Advanced Bash-Scripting Guide:
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# Appendix H. Important System Directories

Sysadmins and anyone else writing administrative scripts should be intimately familiar with the following system directories.

- `/bin`

  Binaries (executables). Basic system programs and utilities (such as bash).

- `/usr/bin` [1]

  More system binaries.

- `/usr/local/bin`

  Miscellaneous binaries local to the particular machine.

- `/sbin`

  System binaries. Basic system administrative programs and utilities (such as fsck).

- `/usr/sbin`

  More system administrative programs and utilities.

- `/etc`

  Et cetera. Systemwide configuration scripts.

  Of particular interest are the `/etc/fstab` (filesystem table), `/etc/mtab` (mounted filesystem table), and the `/etc/inittab` files.

- `/etc/rc.d`

  Boot scripts, on Red Hat and derivative distributions of Linux.

- `/usr/share/doc`

  Documentation for installed packages.

- `/usr/man`

  The systemwide manpages.

- `/dev`

  Device directory. Entries (but not mount points) for physical and virtual devices. See Chapter 29.

- `/proc`

  Process directory. Contains information and statistics about running processes and kernel parameters. See Chapter 29.

- `/sys`

  Systemwide device directory. Contains information and statistics about device and device names. This is newly added to Linux with the 2.6.X kernels.

- `/mnt`

  Mount. Directory for mounting hard drive partitions, such as `/mnt/dos`, and physical devices. In newer Linux distros, the `/media` directory has taken over as the preferred mount point for I/O devices.

- `/media`

  In newer Linux distros, the preferred mount point for I/O devices, such as CD/DVD drives or USB flash drives.

- `/var`

  Variable (changeable) system files. This is a catchall "scratchpad" directory for data generated while a Linux/UNIX machine is running.

- `/var/log`

  Systemwide log files.

- `/var/spool/mail`

  User mail spool.

- `/lib`

  Systemwide library files.

- `/usr/lib`

  More systemwide library files.

- `/tmp`

  System temporary files.

- `/boot`

  System boot directory. The kernel, module links, system map, and boot manager reside here.

  | Altering files in this directory may result in an unbootable system.

### Notes

[1] | Some early UNIX systems had a fast, small-capacity fixed disk (containing `/`, the root partition), and a second drive which was larger, but slower (containing `/usr` and other partitions). The most frequently used programs and utilities therefore resided on the small-but-fast drive, in `/bin`, and the others on the slower drive, in `/usr/bin`. This likewise accounts for the split between `/sbin` and `/usr/sbin`, `/lib` and `/usr/lib`, etc.

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