I. 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.