December 24, 2021

Clojure CLI tools - which execution option to use

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Clojure CLI tools provide a very flexible way to run Clojure, using aliases to include community libraries and tools to enhance clojure projects and provide independent tools. Understand the execution options (exec-opts) on the command line options ensures an effective use of Clojure CLI tools.

Updated on 24th December 2021 with feedback from Alex, Sean and other community members. Thank you.

Clojure CLI tools first documented released in February 2020 with a specific focus on running Clojure code quickly and easily, which also required the management of dependencies from Maven, Git and a local file space.

Since that release the design has evolved to provide clojure.exec (-X) to run any Clojure function not just -main as with clojure.main (-M). In July 2021, the ability to run tools (-T) which are 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.

Summary of use

In very simple terms, the Clojure CLI tools flags are used as follows

-A to configure paths and dependencies when running the Clojure CLI tools REPL

-M using clojure.main to run the -main function of a Clojure project or tool, using the -m flag to specify the namespace containing -main (clojure.main has other features too)

-X for running any fully qualified function from a Clojure project or tool

-P a dry run, downloading all dependencies (must use flag in first position)

-T running a tool separate from a Clojure project (class path)

The tools flag, -T removes the need for other aliases to run tools and eventually all community tools should move to use the -T approach. However, some tools still need to be run using the -M or -X flag (as the -T flag is quite new).

Alias with REPL -A

-A alias is deprecated for running Clojure code via clojure.main. In this case, the -M option should be used.

-A should be used when adding libraries or paths when starting a REPL

For example, adding a dev directory to the class path that contains a user.clj file that will automatically load code from the user namespace defined in that file.

Also adding a dependency to provide hotloading of other dependencies (note: this approach is not official and may change)

{:extra-paths ["dev"]
 :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

clojure.main -M

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/main function looks for a -main function in the given namespace (-m fully-qualified.namespace) or if no main namespace is found then a REPL session is run. This is the approach Leiningen has always used and still uses today.

Running a project using Clojure CLI tools 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.

The same approach is taken when including an alias, in this example :alias/name.

clojure -M:alias/name -m practicalli.sudoku-solver

This command will merge values defined in the alias for :extra-paths and extra-deps into the call.

When the command line includes the -m flag with a namespace it will over-ride the :main-opts value if defined in the alias

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

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

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

clojure -M:project/run

clojure.main has other features, as covered in the REPL and main entrypoints article) on

Chaining aliases together

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

clojure -M:env/dev:env/test:lib/cider:repl/rebel

Using several aliases on the command line will merge their :extra-paths and :extra-deps values.

The :main-opts values are not merged and the value from the last :main-opts in the alias chain is used with clojure.main

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

Aliases can be chained together to merge their configuration, e.g. clojure -M:env/dev:repl/rebel will included the configuration from both :env/dev and :repl/rebel. With the -M flag, only the :main-opts from the last alias in the chain is used. If -m namespace/name is also part of the command, this will be used instead of any values from alias :main-opts keys.

clojure.exec -X

-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

Tools -T

Install, run and remove a tool that can be used independently from a Clojure project configuration.

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 to form a hash-map of arguments to that function

The -T execution option is the same as 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.

-Ttools is a built in tool for install and remove of 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 $HOME/.clojure/tools/ (or $XDG_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.

When using the -X execution option, keys :replace-paths and :replace-deps should be used instead of :extra-paths and :extra-deps so that project paths and dependencies are not included within the alias. Using the -T execution option, these are automatically ignored.

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 :project/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:project/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:project/outdated > outdated-2021-12-24.txt

Example tools include

Prepare -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:project/hotload:env/dev:lib/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.


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.


Tags: clojure-cli tools-deps