What exactly does "/usr/bin/env node" do at the beginning of node files?

Question

I had seen this line #!/usr/bin/env node at the beginning of some examples in nodejs and I had googled without finding any topic that could answer the reason for that line.

The nature of the words makes search it not that easy.

I'd read some javascript and nodejs books recently and I didn't remember seeing it in any of them.

If you want an example, you could see the RabbitMQ official tutorial, they have it in almost all of their examples, here is one of them:

#!/usr/bin/env node

var amqp = require('amqplib/callback_api');

amqp.connect('amqp://localhost', function(err, conn) {
  conn.createChannel(function(err, ch) {
    var ex = 'logs';
    var msg = process.argv.slice(2).join(' ') || 'Hello World!';

    ch.assertExchange(ex, 'fanout', {durable: false});
    ch.publish(ex, '', new Buffer(msg));
    console.log(" [x] Sent %s", msg);
  });

  setTimeout(function() { conn.close(); process.exit(0) }, 500);
});

Could someone explain me what is the meaning of this line?

What is the difference if I put or remove this line? In what cases do I need it?

Jeremie

it basically takes the environment of the calling shell and stuffs that environment into whatever app is specified. in this case, node – Clarineclarinet

Actually no, I am not comming from Windows, but thank you for updating your answer. I am just waiting to see if anyone else cames with a diferent opinion. There is just one thing that I think you didn't mention in your answer, I just find it a couple of hours ago. The things that they mention here seems kind of important but it is not clear enough yet for me. #14518035 (you can update if you want, I will really apreciate that, but don't feel it like an obligation, your answer is good enough right now). – Jeremie

@Gepser: Got it. The short of it is: if you want npm to install a Node.js source script as a (potentially globally available) CLI, you must use a shebang line - and npm will even make that work on Windows; see my once-again updated answer. – Threadfin

"The nature of the words makes search it not that easy" – you may want to try duckduckgo.com for this specific search use case – Antakiya

Possible duplicate of Why do people write the #!/usr/bin/env python shebang on the first line of a Python script?. Same question, different interpreter. – Klaipeda

Answer 1

#!/usr/bin/env node is an instance of a shebang line: the very first line in an executable plain-text file on Unix-like platforms that tells the system what interpreter to pass that file to for execution, via the command line following the magic #! prefix (called shebang).

Note: Windows does not support shebang lines, so they're effectively ignored there; on Windows it is solely a given file's filename extension that determines what executable will interpret it. However, you still need them in the context of npm.^[1]

The following, general discussion of shebang lines is limited to Unix-like platforms:

^{In the following discussion I'll assume that the file containing source code for execution by Node.js is simply named file.}

• You NEED this line, if you want to invoke a Node.js source file directly, as an executable in its own right - this assumes that the file has been marked as executable with a command such as chmod +x ./file, which then allows you to invoke the file with, for instance, ./file, or, if it's located in one of the directories listed in the $PATH variable, simply as file.

  • Specifically, you need a shebang line to create CLIs based on Node.js source files as part of an npm package, with the CLI(s) to be installed by npm based on the value of the "bin" key in a package's package.json file; also see this answer for how that works with globally installed packages. Footnote [1] shows how this is handled on Windows.

• You do NOT need this line to invoke a file explicitly via the node interpreter, e.g., node ./file

---

Optional background information:

#!/usr/bin/env <executableName> is a way of portably specifying an interpreter: in a nutshell, it says: execute <executableName> wherever you (first) find it among the directories listed in the $PATH variable (and implicitly pass it the path to the file at hand).

This accounts for the fact that a given interpreter may be installed in different locations across platforms, which is definitely the case with node, the Node.js binary.

By contrast, the location of the env utility itself can be relied upon to be in the same location across platforms, namely /usr/bin/env - and specifying the full path to an executable is required in a shebang line.

Note that POSIX utility env is being repurposed here to locate by filename and execute an executable in the $PATH.
The true purpose of env is to manage the environment for a command - see env's POSIX spec and Keith Thompson's helpful answer.

---

It's also worth noting that Node.js is making a syntax exception for shebang lines, given that they're not valid JavaScript code (# is not a comment character in JavaScript, unlike in POSIX-like shells and other interpreters).

