Puka

Puka is a cross-platform library for safely passing strings through shells.

Contents

• Introduction
  • Why would I use Puka?
  • How do I use Puka?
  • What's the catch?
• API Documentation
  • Basic API
    • sh
    • unquoted
  • Advanced API
    • quoteForShell
    • quoteForCmd
    • quoteForSh
    • ShellString
  • Secret API
• The sh DSL
  • Syntax
  • Semantics
    • Types of placeholders

Introduction

Why would I use Puka?

When launching a child process from Node, you have a choice between launching directly from the operating system (as with child_process.spawn, if you don't use the { shell: true } option), and running the command through a shell (as with child_process.exec). Using a shell gives you more power, such as the ability to chain multiple commands together or use redirection, but you have to construct your command as a single string instead of using an array of arguments. And doing that can be buggy (if not dangerous) if you don't take care to quote any arguments correctly for the shell you're targeting, and the quoting has to be done differently on Windows and non-Windows shells.

Puka solves that problem by giving you a simple and platform-agnostic way to build shell commands with arguments that pass through your shell unaltered and with no unsafe side effects, whether you are running on Windows or a Unix-based OS.

How do I use Puka?

Puka gives you an sh function intended for tagging template literals, which quotes (if necessary) any values interpolated into the template. A simple example:

const { sh } = require('puka');
const { execSync } = require('child_process');

const arg = 'file with spaces.txt';
execSync(sh`some-command ${arg}`);

But Puka supports more than this! See the sh DSL documentation for a detailed description of all the features currently supported.

What's the catch?

Here are the ones I know about:

Puka does not ensure that the actual commands you're running are cross-platform. If you're running npm programs, you generally won't have a problem with that, but if you want to run sh`cat file` on Windows, you'll need to depend on something like cash-cat.

I searched for days for a way to quote or escape line breaks in arguments to cmd.exe, but couldn't find one (regular ^-prepending and quotation marks don't seem to cut it). If you know of a way that works, please open an issue to tell me about it! Until then, any line break characters (\r or \n) in values being interpolated by sh will cause an error to be thrown on Windows only.

Also on Windows, you may notice quoting mistakes if you run commands that involve invoking a native executable (not a batch file ending in .cmd or .bat). Unfortunately, batch files require some extra escaping on Windows, and Puka assumes all programs are batch files because npm creates batch file shims for programs it installs (and, if you care about cross-platform, you'll be using npm programs in your commands). If this causes problems for you, please open an issue; if your situation is specific enough, there may be workarounds or improvements to Puka to be found.

API Documentation

Basic API

sh

A string template tag for safely constructing cross-platform shell commands.

An sh template is not actually treated as a literal string to be interpolated; instead, it is a tiny DSL designed to make working with shell strings safe, simple, and straightforward. To get started quickly, see the examples below. More detailed documentation is available further down.

Examples

const title = '"this" & "that"';
sh`script --title=${title}`; // => "script '--title=\"this\" & \"that\"'"
// Note: these examples show results for non-Windows platforms.
// On Windows, the above would instead be
// 'script ^^^"--title=\\^^^"this\\^^^" ^^^& \\^^^"that\\^^^"^^^"'.

const names = ['file1', 'file 2'];
sh`rimraf ${names}.txt`; // => "rimraf file1.txt 'file 2.txt'"

const cmd1 = ['cat', 'file 1.txt', 'file 2.txt'];
const cmd2 = ['use-input', '-abc'];
sh`${cmd1}|${cmd2}`; // => "cat 'file 1.txt' 'file 2.txt'|use-input -abc"

Returns String a string formatted for the platform Node is currently running on.

unquoted

This function permits raw strings to be interpolated into a sh template.

IMPORTANT: If you're using Puka due to security concerns, make sure you don't pass any untrusted content to unquoted. This may be obvious, but stray punctuation in an unquoted section can compromise the safety of the entire shell command.

