A simulated bash environment with an in-memory virtual filesystem, written in Elixir.

Designed for AI agents that need a secure, sandboxed bash environment.

Supports optional network access via curl and wget with HTTPS-only enforcement and host allowlists.

Note: This is an Elixir port of just-bash by Vercel. The entire codebase was generated through conversational prompting with Claude Opus 4.5 via OpenCode.

JustBash treats shell code as untrusted and sandboxes it in memory. Custom commands passed via :commands are trusted host-side extensions supplied by the library caller, and JustBash does not sandbox them or provide safety guarantees for them.

• The shell only has access to the provided virtual filesystem
• No access to the real filesystem by default
• No network access by default
• Network access can be enabled with host allowlists — HTTPS-only by default
• Custom commands are outside the sandbox and can bypass the virtual filesystem and network policy

def deps do
  [{:just_bash, "~> 0.1.0"}]
end

bash = JustBash.new()
{_result, bash} = JustBash.exec(bash, ~s(echo "Hello" > greeting.txt))
{result, _bash} = JustBash.exec(bash, "cat greeting.txt")
result.stdout  #=> "Hello\n"
result.exit_code  #=> 0

bash = JustBash.new(
  files: %{"/data/file.txt" => "content"},  # Initial files
  env: %{"MY_VAR" => "value"},              # Environment variables
  cwd: "/app"                                # Starting directory
)

Network access is disabled by default. When enabled, only HTTPS is permitted and an explicit allowlist is required:

# Allow specific hosts (HTTPS only)
bash = JustBash.new(
  network: %{
    enabled: true,
    allow_list: ["api.github.com", "*.example.com"]
  }
)

# Allow all hosts
bash = JustBash.new(
  network: %{enabled: true, allow_list: :all}
)

# Also allow plain HTTP (not recommended)
bash = JustBash.new(
  network: %{enabled: true, allow_list: :all, allow_insecure: true}
)

# Custom HTTP client for testing
bash = JustBash.new(
  network: %{enabled: true, allow_list: :all},
  http_client: MyMockHttpClient
)

Custom commands are trusted extensions supplied by the library caller, not untrusted shell input. JustBash does not sandbox them and does not provide safety guarantees for them.

Register trusted host-side commands with commands::

defmodule MyApp.Commands.Greet do
  @behaviour JustBash.Commands.Command

  @impl true
  def names, do: ["greet", "hello"]

  @impl true
  def execute(bash, args, _stdin) do
    name = Enum.join(args, " ")
    {%{stdout: "Hello, #{name}!\n", stderr: "", exit_code: 0}, bash}
  end
end

bash = JustBash.new(commands: %{"greet" => MyApp.Commands.Greet})
{result, _bash} = JustBash.exec(bash, "hello world")
result.stdout  #=> "Hello, world!\n"

Pass caller data into custom commands with the :context option. It is stored on the JustBash struct as context (default %{}) and is readable inside any custom command as bash.context. Builtins and the interpreter ignore it.

defmodule MyApp.Commands.Whoami do
  @behaviour JustBash.Commands.Command

  @impl true
  def names, do: ["whoami_ctx"]

  @impl true
  def execute(bash, _args, _stdin) do
    user = Map.get(bash.context, :user, "anonymous")
    {%{stdout: "#{user}\n", stderr: "", exit_code: 0}, bash}
  end
end

bash =
  JustBash.new(
    context: %{user: "alice"},
    commands: %{"whoami_ctx" => MyApp.Commands.Whoami}
  )

{result, _bash} = JustBash.exec(bash, "whoami_ctx")
result.stdout  #=> "alice\n"

The :context option seeds caller data at construction. To add or update entries afterward, use the put_context/3 and get_context/3 accessors (modeled on Plug.Conn.put_private/3). Both target the same context map, keys are atoms, and the map is ignored by builtins and the interpreter.

defmodule MyApp.Commands.Counter do
  @behaviour JustBash.Commands.Command

  @impl true
  def names, do: ["counter_ctx"]

  @impl true
  def execute(bash, _args, _stdin) do
    count = JustBash.get_context(bash, :count, 0)
    {%{stdout: "#{count}\n", stderr: "", exit_code: 0}, bash}
  end
end

bash =
  JustBash.new(commands: %{"counter_ctx" => MyApp.Commands.Counter})
  |> JustBash.put_context(:count, 41)

{result, _bash} = JustBash.exec(bash, "counter_ctx")
result.stdout  #=> "41\n"

Important caveats:

• Custom commands run arbitrary Elixir code in the host BEAM process
• They are not restricted by the virtual filesystem or network: policy
• Registration keys must appear in names/0; aliases from names/0 are registered automatically
• Shell functions still win over custom commands at execution time
• Protected stateful builtins such as cd, export, trap, and return cannot be overridden

When a single tool needs many subcommands — acme pr review, acme product list — don't hand-roll a case router, manual --help, and ad-hoc error strings in execute/3. JustBash.CLI is a declarative subcommand layer that gives you routing, typed argument parsing, and auto-generated help, errors, and docs from a single source of truth. A CLI is plain data (a %JustBash.CLI{} tree) that registers like any other command:

alias JustBash.CLI
alias JustBash.Commands.Command

cli =
  CLI.new("acme", doc: "Acme operations toolkit", commands: [
    CLI.command("pr", doc: "Pull request management", commands: [
      CLI.command("review",
        doc: "Review a pull request",
        flags: [
          report:  [type: :integer, required: true, doc: "ID of the report to review"],
          format:  [type: :string, default: "text", values: ~w(text json), doc: "Output format"],
          verbose: [type: :boolean, short: "-v"]
        ],
        run: fn inv ->
          tag = if inv.flags.verbose, do: "[v] ", else: ""
          {Command.ok("#{tag}report #{inv.flags.report} as #{inv.flags.format}\n"), inv.bash}
        end)
    ])
  ])

bash = JustBash.new(commands: %{"acme" => cli})
{result, _} = JustBash.exec(bash, "acme pr review --report 42 --format json")
result.stdout  #=> "report 42 as json\n"

