A number of times I have found myself unable to access a file because some other process is writing to it.
Usually lock files which are used to prevent multiple instances of the same program from running at the same time, or to guard against multiple operations running at the same time that would interfere with each other.
The Linux command which comes to the rescue is called lsof which I guess is short for "list of open files"
It can be used in several different ways:
List All Open Files
lsof
This will list all the God damn files open on your system. There will be lots of other interesting things listed which are not files, things like pipes and network sockets. This is because on Linux, everything is abstracted to a file, even the keyboard.
The output from lsof can be piped to grep to search for bits of file names.
lsof | grep pony
This will perform a very crude search for any file name, path or process with anything to do with 'pony'
List All Open Files for a Given Process (by PID)
For example:
lsof -p 12345
Lists all file handles currently held by process 12345
lsof headings
By default, the lsof output looks like this (without headings)
mysqld 30353 mysql 219u REG 8,2 5976322292 6652117 /mnt/mysql/tagwalk/msg.MYD mysqld 30353 mysql 220u REG 8,2 672237472 6652170 /mnt/mysql/tagwalk/tag.MYD mysqld 30353 mysql 221u REG 8,2 672237472 6652170 /mnt/mysql/tagwalk/tag.MYD mysqld 30353 mysql 222u REG 8,2 5976322292 6652117 /mnt/mysql/tagwalk/msg.MYD (cmd) (pid) (user) (handle) (type) (device)(size/offset) (node) (path)
From left-to-right, the columns are:
- Command (first 9 characters)
- Process ID (PID)
- User
- File Descriptor and Access Mode: r=read, w=write, u=read/write
- Type: REG=Regular File, PIPE=Pipe, IPv4=TCP/IP socket (everything's a file remember!)
- Device: In this case, I'm not sure what "8,2" means
- File Size or Offset (if relevant)
- Node: The node number of this file (an internal file system thing)
- Name: The file name and path of the open file
Hope this helps!
