Use irb3.2 to Test Ruby Code Without a Project

irb3.2 gives you a Ruby prompt for checking an expression, a library or a whole file, without setting up a project first.

The examples use the installed Ruby 3.2.3 and IRB 1.6.2 on this machine. They do not install gems or change system configuration. Allow about ten minutes.

• You need: a shell and the ruby package.
• No sudo. No command in this guide needs elevated privileges.
• Treat pasted code as executable. IRB evaluates Ruby in your user account, especially code copied from an issue, chat or web page.

1. Check the installed command

Confirm which executable will run with these ordinary, read-only checks:

$ command -v irb
/usr/bin/irb
$ irb --version
irb 1.6.2 (2022-12-13)
$ ruby --version
ruby 3.2.3 (2024-01-18 revision 52bb2ac0a6) [x86_64-linux-gnu]

The irb and irb3.2 commands installed here report the same IRB version. irb3.2 identifies the Ruby 3.2 package specifically, while irb is the convenient alias. Use the versioned name when a script, note or runbook must make that choice obvious.

2. Evaluate one expression

Run irb with no arguments for an interactive session. Type an expression, press Enter, and IRB prints its result:

$ irb
irb(main):001:0> 1 + 1
=> 2
irb(main):002:0> "ruby".upcase
=> "RUBY"
irb(main):003:0> exit
$

IRB waits when the input is not syntactically complete, which is why a method definition can span several prompts:

irb(main):001:0> def double(value)
irb(main):002:1>   value * 2
irb(main):003:1> end
=> :double
irb(main):004:0> double(21)
=> 42

The number after the colon is the nesting level. Tip: a prompt ending in > on a new line is usually a request for more Ruby, not a hung terminal. Enter the missing end, closing bracket or quote. Use exit, quit or press Ctrl-D to leave an ordinary session.

3. Use a predictable non-interactive check

For a command that belongs in a shell script or a copy-and-paste check, pipe Ruby source to IRB and suppress prompts:

$ printf '%s\n' '1 + 1' 'exit' | irb --noprompt
Switch to inspect mode.
1 + 1
2
exit

The installed IRB 1.6.2 prints the input lines and the short mode message in this piped form; the useful result is 2. If you need clean machine-readable output, use ruby with a small script instead. IRB is an interactive developer tool, not a stable output protocol.

For a human session with less visual noise, --simple-prompt changes the prompt shape. --noprompt removes prompts, but it does not turn IRB into a general Ruby script runner or remove Ruby's own output and errors.

4. Load a standard library before evaluating code

Use -r to require a library before the session starts. This example loads Ruby's JSON library and calls it:

$ irb -r json
irb(main):001:0> JSON.generate({ok: true})
=> "{\"ok\":true}"
irb(main):002:0> exit

-r json is equivalent to requiring json before your first expression. It searches Ruby's load path, so a missing library produces a load error rather than silently doing nothing. To test a local library directory, use -I DIRECTORY:

$ irb -I /path/to/lib -r example_library

Replace the path and library name with values that exist on your machine, and keep paths quoted when they contain spaces. Do not add a directory merely to make an unexplained load error disappear: inspect what code it contains first.

5. Run a Ruby file when the REPL is not the right tool

IRB accepts a program file followed by arguments, useful when you want IRB's environment or output handling while checking an existing script:

$ irb --noprompt /path/to/check.rb first-argument second-argument

The file path and arguments are placeholders. Confirm the file before running it:

$ test -r /path/to/check.rb && printf 'readable: %s\n' /path/to/check.rb
readable: /path/to/check.rb

6. Control startup configuration

By default, IRB may read personal configuration from ~/.irbrc. If IRBRC is set, it names the configuration file instead. With XDG_CONFIG_HOME set and no IRBRC, IRB also recognises $XDG_CONFIG_HOME/irb/irbrc. Configuration can change prompts, helpers and startup behaviour, which matters when a result must be reproducible.

Use -f to suppress the personal startup file for a clean diagnostic session:

$ irb -f --simple-prompt
>> 1 + 1
=> 2
>> exit

This does not delete or modify the configuration, it only skips loading it for that process. If two users see different prompts or different methods at startup, compare IRBRC, XDG_CONFIG_HOME and the relevant file before comparing Ruby code.

7. Inspect errors without changing anything

A syntax or runtime error is reported inside the session and normally leaves IRB available for another expression:

irb(main):001:0> Integer("not-a-number")
ArgumentError: invalid value for Integer(): "not-a-number"
irb(main):002:0> 6 * 7
=> 42

If the diagnostic is too short, --verbose shows more detail and --tracer displays a trace for each command. --back-trace-limit N limits the top and tail of a backtrace; the installed manpage gives a default of 16. These options affect diagnostics, not the code's permissions or behaviour.

If a command unexpectedly starts with different settings, retry with -f. If a required library is missing, verify the name and load path before trying a different Ruby installation. Elevated privileges will not repair a Ruby load path, and can create root-owned files if the code writes anything.