Below is a list of some of the more useful virtual files in the top-level of the /proc/ directory.
In most cases, the content of the files listed in this section are not the same as those installed on your machine. This is because much of the information is specific to the hardware on which Red Hat Enterprise Linux is running.
This file provides information about the state of the Advanced Power Management (APM) system and is used by the apm command. If a system with no battery is connected to an AC power source, this virtual file would look similar to the following:
1.16 1.2 0x07 0x01 0xff 0x80 -1% -1 ?
Running the apm -v command on such a system results in output similar to the following:
APM BIOS 1.2 (kernel driver 1.16) AC on-line, no system battery
For systems which do not use a battery as a power source, apm is able do little more than put the machine in standby mode. The apm command is much more useful on laptops. For example, the following output is from the command cat /proc/apm on a laptop while plugged into a power outlet:
1.16 1.2 0x03 0x01 0x03 0x09 100% -1 ?
When the same laptop is unplugged from its power source for a few minutes, the contents of the apm file change to something like the following:
1.16 1.2 0x03 0x00 0x00 0x01 99% 1792 min
The apm -v command now yields more useful data, such as the following:
APM BIOS 1.2 (kernel driver 1.16) AC off-line, battery status high: 99% (1 day, 5:52)
This file shows the parameters passed to the kernel at the time it is started. A sample /proc/cmdline file looks like the following:
This tells us that the kernel is mounted read-only (signified by (ro)) off of the second partition on the first IDE device (/dev/hda2).
This virtual file identifies the type of processor used by your system. The following is an example of the output typical of /proc/cpuinfo:
processor : 0 vendor_id : GenuineIntel cpu family : 15 model : 2 model name : Intel(R) Xeon(TM) CPU 2.40GHz stepping : 7 cpu MHz : 2392.371 cache size : 512 KB physical id : 0 siblings : 2 runqueue : 0 fdiv_bug : no hlt_bug : no f00f_bug : no coma_bug : no fpu : yes fpu_exception : yes cpuid level : 2 wp : yes flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm bogomips : 4771.02
processor — Provides each processor with an identifying number. On systems that have one processor, only a 0 is present.
cpu family — Authoritatively identifies the type of processor in the system. For an Intel-based system, place the number in front of "86" to determine the value. This is particularly helpful for those attempting to identify the architecture of an older system such as a 586, 486, or 386. Because some RPM packages are compiled for each of these particular architectures, this value also helps users determine which packages to install.
model name — Displays the common name of the processor, including its project name.
cpu MHz — Shows the precise speed in megahertz for the processor to the thousandth decimal point.
cache size — Displays the amount of level 2 memory cache available to the processor.
siblings — Displays the number of sibling CPUs on the same physical CPU for architectures which use hyper-threading.
flags — Defines a number of different qualities about the processor, such as the presence of a floating point unit (FPU) and the ability to process MMX instructions.
This file displays the various character and block devices currently configured (not including devices whose modules are not loaded). Below is a sample output from this file:
Character devices: 1 mem 2 pty 3 ttyp 4 ttyS 5 cua 7 vcs 10 misc 14 sound 29 fb 36 netlink 128 ptm 129 ptm 136 pts 137 pts 162 raw 254 iscsictl Block devices: 1 ramdisk 2 fd 3 ide0 9 md 22 ide1
The output from /proc/devices includes the major number and name of the device, and is broken into two major sections: Character devices and Block devices.
Character devices are similar to block devices, except for two basic differences:
Block devices have a buffer available, allowing them to order requests before addressing them. This is important for devices designed to store information — such as hard drives — because the ability to order the information before writing it to the device allows it to be placed in a more efficient order. Character devices do not require buffering.
Block devices can send and receive information in blocks of a size configured per device. Character devices send data with no preconfigured size.
For more information about devices refer to the following installed documentation:
This file contains a list of the registered ISA direct memory access (DMA) channels in use. A sample /proc/dma files looks like the following:
This file lists the execution domains currently supported by the Linux kernel, along with the range of personalities they support.
