package main

import (
	"fmt"
	"os"
	"path/filepath"
)

func IsSymlink(path string) (bool, error) {
	var isSymlink bool
	stat, err := os.Lstat(path)

	if err != nil {
		return isSymlink, err
	}

	isSymlink = (stat.Mode() & os.ModeSymlink) == os.ModeSymlink

	return isSymlink, nil
}

func main() {
	// create temporary file
	tmpFile, err := os.CreateTemp("", "tmpfile")

	if err != nil {
		panic(err)
	}

	defer os.Remove(tmpFile.Name())

	fmt.Println("Temp file name:", tmpFile.Name())

	symlinkPath := filepath.Join(os.TempDir(), "tmpsymlink")

	// create symlink
	err = os.Symlink(tmpFile.Name(), symlinkPath)

	if err != nil {
		panic(err)
	}

	defer os.Remove(symlinkPath)

	// check if file is symlink
	isSymlink, err := IsSymlink(symlinkPath)

	if err != nil {
		panic(err)
	}

	fmt.Printf("'%s' is symlink: %v", symlinkPath, isSymlink)

}

// $ go run main.go 
// Temp file name: /tmp/tmpfile1471080551
// '/tmp/tmpsymlink' is symlink: true

package main

import (
	"fmt"
	"net"
	"net/url"
)

func main() {
	s := "mysql://user:pass@host.com:3306/path?key=value#fragment"

	u, err := url.Parse(s)

	if err != nil {
		panic(err)
	}

	fmt.Println("Scheme: " + u.Scheme)

	fmt.Println("Username: " + u.User.Username())
	p, _ := u.User.Password()
	fmt.Println("Password: " + p)

	fmt.Println("Host with port: " + u.Host)

	host, port, err := net.SplitHostPort(u.Host)

	if err != nil {
		panic(err)
	}

	fmt.Println("Host: " + host)
	fmt.Println("Port: " + port)

	fmt.Println("Path: " + u.Path)
	fmt.Println("Fragment: " + u.Fragment)

	fmt.Println("Raw query: " + u.RawQuery)

	m, err := url.ParseQuery(u.RawQuery)

	if err != nil {
		panic(err)
	}

	fmt.Println("Value: " + m["key"][0])
}

// $ go run main.go 
// Scheme: mysql
// Username: user
// Password: pass
// Host with port: host.com:3306
// Host: host.com
// Port: 3306
// Path: /path
// Fragment: fragment
// Raw query: key=value
// Value: value

package main

import (
	"fmt"
	"time"
)

func print(from string) {
	for i := 0; i < 3; i++ {
		fmt.Println(from, ":", i)
	}
}

func main() {
	// Synchronous function call
	print("synchronous call")

	// Asynchronous function call
	// To run this function in a goroutine, use go f(s)
	// This new goroutine will execute concurrently with the calling main goroutine
	go print("asynchronous call")

	// Goroutine can also be execute as an anonymous function
	go func(msg string) {
		fmt.Println(msg)
	}("asynchronous anonymous call")

	// Two previous function calls are now running asynchronously in separate goroutines
	// Wait for them to finish
	time.Sleep(time.Second)
	fmt.Println("done")
}

// $ go run main.go
// synchronous call : 0
// synchronous call : 1
// synchronous call : 2
// asynchronous anonymous call
// asynchronous call : 0
// asynchronous call : 1
// asynchronous call : 2
// done

package main

import "fmt"

func main() {
    // Create unbuffered channel
	unbufferedChannel := make(chan string)

	go func() {
        // Send a value into a channel using the channel<- syntax
		unbufferedChannel <- "unbuffered channel"
	}()

    // The <-channel syntax receives a value from the channel
	fmt.Println(<-unbufferedChannel)

    // Create buffered channel with size 2
	bufferedChannel := make(chan string, 2)

