This does not work, because every process in the tree has a new PID so we can't ensure a consistent symlink path

src/commands/use.rs

@@ -3,6 +3,7 @@ use super::install::Install;
 use crate::fs;
 use crate::installed_versions;
 use crate::outln;
+use crate::symlink_path::DefaultMultishellPathExt;
 use crate::system_version;
 use crate::user_version::UserVersion;
 use crate::version::Version;
@@ -23,8 +24,10 @@ impl Command for Use {
     type Error = Error;
 
     fn apply(self, config: &FnmConfig) -> Result<(), Self::Error> {
-        let multishell_path = config.multishell_path().context(FnmEnvWasNotSourced)?;
-        warn_if_multishell_path_not_in_path_env_var(multishell_path, config);
+        let multishell_path = config
+            .multishell_path_or_default()
+            .context(FnmEnvWasNotSourced)?;
+        warn_if_multishell_path_not_in_path_env_var(&multishell_path, config);
 
         let all_versions =
             installed_versions::list(config.installations_dir()).context(VersionListingError)?;
@@ -78,7 +81,7 @@ impl Command for Use {
             }
         };
 
-        replace_symlink(&version_path, multishell_path).context(SymlinkingCreationIssue)?;
+        replace_symlink(&version_path, &multishell_path).context(SymlinkingCreationIssue)?;
 
         Ok(())
     }
@@ -197,5 +200,5 @@ pub enum Error {
         "You should setup your shell profile to evaluate `fnm env`, see https://github.com/Schniz/fnm#shell-setup on how to do this",
         "Check out our documentation for more information: https://fnm.vercel.app"
     ))]
-    FnmEnvWasNotSourced,
+    FnmEnvWasNotSourced { source: Box<dyn std::error::Error> },
 }

src/current_version.rs

@@ -1,10 +1,12 @@
-use crate::config::FnmConfig;
 use crate::system_version;
 use crate::version::Version;
-use snafu::{OptionExt, ResultExt, Snafu};
+use crate::{config::FnmConfig, symlink_path::DefaultMultishellPathExt};
+use snafu::{ResultExt, Snafu};
 
 pub fn current_version(config: &FnmConfig) -> Result<Option<Version>, Error> {
-    let multishell_path = config.multishell_path().context(EnvNotApplied)?;
+    let multishell_path = config
+        .multishell_path_or_default()
+        .context(CantCreateMultishellPath)?;
 
     if multishell_path.read_link().ok() == Some(system_version::path()) {
         return Ok(Some(Version::Bypassed));
@@ -31,7 +33,7 @@ pub enum Error {
     #[snafu(display(
         "`fnm env` was not applied in this context.\nCan't find fnm's environment variables"
     ))]
-    EnvNotApplied,
+    CantCreateMultishellPath { source: Box<dyn std::error::Error> },
     #[snafu(display("Can't read the version as a valid semver"))]
     VersionError {
         source: semver::Error,

src/main.rs

@@ -33,6 +33,7 @@ mod version_files;
 
 #[macro_use]
 mod log_level;
+mod symlink_path;
 
 fn main() {
     env_logger::init();

src/symlink_path.rs

@@ -0,0 +1,47 @@
+use std::{fs::create_dir_all, path::PathBuf};
+
+use log::info;
+use sysinfo::{get_current_pid, ProcessExt, System, SystemExt};
+
+use crate::config::FnmConfig;
+
+pub fn generate_symlink_token_based_on_pid() -> Result<String, &'static str> {
+    let mut system = System::new();
+    let current_pid = get_current_pid()?;
+
+    system.refresh_process(current_pid);
+    let current_process = system
+        .process(current_pid)
+        .ok_or("Couldn't find the current process in the tree")?;
+
+    let parent_pid = current_process
+        .parent()
+        .ok_or("Couldn't find the parent pid in the process tree")?;
+
+    Ok(format!("pid_{}", parent_pid))
+}
+
+pub(crate) trait DefaultMultishellPathExt {
+    fn multishell_path_or_default(&self) -> Result<PathBuf, Box<dyn std::error::Error>>;
+}
+
+impl DefaultMultishellPathExt for FnmConfig {
+    fn multishell_path_or_default(&self) -> Result<PathBuf, Box<dyn std::error::Error>> {
+        if let Some(given_path) = self.multishell_path() {
+            return Ok(given_path.to_path_buf());
+        }
+
+        let symlink_token = generate_symlink_token_based_on_pid()?;
+        let multishells_path = std::env::temp_dir().join("fnm_multishells");
+        let new_path = multishells_path.join(symlink_token);
+
+        info!(
+            "No multishell path given, generating a new one at {}",
+            new_path.display()
+        );
+
+        create_dir_all(&multishells_path)?;
+
+        Ok(new_path)
+    }
+}