December 24, 2021

Clojure CLI - which execution option to use

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Execution options (-A -M -T -X) define how aliases are used with the Clojure CLI. Aliases are included via one of these execution options and each option can affect how the alias is used.

Since the first documented released which exclusively used the -A execution option to include aliases, the design has evolved to provide specific execution options to run code via clojure.main (-M) and clojure.exec (-X). In July 2021, the ability to run tools (-T) independent from the Clojure project classpath was also introduced.

The exec-opts command line flags have evolved to enable these features, so lets explore those flags and show how each flag is typically used.

Understand aliases in more detail

Quick summary

Use -M for code that should be called via the -main function in a specified namespace, passing string-based arguments.

Use -X to run a specified function, passing arguments as key and value pairs (not hash-maps yet)

Use -T to run a tool independently from any libraries defined in a project. Only the libraries in the alias are included in the ClassPath. The path is defined as "." by default.

Use -P to download libraries on the class path, including those from specified aliases

Use -A in the specific case of running a basic terminal UI REPL with the clojure command (or clj wrapper).

Common aliases will be used to explain the use of these execution options in more detail.

Run a Clojure project with clojure.main

-M execution option uses clojure.main to run Clojure code.

Run a project with the main namespace practicalli.sudoku-solver, without any additional aliases on the command line

clojure -M -m practicalli.sudoku-solver

-M flag instructs Clojure CLI tools to use clojure.main to run the Clojure code.

-m flag is an argument to clojure.main which specifies the namespace to search for a -main function.

Adding a :project/run alias to the project deps.edn file provides a simpler way to run the project on the command line

:project/run {:main-opts ["-m" "practicalli.sudoku-solver"]}

Now the project code can be run using the simple command line form

clojure -M:project/run

How clojure.main works

clojure.main/main function searches for a -main function in the given namespace (-m fully-qualified.namespace)

If no -main function is found or the namespace is not specified, then a REPL session is run. This is the approach Leiningen has always used and still uses today.

clojure.main namespace has been the way Clojure code was run (including a REPL) for most of its history. This is now evolving with the addition of clojure.exec. clojure.main has other features, as covered in the REPL and main entrypoints article) on

Run a rich terminal REPL with clojure.main

Rebel readline provides a terminal UI REPL, providing auto-completion, function signatures, documentation, etc.

:repl/rebel is an alias that includes nrepl, cider-nrepl and rebel-readline libraries, with a :main-opts to run the rebel-readline.main/-main function via clojure.main.

{:extra-deps {nrepl/nrepl                {:mvn/version "0.9.0"}
              cider/cider-nrepl          {:mvn/version "0.28.2"}
              com.bhauman/rebel-readline {:mvn/version "0.1.4"}}
 :main-opts  ["-m" "nrepl.cmdline"
              "--middleware" "[cider.nrepl/cider-middleware]"
              "-f" "rebel-readline.main/-main"]}

Use the :repl/rebel alias with the -M execution option

clojure -M:repl/rebel

Multiple aliases can be specified to include additional paths and libraries. Aliases chained together have their configuration merged

:env/dev adds "dev" as an extra path, with the dev/user.clj file automatically loading its code into the user namespace when the REPL starts

:lib/hotload alias adds the org.clojure/tools.deps.alpha library to provide hotloading of dependencies into the running REPL

Start a REPL process with this alias

clojure -M:env/dev:lib/hotload:repl/rebel

The Rebel REPL UI will start, include the dev directory on the class path and the org.clojure/tools.deps.alpha library loaded into the REPL

Chaining aliases together with clojure.main

Alises can be used together by chaining their names on the command line

clojure -M:env/dev:lib/hotload:repl/rebel

The clojure command will merge the :extra-paths and :extra-deps values from each alias in the chain.

The :main-opts values from the aliases are not merged. Only the :main-opts value from the last alias in the chain is used with clojure.main to run the Clojure code.

If the command line includes the -m flag with a namespace, then that namespace is passed to clojure.main, ignoring all :main-opts values from the aliases. The -i and -e flags for clojure.main also replace :main-opts values.

Run a specific function with clojure.exec

-X flag provides the flexibility to call any fully qualified function, so Clojure code is no longer tied to -main

Any function on the class path can be called and is passed a hash-map as an argument. The argument hash-map is either specified in an alias using :exec-args or assembled into a hash-map from key/value pairs on the command line. Key/values from the command line are merged into the :exec-args map if it exists, with the command line key/values taking precedence.

Call the status function from the namespace practicalli.service, which is on the classpath in the practicalli.service project

clojure -X practicalli.service/status

Pass arguments to a start function in the practicalli.service namespace

clojure -X practicalli.service/start :port 8080 :join? false

As the arguments are key/value pairs, it does not matter in which order the pairs are used in the command line.

Built in clojure.exec functions

Clojure CLI tools has some built in tools under the special :deps alias (not to be confused with the :deps configuration in a deps.edn file)

  • -X:deps mvn-install - install a maven jar to the local repository cache
  • -X:deps find-versions - Find available versions of a library
  • -X:deps prep - prepare source code libraries in the dependency tree

See clojure --help for an overview or man clojure for detailed descriptions

Run an independent Tool -T

-T install, run and remove a tool, by the tool name or an alias.

The -T execution option also uses the clojure.exec approach, although the :deps and :path values from a project deps.edn file are ignored. This isolates the tool from the dependencies in a Clojure project.

