§20 · JULIA CHEAT SHEET · FIRST DAY, THEN NEVER AGAIN

The REPL and Unicode

Modes

Key at an empty prompt	Mode	Prompt
—	Julia	`julia>`
`]`	Pkg	`(env) pkg>`
`?`	Help	`help?>`
`;`	Shell	`shell>`
Backspace	back to Julia

julia> ?sort            # docs, all methods, related functions
julia> ]st              # one-off Pkg command without staying in the mode
julia> ;ls -la          # one-off shell command

Help mode also works on operators, macros, types and unicode characters — ?∘, ?@view, ?Vector, and pasting a character tells you its name and how to type it.

Keybindings

Key	Does
`Ctrl-C`	interrupt the running expression
`Ctrl-D`	exit (at an empty prompt)
`Ctrl-L`	clear screen
`Ctrl-R` / `Ctrl-S`	reverse / forward history search
`Up`/`Down`	history, prefix-filtered by what you’ve typed
`Alt-Enter`	insert a newline without evaluating
`Ctrl-Q`	edit the current line in `$EDITOR`
`Tab`	completion — of names, paths, fields, and LaTeX sequences
`Tab` twice	show all completions, including method signatures

Typing f( then Tab shows the applicable methods. Typing x. then Tab shows fields and properties.

Useful REPL functions

ans                       # value of the last expression
varinfo()                 # what's defined in Main, with sizes
varinfo(MyPkg)
clipboard("text"); clipboard()      # copy out / paste in
edit("file.jl", 10)       # open in $JULIA_EDITOR at a line
@edit f(1)                # open the source of the method that call selects
@which f(1)
@doc f
names(Base; all = false)
methods(f); methodswith(MyType)
apropos("sort")           # full-text search of docstrings
InteractiveUtils.versioninfo()

startup.jl

~/.julia/config/startup.jl runs before every session. Keep it small and fast.

try
    using Revise
catch e
    @warn "Revise failed to load" exception = (e, catch_backtrace())
end

atreplinit() do repl
    try
        @eval using OhMyREPL
    catch
    end
end

ENV["JULIA_EDITOR"] = "zed --wait"

Install these into the shared default environment, not into your projects:

julia -e 'using Pkg;
    Pkg.activate("v$(VERSION.major).$(VERSION.minor)"; shared=true);
    Pkg.add(["Revise", "OhMyREPL", "BenchmarkTools"])'

--startup-file=no skips it — useful for benchmarking and required for reproducible CI.

Command-line flags worth remembering

julia --project                  # activate the nearest Project.toml
julia -t auto                    # all available threads
julia -p 4                       # 4 worker processes (Distributed)
julia -e 'code'                  # evaluate and exit
julia -E 'code'                  # evaluate, print the result, exit
julia -i script.jl               # run then stay interactive
julia --check-bounds=yes         # bounds-check even where @inbounds says not to
julia --startup-file=no
julia --banner=no -q             # quiet
julia --track-allocation=user    # write .mem files with per-line allocation counts
julia --code-coverage=user

Unicode input

Type the LaTeX name and press Tab. Works in the REPL, and in editors with the Julia extension.

Sequence	Char	Sequence	Char
`\alpha`	α	`\le`	≤
`\beta`	β	`\ge`	≥
`\gamma`	γ	`\ne`	≠
`\delta`	δ	`\approx`	≈
`\Delta`	Δ	`\equiv`	≡
`\epsilon`	ϵ	`\in`	∈
`\theta`	θ	`\notin`	∉
`\lambda`	λ	`\subseteq`	⊆
`\mu`	μ	`\cup`	∪
`\pi`	π	`\cap`	∩
`\rho`	ρ	`\cdot`	⋅
`\sigma`	σ	`\times`	×
`\Sigma`	Σ	`\div`	÷
`\phi`	ϕ	`\circ`	∘
`\omega`	ω	`\pm`	±
`\Omega`	Ω	`\sqrt`	√
`\infty`	∞	`\sum`	∑
`\partial`	∂	`\prod`	∏
`\nabla`	∇	`\int`	∫
`\hbar`	ℏ	`\emptyset`	∅
`\euler`	ℯ	`\im`	— (`im` is the literal)

Sub- and superscripts: x\_1 gives x₁, x\^2 gives x², \_a through \_z and \^0 through \^9 mostly exist. These are legal in identifiers, so σ², x₁, Δt are all valid variable names.

Built-in constants: π (also pi), ℯ (also exp(1)), im (imaginary unit), Inf, NaN, missing, nothing.

Operators you can define yourself, because they’re parsed as infix: ⊕ ⊗ ⊙ ⊘ ⋆ ∘ ± ∓ × ⋅ ∪ ∩ ⊆ ⊇ ≈ ≉ ≡ and many more. Precedence follows the ASCII operator each resembles.

⊕(a, b) = a + 2b
3 ⊕ 4       # 11

If you don’t want Unicode in your source, every operator has an ASCII spelling: in, issubset, isapprox, !=, <=, div, dot, cross, union, intersect, pi, sqrt.

Finding out what a character is

Paste it into help mode:

help?> ⊗
"⊗" can be typed by \otimes<tab>

That works for any character, which is the fastest way to read unfamiliar numerical code.