Chapter 6. Operating System Specific Annex

Table of Contents

6.1. Linux
6.1.1. / : Root directory
6.1.2. /bin : Essential user command binaries (for use by all users)
6.1.3. /dev : Devices and special files
6.1.4. /etc : Host-specific system configuration
6.1.5. /proc : Kernel and process information virtual filesystem
6.1.6. /sbin : Essential system binaries
6.1.7. /sys : Kernel and system information virtual filesystem
6.1.8. /usr/include : Header files included by C programs
6.1.9. /usr/src : Source code
6.1.10. /var/spool/cron : cron and at jobs

This section is for additional requirements and recommendations that only apply to a specific operating system. The material in this section should never conflict with the base standard.

6.1. Linux

This is the annex for the Linux operating system.

6.1.1. / : Root directory

On Linux systems, if the kernel is located in /, we recommend using the names vmlinux or vmlinuz, which have been used in recent Linux kernel source packages.

6.1.2. /bin : Essential user command binaries (for use by all users)

Linux systems which require them place these additional files into /bin:

  • setserial

6.1.3. /dev : Devices and special files

The following devices must exist under /dev.


All data written to this device is discarded. A read from this device will return an EOF condition.


This device is a source of zeroed out data. All data written to this device is discarded. A read from this device will return as many bytes containing the value zero as was requested.


This device is a synonym for the controlling terminal of a process. Once this device is opened, all reads and writes will behave as if the actual controlling terminal device had been opened.


Previous versions of the FHS had stricter requirements for /dev. Other devices may also exist in /dev. Device names may exist as symbolic links to other device nodes located in /dev or subdirectories of /dev. There is no requirement concerning major/minor number values.

6.1.4. /etc : Host-specific system configuration

Linux systems which require them place these additional files into /etc.

  • lilo.conf

6.1.5. /proc : Kernel and process information virtual filesystem

The proc filesystem is the de-facto standard Linux method for handling process and system information, rather than /dev/kmem and other similar methods. We strongly encourage this for the storage and retrieval of process information as well as other kernel and memory information.

6.1.6. /sbin : Essential system binaries

Linux systems place commands relating to filesystem maintenance and boot loader management into /sbin.

Optional files for /sbin:

  • Static binaries:

    • ldconfig

    • sln

    • ssync

    Static ln (sln) and static sync (ssync) are useful when things go wrong. The primary use of sln (to repair incorrect symlinks in /lib after a poorly orchestrated upgrade) is no longer a major concern now that the ldconfig program (usually located in /usr/sbin) exists and can act as a guiding hand in upgrading the dynamic libraries. Static sync is useful in some emergency situations. Note that these need not be statically linked versions of the standard ln and sync, but may be.

    The ldconfig binary is optional for /sbin since a site may choose to run ldconfig at boot time, rather than only when upgrading the shared libraries. (It's not clear whether or not it is advantageous to run ldconfig on each boot.) Even so, some people like ldconfig around for the following (all too common) situation:

    1. I've just removed /lib/<file>.

    2. I can't find out the name of the library because ls is dynamically linked, I'm using a shell that doesn't have ls built-in, and I don't know about using "echo *" as a replacement.

    3. I have a static sln, but I don't know what to call the link.

  • Miscellaneous:

    • ctrlaltdel

    • kbdrate

    So as to cope with the fact that some keyboards come up with such a high repeat rate as to be unusable, kbdrate may be installed in /sbin on some systems.

    Since the default action in the kernel for the Ctrl-Alt-Del key combination is an instant hard reboot, it is generally advisable to disable the behavior before mounting the root filesystem in read-write mode. Some init suites are able to disable Ctrl-Alt-Del, but others may require the ctrlaltdel program, which may be installed in /sbin on those systems.

6.1.7. /sys : Kernel and system information virtual filesystem

The sys filesystem is the location where information about devices, drivers, and some kernel features is exposed. Its underlying structure is determined by the particular Linux kernel being used at the moment, and is otherwise unspecified.

6.1.8. /usr/include : Header files included by C programs

These symbolic links are required if a C or C++ compiler is installed and only for systems not based on glibc.

    /usr/include/asm -> /usr/src/linux/include/asm-<arch>
    /usr/include/linux -> /usr/src/linux/include/linux

6.1.9. /usr/src : Source code

For systems based on glibc, there are no specific guidelines for this directory. For systems based on Linux libc revisions prior to glibc, the following guidelines and rationale apply:

The only source code that should be placed in a specific location is the Linux kernel source code. It is located in /usr/src/linux.

If a C or C++ compiler is installed, but the complete Linux kernel source code is not installed, then the include files from the kernel source code must be located in these directories:


<arch> is the name of the system architecture.


/usr/src/linux may be a symbolic link to a kernel source code tree.


It is important that the kernel include files be located in /usr/src/linux and not in /usr/include so there are no problems when system administrators upgrade their kernel version for the first time.

6.1.10. /var/spool/cron : cron and at jobs

This directory contains the variable data for the cron and at programs.