bash Compatibility Reference

June 6, 2026 · View on GitHub

This document details brush's compatibility with bash, including supported features, known limitations, and how to report issues.

Overview

brush aims for high compatibility with bash. We validate this through 1700+ compatibility test cases that compare behavior against bash as an oracle.

Compatibility snapshot: Production-ready for most use cases. Your .bashrc, aliases, functions, and completions should "just work."

Fully Supported Features ✅

Shell Syntax & Control Flow

  • if/then/elif/else/fi conditionals
  • for, while, until loops
  • Arithmetic for loops: for ((i=0; i<10; i++))
  • case/esac pattern matching
  • &&, || conditional execution
  • Subshells () and command grouping {}
  • Pipelines and pipeline negation !
  • Coprocesses: coproc { ... } and coproc NAME { ... }

Expansions

  • Brace expansion: {a,b,c}, {1..10}, {a..z}
  • Parameter expansion: ${var:-default}, ${var:+set}, ${var#pattern}, ${var%pattern}, ${var//find/replace}, etc.
  • Command substitution: $(cmd), `cmd`
  • Arithmetic expansion: $((expr))
  • Process substitution: <(cmd), >(cmd)
  • Tilde expansion: ~, ~user
  • Globbing: *, ?, [...]
  • Extended globbing: ?(pat), *(pat), +(pat), @(pat), !(pat)
  • globstar: ** recursive matching

Builtins (50+)

  • I/O: echo, printf, read, mapfile/readarray
  • Variables: declare, local, export, unset, readonly, typeset
  • Control: break, continue, return, exit
  • Navigation: cd, pushd, popd, dirs, pwd
  • Jobs: jobs, fg, bg, wait, kill
  • Completion: complete, compgen, compopt
  • History: history, fc
  • Testing: test, [, [[
  • Sourcing & introspection: ., source, eval, caller
  • Misc: alias, unalias, hash, type, command, builtin, enable, help, times, ulimit, umask, trap, shopt, set, shift, getopts

Arrays

  • Indexed arrays: arr=(a b c), ${arr[0]}, ${arr[@]}
  • Associative arrays: declare -A map
  • Array slicing: ${arr[@]:start:length}
  • Array operations: ${#arr[@]}, ${!arr[@]}, ${!arr[*]}

Job Control

  • Background execution: cmd &
  • Suspend/resume: Ctrl+Z, fg, bg
  • Job listing: jobs
  • Process groups and pipelines

Dynamic Variables

  • RANDOM, SRANDOM
  • LINENO, FUNCNAME, BASH_SOURCE
  • EPOCHSECONDS, EPOCHREALTIME
  • SECONDS
  • PWD, OLDPWD
  • BASH_VERSINFO, BASH_VERSION

Programmable Completion

  • Compatible with bash-completion
  • Git, Docker, systemctl, etc. completions work out of the box
  • complete, compgen, compopt builtins

Redirection

  • Standard: >, >>, <, 2>&1
  • Here documents: <<EOF, <<-EOF (tab-stripped), <<< (here strings)
  • File descriptor manipulation: >&n, <&n, n>&m
  • Process substitution redirects: >(cmd), <(cmd)
  • Clobber control: >|, set -o noclobber

Partially Supported Features 🔷

Traps

StatusFeature
✅DEBUG trap
✅ERR trap
✅EXIT trap
🔷Signal traps (SIGINT, SIGTERM, etc.) — in progress

Key Bindings (bind)

StatusFeature
✅Basic bind support
✅bind -x for custom key-bound commands
🔷Advanced bind features — in progress

Shell Options

StatusFeature
✅Common options: errexit, pipefail, extglob, globstar, noclobber, nounset, failglob
🔷Less common options — in progress

Not Yet Supported Features 🚧

These features are on our roadmap but not yet implemented:

select Statement

The select builtin for creating menu-driven scripts is not yet implemented.

# Not yet supported
select opt in "Option A" "Option B" "Quit"; do
    case $opt in
        "Option A") echo "A";;
        "Option B") echo "B";;
        "Quit") break;;
    esac
done

wait -n

The wait -n option to wait for the next background job to complete is not implemented.

# Not yet supported
job1 &
job2 &
wait -n  # Wait for whichever finishes first

$! for Background Commands

Bash expands $! to the process ID of the most recent asynchronous command. Brush currently does not reliably provide this value for background commands, so scripts that start a job with cmd & and immediately use $! may see an empty value.

# Not yet fully supported
sleep 10 &
kill "$!"

BASH_COMMAND Variable

The special variable BASH_COMMAND that contains the currently executing command is currently only available in trap contexts.

disown and logout

These job control builtins are not yet implemented.

Known Edge Cases

These areas have known differences from bash in edge cases. Most users won't encounter these, but they're documented for completeness.

IFS (Input Field Separator)

There are ~10 known edge cases where IFS word splitting behavior differs from bash, particularly around:

  • Non-whitespace IFS characters and empty field creation
  • Mixed whitespace and non-whitespace IFS
  • Leading/trailing delimiter handling

printf Format Specifiers

Some advanced printf format specifiers behave differently (~8 known cases), particularly features not supported by the underlying uucore library.

Arithmetic Expressions

  • Division by zero handling may differ in errexit mode
  • $(( exit N )) syntax edge case

Aliases

Some complex alias expansion scenarios differ from bash (see GitHub issues #57, #286).

Test Suite Statistics

  • Total test cases: 1700+
  • Known failures: ~125
  • Most failures are edge cases in IFS handling and printf

The test suite runs on every PR and compares behavior against bash as an oracle.

Version Compatibility

brush targets compatibility with bash 5.3+. Behavior may differ from older bash versions (3.x, 4.x) in some areas.

Reporting Compatibility Issues

Found a script that works in bash but not in brush?

  1. Check existing issues: GitHub Issues
  2. Create a minimal reproducer: Reduce to the smallest failing script
  3. File an issue with:
    • The script or command that fails
    • Expected behavior (what bash does)
    • Actual behavior (what brush does)
    • Your platform (Linux/macOS/etc.)

Tracking Progress

  • GitHub Issues: Track specific compatibility work
  • Test Suite: 1700+ tests run on every PR
  • This Document: Updated as features are implemented