How to Print on Stderr with Swift

How can I output to STDERR with Swift?

For Swift 4.x:

import Darwin
fputs("hello from libc\n", stderr)

How to print on stderr with Swift?

From https://swift.org/blog/swift-linux-port/:

The Glibc Module: Most of the Linux C standard library is available through this module similar to the Darwin module on Apple platforms.

So this should work on all Swift platforms:

#if os(Linux)
    import Glibc
#else
    import Darwin
#endif

public struct StderrOutputStream: OutputStreamType {
    public mutating func write(string: String) { fputs(string, stderr) }
}
public var errStream = StderrOutputStream()

debugPrint("Debug messages...", toStream: &errStream)

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Update for Swift 3:

public struct StderrOutputStream: TextOutputStream {
    public mutating func write(_ string: String) { fputs(string, stderr) }
}
public var errStream = StderrOutputStream()

debugPrint("Debug messages...", to: &errStream) // "Debug messages..."
print("Debug messages...", to: &errStream)      // Debug messages...

swift print doesn't appear in STDOut but 3rd party c library logs do when running in docker on ECS

Had the same issue, I filed a radar, and Apple answered:

When piped to another process print is buffered, so no characters appear until the buffer is filled up. (When piped to the terminal we only buffer until we hit a newline.)

You can get the behavior you want by calling setbuf(stdout, nil) once at startup:

 import Darwin
 setbuf(stdout, nil)

Swift stderr and stout into single file

Have you tried just calling freopen twice? Once with stderr and once with stdout? I think that worked fine for my purposes when I tried it.

Swift Print function formatting

Yes, it is similar to ${} in javaScript, and it is called String Interpolation:

String interpolation is a way to construct a new String value from a mix of constants, variables, literals, and expressions by including their values inside a string literal.

You can find more detailed information in here: Swift Documentation
under the section "String Interpolation".

How to print an array of characters in swift without a line terminator

In a Playground you need to print one final newline, otherwise the output
is not flushed to the output window:

for chrInStr in strTokenizeMe { print(chrInStr, terminator: " ")}
print()

A (not necessarily better) alternative would be to concatenate
the characters before printing them:

print(strTokenizeMe.map(String.init).joined(separator: " "))

The problem does not occur when running a compiled program, because
the standard output is always flushed on program exit.

How can I print some values ​of data? [Swift]

I'd define a type conforming to the Codable protocol and use a JSONDecoder to decode your data to something user friendly.

struct UserData: Codable {
    let userId: String
    let profile: String
}

Here's how to decode your data:

if let httpResponse = response as? HTTPURLResponse {

    guard let data = data else { return }

    let decoder = JSONDecoder()
    do {
        let userData = try decoder.decode(UserData.self, from: data)
        print(userData.profile)
    } catch {
        print("Error: \(error)")
    }
}

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