6. Apps
The app support in Pkg is currently considered experimental and some functionality and API may change.
Some inconveniences that can be encountered are:
- You need to manually make
~/.julia/bin
available on the PATH environment. - The path to the julia executable used is the same as the one used to install the app. If this julia installation gets removed, you might need to reinstall the app.
Apps are Julia packages that are intended to be run as "standalone programs" (by e.g. typing the name of the app in the terminal possibly together with some arguments or flags/options). This is in contrast to most Julia packages that are used as "libraries" and are loaded by other files or in the Julia REPL.
Creating a Julia app
A Julia app is structured similar to a standard Julia library with the following additions:
- A
@main
entry point in the package module (see the Julia help on@main
for details) - An
[apps]
section in theProject.toml
file listing the executable names that the package provides.
A very simple example of an app that prints the reversed input arguments would be:
# src/MyReverseApp.jl
module MyReverseApp
function (@main)(ARGS)
for arg in ARGS
print(stdout, reverse(arg), " ")
end
return
end
end # module
# Project.toml
# standard fields here
[apps]
reverse = {}
The empty table {}
is to allow for giving metadata about the app.
After installing this app one could run:
$ reverse some input string
emos tupni gnirts
directly in the terminal.
Multiple Apps per Package
A single package can define multiple apps by using submodules. Each app can have its own entry point in a different submodule of the package.
# src/MyMultiApp.jl
module MyMultiApp
function (@main)(ARGS)
println("Main app: ", join(ARGS, " "))
end
include("CLI.jl")
end # module
# src/CLI.jl
module CLI
function (@main)(ARGS)
println("CLI submodule: ", join(ARGS, " "))
end
end # module CLI
# Project.toml
# standard fields here
[apps]
main-app = {}
cli-app = { submodule = "CLI" }
This will create two executables:
main-app
that runsjulia -m MyMultiApp
cli-app
that runsjulia -m MyMultiApp.CLI
Configuring Julia Flags
Apps can specify default Julia command-line flags that will be passed to the Julia process when the app is run. This is useful for configuring performance settings, threading, or other Julia options specific to your application.
Default Julia Flags
You can specify default Julia flags in the Project.toml
file using the julia_flags
field:
# Project.toml
[apps]
myapp = { julia_flags = ["--threads=4", "--optimize=2"] }
performance-app = { julia_flags = ["--threads=auto", "--startup-file=yes", "--depwarn=no"] }
debug-app = { submodule = "Debug", julia_flags = ["--check-bounds=yes", "--optimize=0"] }
With this configuration:
myapp
will run with 4 threads and optimization level 2performance-app
will run with automatic thread detection, startup file enabled, and deprecation warnings disableddebug-app
will run with bounds checking enabled and no optimization
Runtime Julia Flags
You can override or add to the default Julia flags at runtime using the --
separator. Everything before --
will be passed as flags to Julia, and everything after --
will be passed as arguments to your app:
# Uses default flags from Project.toml
myapp input.txt output.txt
# Override thread count, keep other defaults
myapp --threads=8 -- input.txt output.txt
# Add additional flags
myapp --threads=2 --optimize=3 --check-bounds=yes -- input.txt output.txt
# Only Julia flags, no app arguments
myapp --threads=1 --
The final Julia command will combine:
- Fixed flags (like
--startup-file=no
and-m ModuleName
) - Default flags from
julia_flags
in Project.toml - Runtime flags specified before
--
- App arguments specified after
--
Overriding the Julia Executable
By default, apps run with the same Julia executable that was used to install them. You can override this globally using the JULIA_APPS_JULIA_CMD
environment variable:
# Use a different Julia version for all apps
export JULIA_APPS_JULIA_CMD=/path/to/different/julia
myapp input.txt
# On Windows
set JULIA_APPS_JULIA_CMD=C:\path\to\different\julia.exe
myapp input.txt
Installing Julia apps
The installation of Julia apps is similar to installing Julia libraries but instead of using e.g. Pkg.add
or pkg> add
one uses Pkg.Apps.add
or pkg> app add
(develop
is also available).