如何在纯Go语言的Linux上实现“ file -s <file>”？

Question

Intent:
Does Go have the functionality (package or otherwise) to perform a special file stat on Linux akin to the command file -s <path>

Example:

[root@localhost ~]# file /proc/uptime
/proc/uptime: empty
[root@localhost ~]# file -s /proc/uptime
/proc/uptime: ASCII text

Use Case:
I have a fileglob of files in /proc/* that I need to very quickly detect if they are truly empty instead of appearing to be empty.

Using The os Package:

Code:

result,_ := os.Stat("/proc/uptime")
fmt.Println("Name:",result.Name()," Size:",result.Size()," Mode:",int(result.Mode()))
fmt.Printf("%q",result)

Result:

Name: uptime  Size: 0  Mode: 292
&{"uptime" '\x00' 'Ĥ' {%!q(int64=63606896088) %!q(int32=413685520) %!q(*time.Location=&{ [] [] 0 0 <nil>})} {'\x03' %!q(uint64=4026532071) '\x01' '脤' '\x00' '\x00' '\x00' '\x00' '\x00' 'Ѐ' '\x00' {%!q(int64=1471299288) %!q(int64=413685520)} {%!q(int64=1471299288) %!q(int64=413685520)} {%!q(int64=1471299288) %!q(int64=413685520)} ['\x00' '\x00' '\x00']}}

Obvious Workaround:
There is the obvious workaround of the following. But it's a little over the top to need to call in a bash shell in order to get file stats.

output,_ := exec.Command("bash","-c","file -s","/proc/uptime").Output()
//parse output etc...

EDIT/MY PRACTICAL USE CASE:
Quickly determining which files are zero size without needing to read each one of them first.

file -s /cgroup/memory/lsf/<cluster>/*/tasks | <clean up commands> | uniq -c
6 /cgroup/memory/lsf/<cluster>/<jobid>/tasks: ASCII text
805 /cgroup/memory/lsf/<cluster>/<jobid>/tasks: empty

So in this case, I know that only those 6 jobs are running and the rest (805) have terminated. Reading the file works like this:

# cat /cgroup/memory/lsf/<cluster>/<jobid>/tasks
#

or

# cat /cgroup/memory/lsf/<cluster>/<jobid>/tasks
12352
53455
...

Answer 1

I'm afraid you might be confusing matters here: file is special in precisely a way it "knows" a set of heuristics to carry out its tasks.

To my knowledge, Go does not have anything like this in its standard library, and I've not came across a 3rd-party package implementing a file-like functionality (though I invite you to search by relevant keywords on http://godoc.org)

On the other hand, Go provides full access to the syscall interface of the underlying OS so when it comes to querying the OS in a way file does it, there's nothing you could not do in plain Go.

So I suggest you to just fetch the source code of file, learn what it does in its mode turned on by the "-s" command-line option and implement that in your Go code. We'll try to have you with specific problems doing that — should you have any.

Update

Looks like I've managed to grasp the OP is struggling with: a simple check:

$ stat -c %s /proc/$$/status && wc -c < $_
0
849

That is, the stat call on a file under /proc shows it has no contents but actually reading from that file returns that contents.

OK, so the solution is simple: instead of doing a call to os.Stat() while traversing the subtree of the filesystem one should instead merely attempt to read a single byte from the file, like in:

var buf [1]byte
f, err := os.Open(fname)
if err != nil {
    // do something, or maybe ignore.
    // A not existing file is OK to ignore
    // (the POSIX error code will be ENOENT)
    // because after the `path/filepath.Walk()` fetched an entry for
    // this file from its directory, the file might well have gone.
}
_, err = f.Read(buf[:])
if err != nil {
    if err == io.EOF {
        // OK, we failed to read 1 byte, so the file is empty.
    }
    // Otherwise, deal with the error
}
f.Close()

You might try to be more clever and first obtain the stat information (using a call to os.Stat()) to see if the file is a regular file—to not attempt reading from sockets etc.