diff --git a/src/config.rs b/src/config.rs
index d3135b58..f20bbfb5 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -18,6 +18,8 @@ use crate::error::Error;
 use std::cell::LazyCell;
 use std::collections::HashMap;
 use std::fmt;
+use std::fs;
+use std::io::{BufRead, BufReader, Read, Write};
 use std::path::PathBuf;
 use std::str::FromStr;
 
@@ -121,10 +123,45 @@ impl<'a> Config<'a> {
         Ok(())
     }
 
-    fn write(&self, _key: &str, _value: ConfigValue, _section: &str) -> Result<(), Error> {
-        // TODO: Copy the file and parse it manually. If the key exists, edit it in place during
-        // parsing. If it doesn't, add it at the end of the section. Replace the original file
-        // atomically.
+    /// Write config file with the least modification to the original file, and replace the
+    /// original.
+    fn write(&self, key: &str, value: ConfigValue, _section: &str) -> Result<(), Error> {
+        // Copy the file
+        let tempfile = self.filename.clone().with_extension("tmp");
+        fs::copy(self.filename.clone(), tempfile.clone())?;
+
+        {
+            // Parse the file, get seek offset
+            let mut file = fs::File::open(tempfile.clone())?;
+            let mut reader = BufReader::new(file);
+            let mut line = String::new();
+            let mut clear_offset: u64 = 0;
+
+            let linestart = format!("{} ", key);
+            loop {
+                let len = reader.read_line(&mut line)?;
+                if line.trim().starts_with(linestart.as_str()) {
+                    // Found offset
+                    break;
+                }
+                clear_offset = clear_offset + u64::try_from(len)?;  // This can fail?
+            }
+
+            // Keep remainder of the file in memory
+            let mut remainder = String::new();
+            reader.read_to_string(&mut remainder);
+
+            let mut file = reader.into_inner();
+
+            // Clear file after the found offset. If not offset was found, clears after EOF, and
+            // the remainder is empty.
+            file.set_len(clear_offset);
+            file.write(format!("{} = {}\n", key, value).as_bytes());
+            file.write(remainder.as_bytes());
+        }
+
+        fs::rename(tempfile, self.filename.clone());
+
         Ok(())
     }
 
diff --git a/src/error.rs b/src/error.rs
index a9baf6fe..84ed56d9 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -18,11 +18,13 @@ use crate::config::ConfigValue;
 use std::error::Error as StdError;
 use std::fmt;
 use std::io;
+use std::num::TryFromIntError;
 use std::path::PathBuf;
 
 #[derive(Debug)]
 pub(crate) enum Error {
     IOError(io::Error),
+    IntError(TryFromIntError),
     UnableToCreateConfigDir,
     InvalidConfigValueType(ConfigValue),
     InvalidValueType(String),
@@ -33,6 +35,7 @@ impl fmt::Display for Error {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
             Error::IOError(e) => write!(f, "io error: {}", e),
+            Error::IntError(e) => write!(f, "int error: {}", e),
             Error::UnableToCreateConfigDir => write!(f, "Unable to create config dir"),
             Error::InvalidConfigValueType(err) => write!(f, "Invalid ConfigValue type: {}", err),
             Error::InvalidValueType(err) => write!(f, "Invalid value type: {}", err),
@@ -50,3 +53,9 @@ impl From<io::Error> for Error {
         Error::IOError(err)
     }
 }
+
+impl From<TryFromIntError> for Error {
+    fn from(err: TryFromIntError) -> Error {
+        Error::IntError(err)
+    }
+}