Each leaf's :run handler takes a single %JustBash.CLI.Invocation{} (flags, args, bash, stdin, path) and returns {result, bash} — the same contract as a plain custom command, so handlers keep full access to bash.fs, bash.context, etc. Use a capture (run: &Acme.PR.review/1) to keep handler logic in named, testable functions.

Help, did you mean suggestions, and usage-bearing errors come for free and are consistent across every CLI — which is exactly what an agent needs to recover from a typo in one turn:

$ acme pr review --help
acme pr review - Review a pull request

Usage: acme pr review --report <int> [--format text|json] [-v]

Options:
  --report <int>       ID of the report to review (required)
  --format text|json   Output format (values: text, json) (default: text)
  -v, --verbose

$ acme pr reviw
acme: unknown command 'pr reviw'
Did you mean 'pr review'?
Run 'acme --help' for available commands.      # exit code 2

$ acme pr review
acme pr review: missing required flag: --report
Usage: acme pr review --report <int> [--format text|json] [-v]   # exit code 2

Because the spec is declarative, you can introspect it to generate the tool documentation that goes into an agent's system prompt — from the same source as the runtime behavior:

JustBash.CLI.describe(cli)
#=> %{name: "acme", doc: "...", commands: [%{path: ["pr", "review"], flags: [...], ...}]}

JustBash.CLI.render_docs(cli, format: :markdown)  # a markdown manual

If you prefer a CLI to live as a module alongside your other command modules, use JustBash.CLI and define spec/0 (conventional use-wiring, not a DSL):

defmodule Acme.CLI do
  use JustBash.CLI

  @impl true
  def spec, do: JustBash.CLI.new("acme", doc: "Acme toolkit", commands: [...])
end

bash = JustBash.new(commands: %{"acme" => Acme.CLI})

For a complete before/after, compare eval/commands/kv.ex (a hand-rolled router with help text duplicated by hand) against eval/commands/kv_cli.ex (the same tool on JustBash.CLI, where only the storage logic remains). CLI handlers carry the same trust model and crash isolation as any custom command — they are host code and are not sandboxed.

# Run a script from the virtual filesystem
bash = JustBash.new(files: %{"/script.sh" => "echo hello"})
{result, bash} = JustBash.exec_file(bash, "/script.sh")

import JustBash.Sigil

result = ~b"echo hello"
result.stdout  #=> "hello\n"

# Modifiers
~b"echo hello"t  # trimmed output
~b"echo hello"s  # stdout only
~b"exit 42"e     # exit code

cat, chmod, chown, cp, du, file, find, ln, ls, mkdir, mktemp, mv, readlink, realpath, rm, stat, touch, tree

awk, base64, comm, cut, diff, expand, fold, grep, head, md5sum, nl, paste, rev, sed, sha256sum, shasum, sort, tac, tail, tr, uniq, wc, xargs

jq (JSON), markdown (Markdown → HTML)

curl, wget

echo, printf, cd, pwd, eval, export, unset, set, test, [, [[, true, false, :, command, source, ., read, exit, return, local, declare, typeset, break, continue, shift, getopts, trap, type

arch, basename, date, dirname, env, hostname, id, nproc, printenv, seq, sleep, tee, uname, which, whoami, yes

• Pipes: cmd1 | cmd2
• Redirections: >, >>, 2>, &>, <, <<<, heredocs
• Command chaining: &&, ||, ;
• Variables: $VAR, ${VAR}, ${VAR:-default}, ${VAR:=value}, ${#VAR}, ${VAR:start:len}, ${VAR#pattern}, ${VAR%pattern}, ${VAR/old/new}, ${VAR^^}, ${VAR,,}
• Brace expansion: {a,b,c}, {1..10}, {a..z}
• Arithmetic: $((expr)) with full operators
• Glob patterns: *, ?, [...]
• Control flow: if/elif/else/fi, for/while/until, case/esac
• Functions: function name { ... } or name() { ... }
• Indexed arrays: arr=(...), ${arr[0]}, ${arr[@]}, ${#arr[@]}
• Associative arrays: declare -A map, map[key]=value, ${map[key]}
• Subshells: (cmd) and command groups { cmd; }

When created without options, JustBash provides a Unix-like directory structure:

• /home/user - Default working directory (and $HOME)
• /bin, /usr/bin - Binary directories
• /tmp - Temporary files

# Create environment
bash = JustBash.new(opts)

# Execute command
{result, bash} = JustBash.exec(bash, "command")
result.stdout      # String
result.stderr      # String
result.exit_code   # Integer
result.env         # Updated environment

# Execute script from virtual filesystem
{result, bash} = JustBash.exec_file(bash, "/path/to/script.sh")

# Parse without executing
{:ok, ast} = JustBash.parse("echo hello")

# Format script
{:ok, formatted} = JustBash.format("if true;then echo yes;fi")

mix deps.get
mix test           # Unit, integration, property-based, and bash-comparison tests
mix dialyzer       # Type checking
mix credo --strict # Linting

MIT