diff --git a/Cargo.lock b/Cargo.lock
index 20a72ed..e6f7af7 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -172,7 +172,7 @@ checksum = "e2abad23fbc42b3700f2f279844dc832adb2b2eb069b2df918f455c4e18cc646"
 
 [[package]]
 name = "leanshot"
-version = "0.4.1"
+version = "0.5.0"
 dependencies = [
  "anyhow",
  "image",
diff --git a/Cargo.toml b/Cargo.toml
index 1e0f67e..8acb5a1 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,7 +1,7 @@
 [package]
 name = "leanshot"
 description = "Screenshot capture for Linux."
-version = "0.4.1"
+version = "0.5.0"
 repository = "https://git.mzhang.io/michael/leanshot"
 license-file = "LICENSE"
 edition = "2018"
diff --git a/src/gui.rs b/src/gui.rs
index e4d4a20..bd11f30 100644
--- a/src/gui.rs
+++ b/src/gui.rs
@@ -223,7 +223,12 @@ impl Gui {
             match evt.response_type() {
                 xcb::KEY_RELEASE => {
                     let evt = unsafe { xcb::cast_event::<xcb::KeyReleaseEvent>(&evt) };
-                    trace!("key released: root={} key={} state={}", evt.root(), evt.detail(), evt.state());
+                    trace!(
+                        "key released: root={} key={} state={}",
+                        evt.root(),
+                        evt.detail(),
+                        evt.state()
+                    );
 
                     if evt.detail() == 9 {
                         if state.dragging {
@@ -255,7 +260,7 @@ impl Gui {
 
                     // left mouse button
                     if state.dragging && evt.detail() == 1 {
-                            state.dragging = false;
+                        state.dragging = false;
                         if let Some(_) = state.rect() {
                             break;
                         }
diff --git a/src/main.rs b/src/main.rs
index 8dfeb2b..ec01cfc 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -4,6 +4,7 @@ extern crate log;
 extern crate anyhow;
 
 mod gui;
+mod singleton;
 
 use std::path::PathBuf;
 use std::process;
@@ -31,6 +32,10 @@ fn main() -> Result<()> {
             capture.save_to(&opt.outfile)?;
         }
         Region::Selection => {
+            info!("Creating lockfile...");
+            let _lockfile = singleton::check_singleton()?;
+            info!("Lockfile: {:?}", _lockfile);
+
             if let Some(rectangle) = gui.interactive_select(&capture)? {
                 capture.save_cropped_to(&opt.outfile, Some(rectangle))?;
             } else {
diff --git a/src/singleton.rs b/src/singleton.rs
new file mode 100644
index 0000000..a33f0c9
--- /dev/null
+++ b/src/singleton.rs
@@ -0,0 +1,73 @@
+use std::fs::{File, self};
+use std::io::{Write, Read};
+use std::path::PathBuf;
+use std::time::{SystemTime, Duration, UNIX_EPOCH};
+use std::process;
+
+use anyhow::{Error, Context, Result};
+
+const PATHS: &[&str] = &["/var/run", "/tmp"];
+
+pub fn check_singleton() -> Result<Lockfile> {
+    let (mut file, path) = get_lock_file()?;
+
+    let current_time = SystemTime::now().duration_since(UNIX_EPOCH)?.as_millis();
+    let pid = process::id();
+    let contents = format!("{}:{}", current_time, pid).as_bytes().to_vec();
+    file.write_all(&contents)?;
+
+    Ok(Lockfile(path))
+}
+
+#[derive(Debug)]
+pub struct Lockfile(PathBuf);
+
+impl Drop for Lockfile {
+    fn drop(&mut self) {
+        info!("Dropping lockfile...");
+        fs::remove_file(&self.0).unwrap();
+    }
+}
+
+fn get_lock_file() -> Result<(File, PathBuf)> {
+    for dir_path in PATHS.iter() {
+        use std::io::ErrorKind::*;
+        let dir_path = PathBuf::from(dir_path);
+        let lockfile_path = dir_path.join("leanshot.pid");
+
+        // check if it already exists
+        match File::open(&lockfile_path) {
+            Ok(mut f) => {
+                let mut contents = String::new();
+                f.read_to_string(&mut contents)?;
+
+                let mut parts = contents.split(":");
+                let timestamp = parts.next().unwrap().parse::<u64>()?;
+                let pid = parts.next().unwrap();
+                let time = UNIX_EPOCH + Duration::from_millis(timestamp);
+
+                // if it hasn't been 5 minutes since the last open, then bail
+                if SystemTime::now() - Duration::from_secs(60 * 5) < time {
+                    bail!("existing lockfile found for pid {} at time {:?}", pid, time);
+                }
+            }
+            // ignore errors
+            Err(e) => match e.kind() {
+                NotFound => {},
+                _ => return Err(Error::from(e).context("could not open lockfile for reading")),
+            }
+        };
+
+        let file = match File::create(&lockfile_path) {
+            Ok(f) => f,
+            Err(e) => match e.kind() {
+                PermissionDenied => continue,
+                _ => return Err(Error::from(e).context("could not open lockfile for writing")),
+            },
+        };
+
+        return Ok((file, lockfile_path));
+    }
+
+    bail!("could not find a suitable place to place lockfile");
+}