Create powerful CLI tools that are fast and memory safe
You can quickly build full-featured command-line interfaces using the Swift Argument Parser library. Define commands by creating types with regular Swift properties. Link multiple commands to create rich command hierarchies.
ArgumentParser provides a detailed help screen, clear error messages with near-miss checking, broad customization, and more.
Swift Argument Parserimport ArgumentParser
@main
struct Repeat: ParsableCommand {
@Argument(help: "The phrase to repeat.")
var phrase: String
@Option(help: "The number of times to repeat 'phrase'.")
var count: Int? = nil
mutating func run() throws {
let repeatCount = count ?? .max
for i in 1...repeatCount {
print(phrase)
}
}
}
In addition to detailed help screens and error messages out of the box, your ArgumentParser CLI tools can provide completion scripts and man pages, as well as extensibility through a JSON rendering of the interface.
$ repeat yes --count 3
yes
yes
yes
$ repeat --count
Error: Missing value for '--count <count>'
Help: --count <count> The number of times to repeat 'phrase'.
Usage: repeat <phrase> [--count <count>]
See 'repeat --help' for more information.
$ repeat -h
USAGE: repeat <phrase> [--count <count>]
ARGUMENTS:
<phrase> The phrase to repeat.
OPTIONS:
--count <count> The number of times to repeat 'phrase'.
-h, --help Show help information..\" "Generated by swift-argument-parser"
.Dd May 21, 2025
.Dt REPEAT 1
.Os
.Sh NAME
.Nm repeat
.Sh SYNOPSIS
.Nm
.Ar subcommand
.Ar phrase
.Op Fl -count Ar count
.Op Fl -help
.Sh DESCRIPTION
.Bl -tag -width 6n
.It Ar phrase
The phrase to repeat.
.It Fl -count Ar count
The number of times to repeat 'phrase'.
.It Fl h , -help
Show help information.
.It Em help
Show subcommand help information.
.Bl -tag -width 6n
.It Ar subcommands...
.El
.El
.Sh "EXIT STATUS"
.Ex -std#compdef repeat
__repeat_complete() {
local -ar non_empty_completions=("${@:#(|:*)}")
local -ar empty_completions=("${(M)@:#(|:*)}")
_describe -V '' non_empty_completions -- empty_completions -P $'\'\''
}
__repeat_custom_complete() {
local -a completions
completions=("${(@f)"$("${command_name}" "${@}" "${command_line[@]}")"}")
if [[ "${#completions[@]}" -gt 1 ]]; then
__repeat_complete "${completions[@]:0:-1}"
fi
}
__repeat_cursor_index_in_current_word() {
if [[ -z "${QIPREFIX}${IPREFIX}${PREFIX}" ]]; then
printf 0
else
printf %s "${#${(z)LBUFFER}[-1]}"
fi
}
_repeat() {
emulate -RL zsh -G
setopt extendedglob nullglob numericglobsort
unsetopt aliases banghist
local -xr SAP_SHELL=zsh
local -x SAP_SHELL_VERSION
SAP_SHELL_VERSION="$(builtin emulate zsh -c 'printf %s "${ZSH_VERSION}"')"
local -r SAP_SHELL_VERSION
local context state state_descr line
local -A opt_args
local -r command_name="${words[1]}"
local -ar command_line=("${words[@]}")
local -ir current_word_index="$((CURRENT - 1))"
local -i ret=1
local -ar arg_specs=(
':phrase:'
'--count[The number of times to repeat '\''phrase'\''.]:count:'
'(-h --help)'{-h,--help}'[Show help information.]'
)
_arguments -w -s -S : "${arg_specs[@]}" && ret=0
return "${ret}"
}
_repeat{
"command" : {
"arguments" : [
{
"abstract" : "The phrase to repeat.",
"isOptional" : false,
"isRepeating" : false,
"kind" : "positional",
"shouldDisplay" : true,
"valueName" : "phrase"
},
{
"abstract" : "The number of times to repeat 'phrase'.",
"isOptional" : true,
"isRepeating" : false,
"kind" : "option",
"names" : [
{
"kind" : "long",
"name" : "count"
}
],
"preferredName" : {
"kind" : "long",
"name" : "count"
},
"shouldDisplay" : true,
"valueName" : "count"
},
{
"abstract" : "Show help information.",
"isOptional" : true,
"isRepeating" : false,
"kind" : "flag",
"names" : [
{
"kind" : "short",
"name" : "h"
},
{
"kind" : "long",
"name" : "help"
}
],
"preferredName" : {
"kind" : "long",
"name" : "help"
},
"shouldDisplay" : true,
"valueName" : "help"
}
],
"commandName" : "repeat",
"shouldDisplay" : true,
"subcommands" : [
{
"abstract" : "Show subcommand help information.",
"arguments" : [
{
"isOptional" : true,
"isRepeating" : true,
"kind" : "positional",
"shouldDisplay" : true,
"valueName" : "subcommands"
},
{
"isOptional" : true,
"isRepeating" : false,
"kind" : "flag",
"names" : [
{
"kind" : "short",
"name" : "h"
},
{
"kind" : "long",
"name" : "help"
},
{
"kind" : "longWithSingleDash",
"name" : "help"
}
],
"preferredName" : {
"kind" : "long",
"name" : "help"
},
"shouldDisplay" : false,
"valueName" : "help"
}
],
"commandName" : "help",
"shouldDisplay" : true,
"superCommands" : [
"repeat"
]
}
]
},
"serializationVersion" : 0
}Noora is Swift package that "distills common CLI patterns into a cohesive design system of themable components, enabling richer and more interactive experiences." Design components range from prompts to progress bars, to alerts.
View Noora on GitHubSubprocess is a Swift library that provides precise, idiomatic control over launching and managing child processes. You can either collect the child process output asynchronously in full, or stream it in real time using AsyncSequence, making it easy to process output line-by-line as it arrives.
Subprocess gives you fine-grained control over environment variables, arguments, and many platform specific parameters, while embracing Swift’s concurrency features and type safety. Whether you’re building CLI tools or server-side Swift applications, swift-subprocess integrates cleanly.
Subprocessimport Subprocess
// Launch Nginx and monitor the log file in parallel
async let monitorResult = run(
.path("/usr/bin/tail"),
arguments: ["-f", "/path/to/nginx.log"]
) { execution, standardOutput in
for try await line in standardOutput.lines(encoding: UTF8.self) {
// Parse the log text
if line.contains("500") {
// Oh no, 500 error
}
}
}
let launchResult = try await run(
.name("nginx"), // Lookup executable by name
arguments: ["-c", "/path/to/nginx.conf"]
)
if !launchResult.terminationStatus.isSuccess {
print("Nginx failed to launch: \(launchResult.terminationStatus)")
} else {
print("Nginx launched with PID \(launchResult.processIdentifier)")
}