Parameters

• value any value (it will be treated as a string)

Examples

const both = true;
sh`foo ${unquoted(both ? '&&' : '||')} bar`; // => 'foo && bar'

Advanced API

If these functions make life easier for you, go ahead and use them; they are just as well supported as the above. But if you aren't certain you need them, you probably don't.

quoteForShell

Quotes a string for injecting into a shell command.

This function is exposed for some hypothetical case when the sh DSL simply won't do; sh is expected to be the more convenient option almost always. Compare:

console.log('cmd' + args.map(a => ' ' + quoteForShell(a)).join(''));
console.log(sh`cmd ${args}`); // same as above

console.log('cmd' + args.map(a => ' ' + quoteForShell(a, true)).join(''));
console.log(sh`cmd "${args}"`); // same as above

Additionally, on Windows, sh checks the entire command string for pipes, which subtly change how arguments need to be quoted. If your commands may involve pipes, you are strongly encouraged to use sh and not try to roll your own with quoteForShell.

Parameters

• text String to be quoted
• forceQuote Boolean? whether to always add quotes even if the string is already safe. Defaults to false.
• platform String? a value that process.platform might take: 'win32', 'linux', etc.; determines how the string is to be formatted. When omitted, effectively the same as process.platform.

Returns String a string that is safe for the current (or specified) platform.

quoteForCmd

A Windows-specific version of quoteForShell.

Parameters

• text String to be quoted
• forceQuote Boolean? whether to always add quotes even if the string is already safe. Defaults to false.

quoteForSh

A Unix-specific version of quoteForShell.

Parameters

• text String to be quoted
• forceQuote Boolean? whether to always add quotes even if the string is already safe. Defaults to false.

ShellString

A ShellString represents a shell command after it has been interpolated, but before it has been formatted for a particular platform. ShellStrings are useful if you want to prepare a command for a different platform than the current one, for instance.

To create a ShellString, use ShellString.sh the same way you would use top-level sh.

toString

A method to format a ShellString into a regular String formatted for a particular platform.

Parameters

• platform String? a value that process.platform might take: 'win32', 'linux', etc.; determines how the string is to be formatted. When omitted, effectively the same as process.platform.

Returns String

sh

ShellString.sh is a template tag just like sh; the only difference is that this function returns a ShellString which has not yet been formatted into a String.

Returns ShellString

Secret API

Some internals of string formatting have been exposed for the ambitious and brave souls who want to try to extend Puka to handle more shells or custom interpolated values. This ‘secret’ API is partially documented in the code but not here, and the semantic versioning guarantees on this API are bumped down by one level: in other words, minor version releases of Puka can change the secret API in backward-incompatible ways, and patch releases can add or deprecate functionality.

If it's not even documented in the code, use at your own risk—no semver guarantees apply.

The sh DSL

Syntax

An sh template comprises words, separated by whitespace. Words can contain:

• text, which is composed of any characters that are not whitespace, single or double quotes, or any of the special characters # $ & ( ) ; < > \ ` |;
• quotations, which are matching single or double quotes surrounding any characters other than the delimiting quote; and
• placeholders, using the standard JavaScript template syntax (${}). (Placeholders may also appear inside quotations.)

The special characters # $ & ( ) ; < > \ ` |, if unquoted, form their own words.

Redirect operators (<, >, >>, 2>, etc.) receive their own special handling, as do semicolons. Other than these two exceptions, no attempt is made to understand any more sophisticated features of shell syntax.

Standard JavaScript escape sequences, such as \t, are honored in the template literal, and are treated equivalently to the characters they represent. There is no further mechanism for escaping within the sh DSL itself; in particular, if you want to put quotes inside quotes, you have to use interpolation, like this:

sh`echo "${'single = \', double = "'}"` // => "echo 'single = '\\'', double = \"'"

Semantics

Words that do not contain placeholders are emitted mostly verbatim to the output string. Quotations are formatted in the expected style for the target platform (single quotes for Unix, double quotes for Windows) regardless of the quotes used in the template literal—as with JavaScript, single and double quotes are interchangeable, except for the requirement to pair like with like. Unquoted semicolons are translated to ampersands on Windows; all other special characters (as enumerated above), when unquoted, are passed as-is to the output for the shell to interpret.