0-0 Linux [kernel]
Think of execution domains as the "personality" for an operating system. Because other binary formats, such as Solaris, UnixWare, and FreeBSD, can be used with Linux, programmers can change the way the operating system treats system calls from these binaries by changing the personality of the task. Except for the PER_LINUX execution domain, different personalities can be implemented as dynamically loadable modules.
This file contains a list of frame buffer devices, with the frame buffer device number and the driver that controls it. Typical output of /proc/fb for systems which contain frame buffer devices looks similar to the following:
0 VESA VGA
This file displays a list of the file system types currently supported by the kernel. Sample output from a generic /proc/filesystems looks similar to the following:
nodev rootfs nodev bdev nodev proc nodev sockfs nodev tmpfs nodev shm nodev pipefs ext2 nodev ramfs iso9660 nodev devpts ext3 nodev autofs nodev binfmt_misc
The first column signifies whether the file system is mounted on a block device. Those beginning with nodev are not mounted on a device. The second column lists the names of the file systems supported.
The mount command cycles through the file systems listed here when one is not specified as an argument.
This file records the number of interrupts per IRQ on the x86 architecture. A standard /proc/interrupts looks similar to the following:
CPU0 0: 80448940 XT-PIC timer 1: 174412 XT-PIC keyboard 2: 0 XT-PIC cascade 8: 1 XT-PIC rtc 10: 410964 XT-PIC eth0 12: 60330 XT-PIC PS/2 Mouse 14: 1314121 XT-PIC ide0 15: 5195422 XT-PIC ide1 NMI: 0 ERR: 0
For a multi-processor machine, this file may look slightly different:
CPU0 CPU1 0: 1366814704 0 XT-PIC timer 1: 128 340 IO-APIC-edge keyboard 2: 0 0 XT-PIC cascade 8: 0 1 IO-APIC-edge rtc 12: 5323 5793 IO-APIC-edge PS/2 Mouse 13: 1 0 XT-PIC fpu 16: 11184294 15940594 IO-APIC-level Intel EtherExpress Pro 10/100 Ethernet 20: 8450043 11120093 IO-APIC-level megaraid 30: 10432 10722 IO-APIC-level aic7xxx 31: 23 22 IO-APIC-level aic7xxx NMI: 0 ERR: 0
The first column refers to the IRQ number. Each CPU in the system has its own column and its own number of interrupts per IRQ. The next column reports the type of interrupt, and the last column contains the name of the device that is located at that IRQ.
Each of the types of interrupts seen in this file, which are architecture-specific, mean something different. For x86 machines, the following values are common:
XT-PIC — This is the old AT computer interrupts.
IO-APIC-edge — The voltage signal on this interrupt transitions from low to high, creating an edge, where the interrupt occurs and is only signaled once. This kind of interrupt, as well as the IO-APIC-level interrupt, are only seen on systems with processors from the 586 family and higher.
IO-APIC-level — Generates interrupts when its voltage signal is high until the signal is low again.
This file shows you the current map of the system's memory for each physical device:
00000000-0009fbff : System RAM 0009fc00-0009ffff : reserved 000a0000-000bffff : Video RAM area 000c0000-000c7fff : Video ROM 000f0000-000fffff : System ROM 00100000-07ffffff : System RAM 00100000-00291ba8 : Kernel code 00291ba9-002e09cb : Kernel data e0000000-e3ffffff : VIA Technologies, Inc. VT82C597 [Apollo VP3] e4000000-e7ffffff : PCI Bus #01 e4000000-e4003fff : Matrox Graphics, Inc. MGA G200 AGP e5000000-e57fffff : Matrox Graphics, Inc. MGA G200 AGP e8000000-e8ffffff : PCI Bus #01 e8000000-e8ffffff : Matrox Graphics, Inc. MGA G200 AGP ea000000-ea00007f : Digital Equipment Corporation DECchip 21140 [FasterNet] ea000000-ea00007f : tulip ffff0000-ffffffff : reserved
The first column displays the memory registers used by each of the different types of memory. The second column lists the kind of memory located within those registers and displays which memory registers are used by the kernel within the system RAM or, if the network interface card has multiple Ethernet ports, the memory registers assigned for each port.
