What is the lock file pattern?

Create a file with O_CREAT|O_EXCL (exclusive create). If it fails with EEXIST, another instance is running. Store your PID in the file for cleanup.

How do I handle EEXIST in scripts?

Use || true to ignore: mkdir /path 2>/dev/null || true. Or use -p flag for mkdir. For other commands, check and handle the error explicitly.

Linux EEXIST (errno 17) — File or Resource Already Exists (Deep Dive)

Informationalerrno

Overview

Deep dive into Linux EEXIST covering mkdir/symlink conflicts, atomic file creation, lock file patterns, and handling EEXIST in scripts and applications.

Key Details

EEXIST (errno 17) occurs when creating a file, directory, or resource that already exists
Common with mkdir, symlink, link, mknod, and open with O_CREAT|O_EXCL flags
O_CREAT|O_EXCL is the standard pattern for atomic lock file creation
In shell: mkdir -p avoids EEXIST by not failing if directory already exists
Socket bind() returns EEXIST if the address is already bound (actually EADDRINUSE on Linux)

Common Causes

Creating a directory that already exists without -p flag
Creating a symbolic link where one already exists
Lock file from a previous process instance still present
Race condition: two processes trying to create the same resource simultaneously
Package installation trying to create a file that already exists from another package

Steps

1For directories: use mkdir -p which succeeds even if the directory exists
2For symlinks: remove first, then create: rm -f /path/to/link && ln -s target /path/to/link
3For lock files: check if the owning process is still running (check PID in lock file)
4For atomic creation: use open(path, O_CREAT|O_EXCL) which fails atomically if file exists
5Use ln -sf for force-creating symlinks (removes existing link first)