Calling Tools on the command line has the general form:

clojure -Ttool-name function-name :key "value" ,,,

A tool may provide many functions, so the specific function name is provided when calling the tool.

key/value pairs can be passed as arguments to that function (as with the -X execution option)

-Ttools is a built-in tool to install and remove other tools, with the :as directive providing a specific name for the tool.

In this example, the antq tool is installed using the name antq

clojure -Ttools install com.github.liquidz/antq '{:git/tag "1.3.1"}' :as antq

Installing a tool adds an EDN configuration file using the name of the tool in $XDG_HOME/.clojure/tools/ or $HOME/.clojure/tools/ directory.

Once a tool is installed, run by using the name of the tool.

clojure -Tantq outdated

Options to the tool are passed as key/value pairs (as the tool is called by clojure.exec)

clojure -Tantq outdated :upgrade true

-Ttools remove will remove the configuration of the tool of the given name

clojure -Ttools remove :tool antq

Tools install or aliases

Tools can also be defined in an alias with :exec-fn can be run via -T:alias-name as they are both executed using clojure.exec.

-X execution option can emulate -T behaviour when an alias uses :replace-paths and :replace-deps keys, instead of :extra-paths and :extra-deps, so project paths and dependencies are not included loaded by the alias.

Using an alias for a tool has the advantage allowing a use to define their preferred default arguments that are passed to the :exec-fn, using the :exec-args key.

Default arguments could be included in the deps.edn of the installed tool itself, although this is controlled by the developer of that tool project.

The :search/outdated alias defined in the practicalli/clojure-deps-edn user level configuration is an example of a tool alias with default arguments

  {:replace-paths ["."]
   :replace-deps  {com.github.liquidz/antq {:mvn/version "1.3.1"}
                   org.slf4j/slf4j-nop     {:mvn/version "1.7.32"}}
   :main-opts     ["-m" "antq.core"]
   :exec-fn antq.tool/outdated
   :exec-args {:directory ["."] ; default
               :exclude ["com.cognitect/rebl"
               ;; :focus ["com.github.liquidz/antq"]
               :skip ["boot" "leiningen"]
               :reporter "table" ; json edn format
               :verbose false
               :upgrade false
               :force   false}}

This alias is called using clojure -T:search/outdated and is the same as calling clojure -Tantq outdated ,,, ,,, with a long list of key value options that represent the arguments in the alias.

As the output is a table of results, the command output is typically pushed to a file: clojure -T:search/outdated > outdated-2021-12-24.txt

Example tools include

Prepare by downloading dependencies -P

-P flag instructs the clojure command to download all library dependencies to the local cache and then stop without executing a function call.

The -P flag is often used with Continuous Integration workflows and to create pre-populated Container images, to avoid repeatedly downloading the same library jar files.

If used with just a project, then the Maven dependencies defined in the project deps.edn file will be downloaded, if not already in the users local cache (~/.m2/repository/).

If :git or :local/root dependencies are defined, the respective code will be downloaded and added to the classpath.

Prepare flag by itself download dependencies defined in the :deps section of the deps.edn file of the current project.

clojure -P

Including one or more aliases will preparing all the dependencies from every alias specified

clojure -P -M:env/dev:lib/hotload:repl/cider

-P flag must be used before any subsequent arguments, i.e. before -M, -X, -T

As prepare is essentially a dry run, then the clojure command does not call :main-opts or :exec-fn functions, even if they exist in an alias or on the command line.

-P will warn if a project has dependencies that require building from source (i.e Java code) or resource file manipulation. If so then clojure -X:deps prep will prepare these source based dependencies.

Alias with built-in terminal UI REPL -A

-A is stated as the official way to include an alias when running a REPL terminal UI clojure or clj.

Practicalli recommends using Rebel Readline which uses -M execution option, so -A execution option is rarely used by Practicalli.

The :env/dev alias adds "dev" directory to the class path, typically used to add a user.clj that will automatically load code from the user namespace defined in that file.

clojure -A:env/dev

The alias definition is :env/dev {:extra-paths ["dev"]}

Aliases can be chained together and their configuration will be merged

:lib/hotload adds a dependency to provide hotloading of other dependencies

{:extra-deps {org.clojure/tools.deps.alpha
                {:git/url ""
                 :sha     "d77476f3d5f624249462e275ae62d26da89f320b"}
              org.slf4j/slf4j-nop {:mvn/version "1.7.32"}}}

Start a REPL process with this alias

clojure -A:env/dev:lib/hotload

Using an alias that contains a :main-opts key with -A will fail to run a REPL and print a warning to use -M execution option The :main-opts configuration for -A execution option is deprecated (although currently works in 1.10.x). To run Clojure code via clojure.main the -M option should be with aliases that includes :main-opts.


There are many options when it comes to running Clojure CLI tools that are not covered here, however, this guide gives you the most common options used so far.

Practicalli recommends using the -X execution option where possible, as arguments follow the data approach of Clojure design.

The -J and :jvm-opts are useful to configure the Java Virtual machine and deserve an article to themselves as there are many possible options.

The -T tools is an exciting and evolving approach and it will be interesting to see how the Clojure community adopt this model.

See the Deps and CLI Reference Rationale for more details and description of these options.


Thank you

practicalli GitHub profile I @practical_li

Tags: clojure-cli tools-deps