The output of /proc/ioports provides a list of currently registered port regions used for input or output communication with a device. This file can be quite long. The following is a partial listing:
0000-001f : dma1 0020-003f : pic1 0040-005f : timer 0060-006f : keyboard 0070-007f : rtc 0080-008f : dma page reg 00a0-00bf : pic2 00c0-00df : dma2 00f0-00ff : fpu 0170-0177 : ide1 01f0-01f7 : ide0 02f8-02ff : serial(auto) 0376-0376 : ide1 03c0-03df : vga+ 03f6-03f6 : ide0 03f8-03ff : serial(auto) 0cf8-0cff : PCI conf1 d000-dfff : PCI Bus #01 e000-e00f : VIA Technologies, Inc. Bus Master IDE e000-e007 : ide0 e008-e00f : ide1 e800-e87f : Digital Equipment Corporation DECchip 21140 [FasterNet] e800-e87f : tulip
The first column gives the I/O port address range reserved for the device listed in the second column.
This file lists installed Plug and Play (PnP) cards in ISA slots on the system. This is most often seen with sound cards but may include any number of devices. The following is an example /proc/isapnp file with a sound card installed:
Card 1 'CTL0070:Creative ViBRA16C PnP' PnP version 1.0 Product version 1.0 Logical device 0 'CTL0001:Audio' Device is not active Active port 0x220,0x330,0x388 Active IRQ 5 [0x2] Active DMA 1,5 Resources 0 Priority preferred Port 0x220-0x220, align 0x0, size 0x10, 16-bit address decoding Port 0x330-0x330, align 0x0, size 0x2, 16-bit address decoding Port 0x388-0x3f8, align 0x0, size 0x4, 16-bit address decoding IRQ 5 High-Edge DMA 1 8-bit byte-count compatible DMA 5 16-bit word-count compatible Alternate resources 0:1 Priority acceptable Port 0x220-0x280, align 0x1f, size 0x10, 16-bit address decoding Port 0x300-0x330, align 0x2f, size 0x2, 16-bit address decoding Port 0x388-0x3f8, align 0x0, size 0x4, 16-bit address decoding IRQ 5,7,2/9,10 High-Edge DMA 1,3 8-bit byte-count compatible DMA 5,7 16-bit word-count compatible
This file can be quite long, depending on the number of devices displayed and their resource requirements.
Each card lists its name, PnP version number, and product version number. If the device is active and configured, this file also reveals the port and IRQ numbers for the device. In addition, to ensure better compatibility, the card specifies preferred and acceptable values for a number of different parameters. The goal here is to allow the PnP cards to work around one another and avoid IRQ and port conflicts.
This file represents the physical memory of the system and is stored in the core file format. Unlike most /proc/ files, kcore displays a size. This value is given in bytes and is equal to the size of the physical memory (RAM) used plus 4KB.
The contents of this file are designed to be examined by a debugger, such as gdb, and is not human readable.
Do not view the /proc/kcore virtual file. The
contents of the file scrambles text output on the terminal. If
this file is accidentally viewed, press
This file is used to hold messages generated by the kernel. These messages are then picked up by other programs, such as /sbin/klogd or /bin/dmesg.
This file contains the symbol definitions used by the module tools to dynamically link and bind kernel modules.
e003def4 speedo_debug [eepro100] e003b04c eepro100_init [eepro100] e00390c0 st_template [st] e002104c RDINDOOR [megaraid] e00210a4 callDone [megaraid] e00226cc megaraid_detect [megaraid]
The first column lists the memory address for the kernel function, the second column refers to the name of the function, and the last column reveals the name of the loaded module.