    // Send a value to a buffered channel. The operation in non blocking
	bufferedChannel <- "buffered"
	bufferedChannel <- "channel"

	fmt.Println(<-bufferedChannel)
	fmt.Println(<-bufferedChannel)
}

// go run main.go 
// unbuffered channel
// buffered
// channel

package main

import (
    "fmt"
    "runtime"
)

func main() {
    // The runtime.GOOS constant can be used to detect the OS at runtime,
    // since this constant is only set at runtime.
    os := runtime.GOOS

    switch os {
    case "windows":
        fmt.Println("Windows")
    case "darwin":
        fmt.Println("MacOS")
    case "linux":
        fmt.Println("Linux")
    default:
        fmt.Printf("%s.\n", os)
    }

    // The runtime.GOARCH constant can be used to determine the target architecture of a running program.
    fmt.Println(runtime.GOARCH)
}

// Output:
// Linux
// amd64

package main

import (
    "fmt"
    "os"
    "path/filepath"
)

func checkErr(err error) {
    if err != nil {
        panic(err)
    }
}

func main() {
    // The simplest way to create a temporary file is to call os.CreateTemp.
    // It will create and open the file for reading and writing.
    // We used "" as the first argument, so os.CreateTemp will create a file in the default directory.
    tmpFile, err := os.CreateTemp("", "tmpfile")
    checkErr(err)
    defer os.Remove(tmpFile.Name())

    fmt.Println("Temp file name:", tmpFile.Name())

    // Write some data to the temporary file
    _, err = tmpFile.Write([]byte{1, 2, 3, 4, 5})
    checkErr(err)

    // If we intend to write a lot of temporary files, we may prefer to create a temporary directory.
    // The arguments to os.MkdirTemp are the same as for os.CreateTemp, but it returns the directory name rather than the opened file.
    dName, err := os.MkdirTemp("", "tmpdir")
    checkErr(err)
    defer os.RemoveAll(dName)
    
    fmt.Println("Temp directory name:", dName)

    fName := filepath.Join(dName, "testFile")
    err = os.WriteFile(fName, []byte{1, 2, 3, 4, 5}, 0666)
    checkErr(err)
    defer os.Remove(fName)
}

// Output:
// Temp file name: /tmp/tmpfile3400905374
// Temp directory name: /tmp/tmpdir2812568099

# File management using context manager
class FileManager():
    def __init__(self, filename, mode):
        self.filename = filename
        self.mode = mode
        self.file = None
  
    # The __enter__ method opens the file and returns a file object
    def __enter__(self):
        print('Open file: {}'.format(self.filename))
        self.file = open(self.filename, self.mode)
        return self.file

    # The __exit__ method takes care of closing the file on exiting the with block 
    def __exit__(self, etype, value, traceback):
        print('Close file: {}'.format(self.filename))
        self.file.close()

# A FileManager object is created with test.txt as the filename and "write" mode
# when __init__ method is executed
with FileManager('test.txt', 'w') as f:
    f.write('First line\n')
    f.write('Second line\n')

# The file is already closed thanks to the automatic call to the __exit__ method
print('File closed: {}\n'.format(f.closed))

with FileManager('test.txt', 'r') as f:
    for line in f:
        print(line.rstrip())

# Output
#Open file: test.txt
#Close file: test.txt
#File closed: True
#
#Open file: test.txt
#First line
#Second line
#Close file: test.txt

package main

import (
    "bufio"
    "fmt"
    "os"
)

func checkErr(err error) {
    if err != nil {
        panic(err)
    }
}

func main() {
    content := "Some content"
    // write a string (or just bytes) into a file
    err := os.WriteFile("/tmp/dfile1", []byte(content), 0644)
    checkErr(err)

    // open a file for writing
    file, err := os.Create("/tmp/dfile2")
    checkErr(err)
    // defer a Close immediately after opening a file
    defer file.Close()

    bytesCount, err := file.Write([]byte(content))
    checkErr(err)
    fmt.Printf("wrote %d bytes\n", bytesCount)

    bytesCount, err = file.WriteString("Some other content\n")
    checkErr(err)
    fmt.Printf("wrote %d bytes\n", bytesCount)
    // flush writes to a stable storage (file system for example)
    file.Sync()

