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Groovy makes file I/O concise.

Reading a file

def file = new File("data.txt")
file.text                    // entire content as String
file.bytes                   // entire content as byte[]
file.readLines()             // List<String>
file.eachLine { line, num ->
    println "$num: $line"
}

Writing

new File("out.txt").text = "hello\n"     // overwrite
new File("log.txt") << "appended\n"      // append
new File("data.bin").bytes = someByteArray

The << operator on a File appends — concise but mind the encoding.

Stream-style for big files

new File("big.csv").withReader { reader ->
    def line
    while ((line = reader.readLine()) != null) {
        process(line)
    }
}

withReader ensures the reader is closed after the block — idiomatic resource management.

Walking a directory

new File("src").eachFileRecurse { f ->
    if (f.name.endsWith(".groovy")) {
        println f.absolutePath
    }
}

new File("src").eachFile { f ->
    println f.name        // direct children only
}

stdin / stdout

For scripts (which is most Groovy code):

System.in.eachLine { line ->
    println line.toUpperCase()
}

// Or read all at once:
def text = System.in.text

// Or single line:
def line = System.in.newReader().readLine()

printf / println

println "hello"                    // line + newline
print "no newline"
printf("%-10s %d\n", "hi", 42)     // C-style
println "item: " + value

File system queries

def f = new File("data.txt")
f.exists()
f.size()                    // bytes
f.canRead()
f.canWrite()
f.isDirectory()
f.isFile()
f.lastModified()             // ms since epoch

Process execution

def proc = "git status".execute()
proc.waitForOrKill(5000)
println proc.text                  // stdout
println proc.errStr                // stderr
println proc.exitValue()           // 0/non-zero

// Pipe-like:
def out = ("ls -la" | "grep .groovy").text

Configurable I/O

file.withReader("UTF-8") { ... }      // explicit encoding
file.append("more\n", "UTF-8")
file.write("new content", "UTF-16")

For scripts that aren't files-on-disk (like Judge0), most of this is conceptual — but the same idioms apply when you have a real filesystem.