The Unix/Linux ls command can provide all kinds of details about a file or a directory
or symbolic link or some other things that can be on the file-system. Specifically using the -l
flag it can show the type of thing we are listing (file/directory/symbolic link/etc), whether it
is readable, writable, executable etc.
In general, it can fetch information from the inode table, which is not the Apple version of Node.js.
Before attempting to re-implement the ls unix command, let's see how can we fetch the details
of a single file-system entry using Node.js.
The fs library that comes with Node.js provides a non-blocking method called stat, that given a path to something on he file-system, will fetch the information from the inode of that thing and when done, will execute the callback provided to the method. It will path a fs.Stats object to the callback.
There is also a synchronous version called statSync that will, return the fs.Stats object, once the data was read from the file-system.
In this script we can see how to use the asynchronous version of this method:
var fs = require('fs');
if (process.argv.length <= 2) {
console.log("Usage: " + __filename + " path/to");
process.exit(-1);
}
var path = process.argv[2];
fs.stat(path, function(err, stats) {
console.log(path);
console.log();
console.log(stats);
console.log();
if (stats.isFile()) {
console.log(' file');
}
if (stats.isDirectory()) {
console.log(' directory');
}
console.log(' size: ' + stats["size"]);
console.log(' mode: ' + stats["mode"]);
console.log(' others eXecute: ' + (stats["mode"] & 1 ? 'x' : '-'));
console.log(' others Write: ' + (stats["mode"] & 2 ? 'w' : '-'));
console.log(' others Read: ' + (stats["mode"] & 4 ? 'r' : '-'));
console.log(' group eXecute: ' + (stats["mode"] & 10 ? 'x' : '-'));
console.log(' group Write: ' + (stats["mode"] & 20 ? 'w' : '-'));
console.log(' group Read: ' + (stats["mode"] & 40 ? 'r' : '-'));
console.log(' owner eXecute: ' + (stats["mode"] & 100 ? 'x' : '-'));
console.log(' owner Write: ' + (stats["mode"] & 200 ? 'w' : '-'));
console.log(' owner Read: ' + (stats["mode"] & 400 ? 'r' : '-'));
console.log(' file: ' + (stats["mode"] & 0100000 ? 'f' : '-'));
console.log(' directory: ' + (stats["mode"] & 0040000 ? 'd' : '-'));
});
The expected use of the script is node examples/node/stats.js path/to/file.
For example I ran node examples/node/stats.js examples providing the 'examples'
directory as the parameter and got the following output:
examples
{ dev: 16777220,
mode: 16877,
nlink: 11,
uid: 501,
gid: 20,
rdev: 0,
blksize: 4096,
ino: 32548075,
size: 374,
blocks: 0,
atime: Sat Jan 31 2015 10:56:30 GMT+0200 (IST),
mtime: Sat Jan 31 2015 10:52:13 GMT+0200 (IST),
ctime: Sat Jan 31 2015 10:52:13 GMT+0200 (IST) }
directory
size: 374
mode: 16877
others eXecute: x
others Write: -
others Read: r
group eXecute: x
group Write: w
group Read: r
owner eXecute: x
owner Write: w
owner Read: r
file: -
directory: d
We can compare that with the output of the appropriate Unix ls command:
$ ls -ld examples
drwxr-xr-x 11 gabor staff 374 Jan 31 10:52 examples
Let's take that script apart:
var fs = require('fs');
if (process.argv.length <= 2) {
console.log("Usage: " + __filename + " path/to");
process.exit(-1);
}
var path = process.argv[2];
After loading the fs module, we check the number of arguments passed on the command line.
If it is 2 or fewer (and I am not even sure fewer would be possible), that means the user has not give any command line parameters.
(If the user wrote node examples/node/stats.js then this number will be 2.) In that case we print out a usage-message that looks like this:
$ node examples/node/stats.js
Usage: /home/gabor/code-maven/examples/node/stats.js path/to
The global variable __filename (starting with two underscores) contains the full path to the current JavaScript file.
Then we call process.exit() to leave the Node.js script early.
Before reaching the end of the file.
The last step in this part of the code is to fetch element 3 of the argv which is the value the user has passed on the
command line, and assign it to a variable called path.
Calling fs.stat
Then we call the stat method, passing
the path variable and the callback. This callback function will receive and error-object - if there was an error,
and a fs.Stats object.
fs.stat(path, function(err, stats) {
The Stats object contains some data retrieved from the inode table (in our case it looked like this:)
{ dev: 16777220,
mode: 16877,
nlink: 11,
uid: 501,
gid: 20,
rdev: 0,
blksize: 4096,
ino: 32548075,
size: 374,
blocks: 0,
atime: Sat Jan 31 2015 10:56:30 GMT+0200 (IST),
mtime: Sat Jan 31 2015 10:52:13 GMT+0200 (IST),
ctime: Sat Jan 31 2015 10:52:13 GMT+0200 (IST) }
and it provides a few methods for more convenience.
Among the values that we got there dev is the device number. It might be interesting if you have
multiple disks or partitions mounted.
mode contains a lot of information, including the type of the thing (file/directory/symbolic link)
and the permissions on that thing.
uid is the user-id of the owner of this thing.
gid is the group-id of the owner of this thing.
size is, the size of the thing in bytes.
atime, mtime, and ctime are 3 different timestamps representing the last
access time, the last modify time and the create time of the thing.
Before checking out the value of mode, let's see a few helper functions:
isFile() will return True if the thing is a file.
isDirectory() will return True it the thing is a directory.
There are a few more such helper functions listed in documentation of fs.Stat.
The other values of the stat object can be accessed just as regular members of any JavaScript object. For example it is easy to access the size of the file:
Size of a file in Node.js
console.log(' size: ' + stats["size"]);
Mode and file access rights
man 2 stat provides information on how to interpret
the values in mode which was 16877 in our case.
We need to use special bitwise masks on that number to check if specific bits are on or off in that number.
For example mode & 1 will be 1 if the right-most bit in mode was on. Otherwise this will be 0.
mode & 2 will be 2 if he second bit from the right was on, and 0 if it was not.
mode & 4 will be 4 if he third(!) bit from the right was on, and 0 if it was not.
Luckily numbers, except of 0, are considered True in JavaScript. So we could use the
ternary operator ?: to returns some interesting character if the expression is different from 0
and return - if the expression was 0.
That's how our output resembles (in the usage of rwx- characters) to the output of ls -ld.
Besides the read-write-execute flags, we can also extract the file-type from the the mode
value, but for those we have already seen a set of more readable convenience methods.
Comments
I read the article several times, but I could not associate the same in my need, I need inside a folder to get the name of the last created file
It is much easier to use stat-mode in order to get the user/group/others permissions https://www.npmjs.com/package/stat-mode
Can anyone Answer me? I have List of files in the folder. i need to read the file from the folder while if i met criteria , i have to come out from the function and update the document.Ultimately i dont need to read all the files when condition met. using Node.js?
Group and Owner permissions check looks wrong. Group execute should be stats["mode"] & 8, write should be stats["mode"] & 16 and so forth.
