}`, with `$var`/quoted prefixes	✅
Globbing	`*` `?` `[…]`, sorted matches, no-match passthrough	✅
Options/traps	`set -o pipefail`, `set -e`/`-u`/`-o`, `trap` (EXIT/INT/TERM)	✅
Job control	`bg`/`fg`/`jobs`/`kill`, `&`, dirstack `pushd`/`popd`/`dirs`	✅
Builtins	`getopts`, `read`, `printf`, `alias`/`unalias`, `hash`, `type`, `local`, …	✅
Arrays	indexed `a=(…)`, `a[i]=v`, `${a[i]}`, `${a[@]}`, `declare -a`	🚧 (Wk2–3)
`set -e`	abort on failure in a multi-stage pipeline (simple commands ✅)	🚧 (Wk3)
Assoc. arrays	`declare -A`, string keys	📋
Pattern/misc	`${v/pat/repl}`, `var+=value` (string append), `extglob`, `[[ str == glob ]]` RHS, `mapfile`/namerefs/`coproc`	📋

Honesty matters more than a green wall: the 🚧/📋 rows are real gaps, tracked openly. The ✅ rows are each locked behind a regression test that's diffed against bash --posix.

Arrays — the headline of this cycle 🚧¶

Arrays are the single biggest compatibility unlock (a huge fraction of real bash scripts use them), and they're the current focus.

# Target syntax (Weeks 2–3)
declare -a fruits=(apple banana cherry)
fruits[3]=date
echo "${fruits[@]}"      # apple banana cherry date
echo "${#fruits[@]}"     # 4
echo "${!fruits[@]}"     # 0 1 2 3
fruits+=(elderberry)     # append
for f in "${fruits[@]}"; do echo "$f"; done   # one field per element, even quoted

Why it's a real change, not a bolt-on: hellish's variable store is currently one string per name. Arrays add a value-vector + type tag to that store, subscript parsing on the assignment side, and @/* handling (with the subtle ""${a[@]}" splits to one field per element even when quoted" rule) on the expansion side — plus a declare/typeset builtin. The good news: bash arrays are shell-local (never exported to the environment), so the execve path is untouched.

How compatibility is guaranteed¶

Every fixed construct gets a permanent regression case, and a conformance gate diffs each construct under hellish vs. bash --posix:

make -C vendor/42sh test          # the full suite (2481+ cases, growing)
HELLISH=… bash scripts/conformance.sh   # construct-by-construct vs bash → 0 divergences

If a behavior diverges from bash, it shows up here before it ships. That's the contract behind every ✅ above.