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lsof

The lsof command in Linux stands for "list open files". It is a powerful tool used to list information about files that are currently opened by processes. Here’s a detailed explanation of how to use lsof and what information it provides:

Usage of lsof

Basic Usage

To use lsof, open a terminal and type:

lsof

By default, lsof lists all open files for all active processes on the system.

Options and Output

lsof provides output with different columns representing various file and process attributes:

  1. Columns

    • COMMAND: The name of the command that opened the file.
    • PID: Process ID of the command.
    • USER: User name of the user who owns the process.
    • FD: File descriptor of the open file.
    • TYPE: Type of the node associated with the file (e.g., DIR for directory, REG for regular file).
    • DEVICE: Device number on which the file resides.
    • SIZE/OFF: Size of the file or offset into the file.
    • NODE: Inode number of the file.
    • NAME: Name of the file or file descriptor.

    Example output:

    COMMAND  PID  USER   FD   TYPE DEVICE SIZE/OFF  NODE NAME
systemd  1    root  cwd    DIR  253,0    4096      2 /
sshd     1234 root  cwd    DIR  253,0    4096      2 /
sshd     1234 root  txt    REG  253,0  100800  12345 /usr/sbin/sshd

  2. Options

    • -i: List files opened by Internet address (e.g., -i :80 for files opened on port 80).
    • -p <PID>: List files opened by a specific process ID.
    • -u <username>: List files opened by a specific username.
    • -c <command>: List files opened by a specific command.
    • -t <type>: List files of a specific type (e.g., -t DIR for directories).
    • +d <directory>: List files opened within a specific directory.

    Example:

    lsof -i :80     # List files opened on port 80
lsof -p 1234    # List files opened by process ID 1234
lsof -u username    # List files opened by user 'username'

Use Cases

  • Troubleshooting: lsof helps in troubleshooting issues related to file locks, network connections, and file access permissions.

  • System Monitoring: Useful for monitoring file access patterns and detecting unauthorized file access.

  • Resource Management: Assists in managing resources by identifying processes consuming excessive file descriptors.

Conclusion

lsof is a versatile command-line tool for listing open files and identifying which processes have opened them. It provides crucial insights into file system activity and helps in diagnosing various system issues related to file access. By understanding its output and options, administrators and users can effectively monitor and manage file resources on Linux systems.

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