ServerBuddies Support Blog

Linux assigns the file permissions of the new file using your default UID and GID. To allow others access to the file, you need to either change the security permissions for the everyone security group or assign the file a different default group that contains other users.

This can be a pain in a large environment if you want to create and share documents among several people. Fortunately, there’s a simple solution for how to solve this problem.

There are three additional bits of information that Linux stores for each file and directory.

The set user id (SUID): When a file is executed by a user, the program runs under the permissions of the file owner.

The set group id (SGID): For a file, the program runs under the permissions of the file group. For a directory, new files created in the directory use the directory group as the default group.

The sticky bit: The file remains (sticks) in memory after the process ends.

The SGID bit is important for sharing files. By enabling the SGID bit, you can force all new files created in a shared directory to be owned by the directory’s group and now the individual user’s group.

The SGID is set using the chmod command. It’s added to the beginning of the standard three digit octal value (making a four-digit octal value), or you can use the symbol s in symbolic mode.

The Linux system identifies hardware devices as special files, called device files. There are three
different classifications of device files:

? Character
? Block
? Network

Character device files are for devices that can only handle data one character at a time. Most types
of modems and terminals are created as character files. Block files are for devices that can handle
data in large blocks at a time, such as disk drives.

The network file types are used for devices that use packets to send and receive data. This includes network cards and a special loopback device that allows the Linux system to communicate with itself using common network programming protocols.

To list all open files, use:

To list all open Internet, x.25 (HP-UX), and UNIX domain files, use:

To list all open IPv4 network files in use by the process whose PID is 1234, use:

Presuming the UNIX dialect supports IPv6, to list only open IPv6 network files, use:

To list all files using any protocol on ports 513, 514, or 515 of host wonderland.cc.pur-due.edu, use:

To list all open files for login name “abe”, or user ID 1234, or process 456, or pro-cess 123, or process 789, use:

To list all open files on device /dev/hd4, use:

To find the process that has /u/abe/foo open, use:

To send a SIGHUP to the processes that have /u/abe/bar open, use:

To find any open file, including an open UNIX domain socket file, with the name /dev/log,use:

fuser - identify processes using files or sockets

In order to look up processes using TCP and UDP sockets, the corresponding name space has to be selected with the -n option. By default fuser will look in both IPv6 and IPv4 sock-ets. To change the default, behavior, use the -4 and -6 options. The socket(s) can be specified by the local and remote port, and the remote address. All fields are optional, but commas in front of missing fields must be present:

Either symbolic or numeric values can be used for IP addresses and port numbers.

fuser outputs only the PIDs to stdout, everything else is sent to stderr.

FILES

/proc location of the proc file system

EXAMPLES

fuser -km /home kills all processes accessing the file system /home in any way.

if fuser -s /dev/ttyS1; then :; else something; fi invokes something if no other process
is using /dev/ttyS1.

fuser telnet/tcp shows all processes at the (local) TELNET port.

Kinds of device files in Linux operating systems

There are two general kinds of device files in Unix-like operating systems, known as character special files and block special files. The difference between them lies in how data written to them and read from them is processed by the operating system and hardware.

These together can be called device special files in contrast to named pipes, which are not connected to a device but are not ordinary files either.