How to Append Text to a File

How do I append text to a file?

cat >> filename
This is text, perhaps pasted in from some other source.
Or else entered at the keyboard, doesn't matter. 
^D

Essentially, you can dump any text you want into the file. CTRL-D sends an end-of-file signal, which terminates input and returns you to the shell.

How do I append to a file?

Set the mode in open() to "a" (append) instead of "w" (write):

with open("test.txt", "a") as myfile:
    myfile.write("appended text")

The documentation lists all the available modes.

How do I append text to a file with python?

For Append File:

with open("newfile.txt", "a+") as file:
    file.write("I am adding in more lines\n")
    file.write("And more…")

For Read File:

with open('newfile.txt') as f:
    lines = f.readlines()
     print(lines)

How to append text to an existing file in Java?

Are you doing this for logging purposes? If so there are several libraries for this. Two of the most popular are Log4j and Logback.

Java 7+

For a one-time task, the Files class makes this easy:

try {
    Files.write(Paths.get("myfile.txt"), "the text".getBytes(), StandardOpenOption.APPEND);
}catch (IOException e) {
    //exception handling left as an exercise for the reader
}

Careful: The above approach will throw a NoSuchFileException if the file does not already exist. It also does not append a newline automatically (which you often want when appending to a text file). Another approach is to pass both CREATE and APPEND options, which will create the file first if it doesn't already exist:

private void write(final String s) throws IOException {
    Files.writeString(
        Path.of(System.getProperty("java.io.tmpdir"), "filename.txt"),
        s + System.lineSeparator(),
        CREATE, APPEND
    );
}

However, if you will be writing to the same file many times, the above snippets must open and close the file on the disk many times, which is a slow operation. In this case, a BufferedWriter is faster:

try(FileWriter fw = new FileWriter("myfile.txt", true);
    BufferedWriter bw = new BufferedWriter(fw);
    PrintWriter out = new PrintWriter(bw))
{
    out.println("the text");
    //more code
    out.println("more text");
    //more code
} catch (IOException e) {
    //exception handling left as an exercise for the reader
}

Notes:

• The second parameter to the FileWriter constructor will tell it to append to the file, rather than writing a new file. (If the file does not exist, it will be created.)
• Using a BufferedWriter is recommended for an expensive writer (such as FileWriter).
• Using a PrintWriter gives you access to println syntax that you're probably used to from System.out.
• But the BufferedWriter and PrintWriter wrappers are not strictly necessary.

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Older Java

try {
    PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("myfile.txt", true)));
    out.println("the text");
    out.close();
} catch (IOException e) {
    //exception handling left as an exercise for the reader
}

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Exception Handling

If you need robust exception handling for older Java, it gets very verbose:

FileWriter fw = null;
BufferedWriter bw = null;
PrintWriter out = null;
try {
    fw = new FileWriter("myfile.txt", true);
    bw = new BufferedWriter(fw);
    out = new PrintWriter(bw);
    out.println("the text");
    out.close();
} catch (IOException e) {
    //exception handling left as an exercise for the reader
}
finally {
    try {
        if(out != null)
            out.close();
    } catch (IOException e) {
        //exception handling left as an exercise for the reader
    }
    try {
        if(bw != null)
            bw.close();
    } catch (IOException e) {
        //exception handling left as an exercise for the reader
    }
    try {
        if(fw != null)
            fw.close();
    } catch (IOException e) {
        //exception handling left as an exercise for the reader
    }
}

How to append text to a text file in C++?

You need to specify the append open mode like

#include <fstream>

int main() {  
  std::ofstream outfile;

  outfile.open("test.txt", std::ios_base::app); // append instead of overwrite
  outfile << "Data"; 
  return 0;
}

How to append two lists in order into a text file?

Try the following:

a = [1, 2, 3, 4]

b = ["hello", "how", "iz", "life"]


c = zip(a,b)

with open("sample.txt", 'w') as outfile:
    for elem in c:
        outfile.write(str(elem[0]) + " " + elem[1] +"\n")

This will output:

1 hello
2 how
3 iz
4 life

to a file called sample.txt.

Edit:
Kelly makes a great point you can just simplify the line as print(*elem, file=outfile). I looked at the docs for print and I was surprised to find that the print function defines a separator for elements given in the *objects argument and ends the line with a newline through default arguments, which provides us with the same desired output.

Append text to a Text file without replacing it Python

When you write to a file it always effectively overwrites the bytes inside the file stream. What you might want to do instead, is read the file first, and write the necessary parts, and then write your original contents back:

with open(filename,'r+',encoding="UTF-8") as file:
    data = file.read()
    file.write('test\n')
    file.write(data)

This should be all you need. Remove the f = open(filename) and file_contents = f.read() lines, because you are opening the same file twice.

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