    // bufio provides buffered writers
    writer := bufio.NewWriter(file)
    bytesCount, err = writer.WriteString("Some othe buffered content\n")
    checkErr(err)
    fmt.Printf("wrote %d bytes\n", bytesCount)
    // ensure all buffered operations have been applied to the underlying writer
    writer.Flush()
}

package main

import (
    "fmt"
    "time"
)

func main() {
    // in this example we will choose between two channels
    c1 := make(chan string)
    c2 := make(chan string)

    go func() {
        time.Sleep(1 * time.Second)
        c1 <- "one"
    }()

    // each channel will receive a value after some time
    go func() {
        time.Sleep(2 * time.Second)
        c2 <- "two"
    }()

    // use select statement to wait for both values ​​at the same time,
    // printing each one as it arrives
    for i := 0; i < 2; i++ {
        select {
        case msg1 := <-c1:
            fmt.Println("received", msg1)
        case msg2 := <-c2:
            fmt.Println("received", msg2)
        }
    }
}

package main

import (
    "fmt"
    "time"
)

// this is a worker that we will run in several parallel instances
// these workers will receive tasks through the 'jobs' channel and send the results to 'results'
// we will wait for one second for each task to simulate heavy requests
func worker(id int, jobs <-chan int, results chan<- int) {
    for j := range jobs {
        fmt.Println("worker", id, "started job", j)
        time.Sleep(time.Second)
        fmt.Println("worker", id, "finished job", j)
        results <- j * 2
    }
}

func main() {
    // to use worker pool, we need to send a task and receive the execution results
    // therefore 2 channels are created
    jobs := make(chan int, 100)
    results := make(chan int, 100)

    // start 3 workers, initially blocked because no assignments yet
    for w := 1; w <= 3; w++ {
        go worker(w, jobs, results)
    }

    // send 5 jobs and then close the channel, notofying that all jobs have been sent
    for j := 1; j <= 5; j++ {
        jobs <- j
    }

    close(jobs)

    // collect all the results
    // this also ensures that the goroutines have ended
    for a := 1; a <= 5; a++ {
        <-results
    }
}

package main

import (
    "fmt"
    "time"
)

// the channel is used to notify main goroutine that the function completed successfully
func worker(done chan bool) {
    fmt.Println("working...")
    time.Sleep(time.Second)
    fmt.Println("done")

    done <- true // send a value to indicate that the function completed successfully
}

func main() {
    done := make(chan bool, 1) // create a buffered channel for notification
    go worker(done) // run worker

    <-done // blocked until a notification is received from the worker from the channel
           // if you remove the line <- done the program will close before the worker starts
}

package main

import (
	"errors"
	"fmt"
	"os"
)

func main() {
	filePath := "file.txt"

	if _, err := os.Stat(filePath); errors.Is(err, os.ErrNotExist) {
		fmt.Println("file not exists")
	} else {
		fmt.Println("file exists")
	}
}

#!/bin/bash

die()
{
    echo "ERROR: $*" >&2
    exit 1
}

detect_os ()
{
    OS="$(uname -s)"
    ARCH="$(uname -m)"

    case "$OS" in
        Linux)
            if [ -e /etc/lsb-release ]; then
                source /etc/lsb-release

                DIST_ID="${DISTRIB_ID}"
                OS_VERSION="${DISTRIB_RELEASE}"
                OS_CODENAME="${DISTRIB_CODENAME}"
            elif [ -e /etc/os-release]; then
                source /etc/os-release

                DIST_ID="${ID}"
                OS_VERSION="${VERSION_ID}"
                OS_CODENAME="${VERSION_CODENAME}"

            elif [ $(which lsb_release 2>/dev/null) ]; then
                DIST_ID="$(lsb_release -s -i)"
                OS_VERSION="$(lsb_release -s -r)"
                OS_CODENAME="$(lsb_release -s -c)"
            else
                die "Colud not get OS information"
            fi

            case "$DIST_ID" in
                RedHat*)
                    OS_NAME="RedHat" ;;
                debian)
                    OS_NAME="Debian" ;;
                *)
                    OS_NAME="${DIST_ID}" ;;
            esac
            ;;
        *)
            die "Unsupported OS family: $OS"
            ;;
    esac

    echo "${OS}"
    echo "${OS_NAME}"
    echo "${OS_VERSION}"
    echo "${OS_CODENAME}"
}