---

^{[1] In the interest of cross-platform consistency, npm creates wrapper *.cmd files (batch files) on Windows when installing executables specified in a package's package.json file (via the "bin" property). Essentially, these wrapper batch files mimic Unix shebang functionality: they invoke the target file explicitly with the executable specified in the shebang line - thus, your scripts must include a shebang line even if you only ever intend to run them on Windows - see this answer of mine for details.
Since *.cmd files can be invoked without the .cmd extension, this makes for a seamless cross-platform experience: on both Windows and Unix you can effectively invoke an npm-installed CLI by its original, extension-less name.}

Threadfin

Can you provide an explanation or summary for dummies like me? – Dachshund

@AndrewLam: On Windows, filename extensions such as .cmd and .py determine what program will be used to execute such files. On Unix, the shebang line performs that function. To make npm work on all supported platforms, you need the shebang line even on Windows. – Threadfin

Answer 2

Scripts that are to be executed by an interpreter normally have a shebang line at the top to tell the OS how to execute them.

If you have a script named foo whose first line is #!/bin/sh, the system will read that first line and execute the equivalent of /bin/sh foo. Because of this, most interpreters are set up to accept the name of a script file as a command-line argument.

The interpreter name following the #! has to be a full path; the OS won't search your $PATH to find the interpreter.

If you have a script to be executed by node, the obvious way to write the first line is:

#!/usr/bin/node

but that doesn't work if the node command isn't installed in /usr/bin.

A common workaround is to use the env command (which wasn't really intended for this purpose):

#!/usr/bin/env node

If your script is called foo, the OS will do the equivalent of

/usr/bin/env node foo

The env command executes another command whose name is given on its command line, passing any following arguments to that command. The reason it's used here is that env will search $PATH for the command. So if node is installed in /usr/local/bin/node, and you have /usr/local/bin in your $PATH, the env command will invoke /usr/local/bin/node foo.

The main purpose of the env command is to execute another command with a modified environment, adding or removing specified environment variables before running the command. But with no additional arguments, it just executes the command with an unchanged environment, which is all you need in this case.

There are some drawbacks to this approach. Most modern Unix-like systems have /usr/bin/env, but I worked on older systems where the env command was installed in a different directory. There might be limitations on additional arguments you can pass using this mechanism. If the user doesn't have the directory containing the node command in $PATH, or has some different command called node, then it could invoke the wrong command or not work at all.

Other approaches are:

• Use a #! line that specifies the full path to the node command itself, updating the script as needed for different systems; or
• Invoke the node command with your script as an argument.

See also this question (and my answer) for more discussion of the #!/usr/bin/env trick.

Incidentally, on my system (Linux Mint 17.2), it's installed as /usr/bin/nodejs. According to my notes, it changed from /usr/bin/node to /usr/bin/nodejs between Ubuntu 12.04 and 12.10. The #!/usr/bin/env trick won't help with that (unless you set up a symlink or something similar).

UPDATE: A comment by mtraceur says (reformatted):

A workaround for the nodejs vs node problem is to start the file with the following six lines:

#!/bin/sh -
':' /*-
test1=$(nodejs --version 2>&1) && exec nodejs "$0" "$@"
test2=$(node --version 2>&1) && exec node "$0" "$@"
exec printf '%s\n' "$test1" "$test2" 1>&2
*/

This will first try nodejs and then try node, and only print the error messages if both of them are not found. An explanation is out of scope of these comments, I'm just leaving it here in case it helps anyone deal with the problem since this answer brought the problem up.

I haven't used NodeJS lately. My hope is that the nodejs vs. node issue has been resolved in the years since I first posted this answer. On Ubuntu 18.04, the nodejs package installs /usr/bin/nodejs as a symlink to /usr/bin/node. On some earlier OS (Ubuntu or Linux Mint, I'm not sure which), there was a nodejs-legacy package that provided node as a symlink to nodejs. No guarantee that I have all the details right.

Overarch

A very thorough answer providing why of things. – Ehudd

A workaround for the nodejs vs node problem is to start the file with the following six lines: 1) #!/bin/sh -, 2) ':' /*- 3) test1=$(nodejs --version 2>&1) && exec nodejs "$0" "$@" 4) test2=$(node --version 2>&1) && exec node "$0" "$@", 5) exec printf '%s\n' "$test1" "$test2" 1>&2 6) */. This will first try nodejs and then try node, and only print the error messages if both of them are not found. An explanation is out of scope of these comments, I'm just leaving it here in case it helps anyone deal with the problem since this answer brought the problem up. – Rabbin

@mtraceur: I've incorporated your comment into my answer. Why the - on the #! line? – Overarch

The - in #!/bin/sh - is just a habit that makes sure the shell behaves right in the extremely narrow and unlikely set of circumstances that the script name or relative path that the shell sees starts with a -. (Also, yes, it looks like every mainstream distro had converged back to node as the primary name. I didn't go digging to check when making my comment, but as far as I know only the Debian distro family tree used nodejs, and it looks like they all reverted back to also supporting node once Debian did.) – Rabbin