Puka may still quote words not containing the above special characters, if they contain characters that need quoting on the target platform. For example, on Windows, the character % is used for variable interpolation in cmd.exe, and Puka quotes it on on that platform even if it appears unquoted in the template literal. Consequently, there is no need to be paranoid about quoting anything that doesn't look alphanumeric inside a sh template literal, for fear of being burned on a different operating system; anything that matches the definition of ‘text’ above will never need manual quoting.

Types of placeholders

Strings

If a word contains a string placeholder, then the value of the placeholder is interpolated into the word and the entire word, if necessary, is quoted. If the placeholder occurs within quotes, no further quoting is performed:

sh`script --file="${'herp derp'}.txt"`; // => "script --file='herp derp.txt'"

This behavior can be exploited to force consistent quoting, if desired; but both of the examples below are safe on all platforms:

const words = ['oneword', 'two words'];
sh`minimal ${words[0]}`; // => "minimal oneword"
sh`minimal ${words[1]}`; // => "minimal 'two words'"
sh`consistent '${words[0]}'`; // => "consistent 'oneword'"
sh`consistent '${words[1]}'`; // => "consistent 'two words'"

Arrays and iterables

If a word contains a placeholder for an array (or other iterable object), then the entire word is repeated once for each value in the array, separated by spaces. If the array is empty, then the word is not emitted at all, and neither is any leading whitespace.

const files = ['foo', 'bar'];
sh`script ${files}`; // => "script foo bar"
sh`script --file=${files}`; // => "script --file=foo --file=bar"
sh`script --file=${[]}`; // => "script"

Note that, since special characters are their own words, the pipe operator here is not repeated:

const cmd = ['script', 'foo', 'bar'];
sh`${cmd}|another-script`; // => "script foo bar|another-script"

Multiple arrays in the same word generate a Cartesian product:

const names = ['foo', 'bar'], exts = ['log', 'txt'];
// Same word
sh`... ${names}.${exts}`; // => "... foo.log foo.txt bar.log bar.txt"
sh`... "${names} ${exts}"`; // => "... 'foo log' 'foo txt' 'bar log' 'bar txt'"

// Not the same word (extra space just for emphasis):
sh`... ${names}   ${exts}`; // => "... foo bar   log txt"
sh`... ${names};${exts}`; // => "... foo bar;log txt"

Finally, if a placeholder appears in the object of a redirect operator, the entire redirect is repeated as necessary:

sh`script > ${['foo', 'bar']}.txt`; // => "script > foo.txt > bar.txt"
sh`script > ${[]}.txt`; // => "script"

unquoted

The unquoted function returns a value that will skip being quoted when used in a placeholder, alone or in an array.

const cmd = 'script < input.txt';
const fields = ['foo', 'bar'];
sh`${unquoted(cmd)} | json ${fields}`; // => "script < input.txt | json foo bar"

ShellString

If ShellString.sh is used to construct an unformatted ShellString, that value can be used in a placeholder to insert the contents of the ShellString into the outer template literal. This is safer than using unquoted as in the previous example, but unquoted can be used when all you have is a string from another (trusted!) source.

const url = 'http://example.com/data.json?x=1&y=2';
const curl = ShellString.sh`curl -L ${url}`;
const fields = ['foo', 'bar'];
sh`${curl} | json ${fields}`; // => "curl -L 'http://example.com/data.json?x=1&y=2' | json foo bar"

Anything else

... is treated like a string—namely, a value x is equivalent to '' + x, if not in one of the above categories.