Technically the single dash as first argument did not mean "end of options" - it originally meant "turn off -x and -v", but since the early Bourne-likes only parsed the first argument as possible options, and since the shell starts with those options off, it was abusable to cause the shell to not try to parse the script name since the original, and remains thusly abusable because the behavior is maintained in modern Bourne-likes for compatibility reasons. If I remember all my Bourne history and portability trivia right. – Rabbin

I think every modern Bourne-like shell supports the more recognizable convention of -- being used to signal the end of options, but I am afflicted with a rare brain disease that forces me to prefer shell scripting habits that will work on the widest range of Bourne-like shells unless there is a compelling reason otherwise. I tend to not remember or think about the interdependencies between those habits, which is why I stuck with - instead of -- even though I used the $( substitution instead of backticks and did "$@" without handling the ancient bug in the case of no arguments. – Rabbin

Answer 3

The exec system call of the Linux kernel understands shebangs (#!) natively

When you do on bash:

./something

on Linux, this calls the exec system call with the path ./something.

This line of the kernel gets called on the file passed to exec: https://github.com/torvalds/linux/blob/v4.8/fs/binfmt_script.c#L25

if ((bprm->buf[0] != '#') || (bprm->buf[1] != '!'))

It reads the very first bytes of the file, and compares them to #!.

If the comparison is true, then the rest of the line is parsed by the Linux kernel, which makes another exec call with:

• executable: /usr/bin/env
• first argument: node
• second argument: script path

therefore equivalent to:

/usr/bin/env node /path/to/script.js

env is an executable that searches PATH to e.g. find /usr/bin/node, and then finally calls:

/usr/bin/node /path/to/script.js

The Node.js interpreter does see the #! line in the file, but it must be programmed to ignore that line even though # is not in general a valid comment character in Node (unlike many other languages such as Python where it is), see also: Pound Sign (#) As Comment Start In JavaScript?

And yes, you can make an infinite loop with:

printf '#!/a\n' | sudo tee /a
sudo chmod +x /a
/a

Bash recognizes the error:

-bash: /a: /a: bad interpreter: Too many levels of symbolic links

#! just happens to be human readable, but that is not required.

If the file started with different bytes, then the exec system call would use a different handler. The other most important built-in handler is for ELF executable files: https://github.com/torvalds/linux/blob/v4.8/fs/binfmt_elf.c#L1305 which checks for bytes 7f 45 4c 46 (which also happens to be human readable for .ELF). Let's confirm that by reading the 4 first bytes of /bin/ls, which is an ELF executable:

head -c 4 "$(which ls)" | hd 

output:

00000000  7f 45 4c 46                                       |.ELF|
00000004                                                                 

So when the kernel sees those bytes, it takes the ELF file, puts it into memory correctly, and starts a new process with it. See also: How does kernel get an executable binary file running under linux?

Finally, you can add your own shebang handlers with the binfmt_misc mechanism. For example, you can add a custom handler for .jar files. This mechanism even supports handlers by file extension. Another application is to transparently run executables of a different architecture with QEMU.

I don't think POSIX specifies shebangs however: https://unix.stackexchange.com/a/346214/32558 , although it does mention in on rationale sections, and in the form "if executable scripts are supported by the system something may happen". macOS and FreeBSD also seem to implement it however.

PATH search motivation

Likely, one big motivation for the existence of shebangs is the fact that in Linux, we often want to run commands from PATH just as:

basename-of-command

instead of:

/full/path/to/basename-of-command

But then, without the shebang mechanism, how would Linux know how to launch each type of file?

Hardcoding the extension in commands:

 basename-of-command.js

or implementing PATH search on every interpreter:

node basename-of-command

would be a possibility, but this has the major problem that everything breaks if we ever decide to refactor the command into another language.

Shebangs solve this problem beautifully.

Perish

You have mentioned /usr/bin/node /path/to/script.py , but i think you actually meant /usr/bin/node /path/to/script.js. Please correct the snippet. Currently edit queue is full. Or I could have edited. – Serrulate

Answer 4

Short answer: It is the path to the interpreter.

EDIT (Long Answer): The reason there is no slash before "node" is because you can not always guarantee the reliability of #!/bin/ . The "/env" bit makes the program more cross-platform by running the script in a modified environment and more reliably being able to find the interpreter program.

You do not necessarily need it, but it is good to use to ensure portability (and professionalism)

Ooze

The /usr/bin/env bit doesn't modify the environment. It's just a command at a (mostly) known location that invokes another command given as an argument, and searches $PATH to find it. The point is that the #! line requires the full path to the command being invoked, and you don't necessarily know where node is installed. – Overarch

That was what I was going at, thanks for clarifying! – Ooze