This file provides a look the load average in regards to both the CPU and IO over time, as well as additional data used by uptime and other commands. A sample /proc/loadavg file looks similar to the following:
0.20 0.18 0.12 1/80 11206
The first three columns measure CPU and IO utilization of the last 1, 5, and 10 minute periods. The fourth column shows the number of currently running processes and the total number of processes. The last column displays the last process ID used.
This file displays the files currently locked by the kernel. The contents of this file contain internal kernel debugging data and can vary tremendously, depending on the use of the system. A sample /proc/locks file for a lightly loaded system looks similar to the following:
1: FLOCK ADVISORY WRITE 807 03:05:308731 0 EOF c2a260c0 c025aa48 c2a26120 2: POSIX ADVISORY WRITE 708 03:05:308720 0 EOF c2a2611c c2a260c4 c025aa48
Each lock has its own line which starts with a unique number. The second column refers to the class of lock used, with FLOCK signifying the older-style UNIX file locks from a flock system call and POSIX representing the newer POSIX locks from the lockf system call.
The third column can have two values: ADVISORY or MANDATORY. ADVISORY means that the lock does not prevent other people from accessing the data; it only prevents other attempts to lock it. MANDATORY means that no other access to the data is permitted while the lock is held. The fourth column reveals whether the lock is allowing the holder READ or WRITE access to the file. The fifth column shows the ID of the process holding the lock. The sixth column shows the ID of the file being locked, in the format of MAJOR-DEVICE:MINOR-DEVICE:INODE-NUMBER. The seventh column shows the start and end of the file's locked region. The remaining columns point to internal kernel data structures used for specialized debugging and can be ignored.
This file contains the current information for multiple-disk, RAID configurations. If the system does not contain such a configuration, then /proc/mdstat looks similar to the following:
Personalities : read_ahead not set unused devices: <none>
This file remains in the same state as seen above unless a software RAID or md device is present. In that case, view /proc/mdstat to find the current status of mdX RAID devices.
The /proc/mdstat file below shows a system with its md0 configured as a RAID 1 device, while it is currently re-syncing the disks:
Personalities : [linear] [raid1] read_ahead 1024 sectors md0: active raid1 sda2 sdb2 9940 blocks [2/2] [UU] resync=1% finish=12.3min algorithm 2 [3/3] [UUU] unused devices: <none>
This is one of the more commonly used files in the /proc/ directory, as it reports a large amount of valuable information about the systems RAM usage.
The following sample /proc/meminfo virtual file is from a system with 256MB of RAM and 384MB of swap space:
total: used: free: shared: buffers: cached: Mem: 128692224 121212928 7479296 0 9293824 47964160 Swap: 1103093760 32772096 1070321664 MemTotal: 125676 kB MemFree: 7304 kB MemShared: 0 kB Buffers: 9076 kB Cached: 34204 kB SwapCached: 12636 kB Active: 79352 kB ActiveAnon: 57308 kB ActiveCache: 22044 kB Inact_dirty: 240 kB Inact_laundry: 17468 kB Inact_clean: 984 kB Inact_target: 19608 kB HighTotal: 0 kB HighFree: 0 kB LowTotal: 125676 kB LowFree: 7304 kB SwapTotal: 1077240 kB SwapFree: 1045236 kB HugePages_Total: 2 HugePages_Free: 2 Hugepagesize: 2096 kB
Much of the information here is used by the free, top, and ps commands. In fact, the output of the free command is similar in appearance to the contents and structure of /proc/meminfo. But by looking directly at /proc/meminfo, more details are revealed:
Mem — The current state of physical RAM in the system, including a full breakdown of total, used, free, shared, buffered, and cached memory utilization in bytes.
Swap — The total, used, and free amounts of swap space, in bytes.
MemTotal — Total amount of physical RAM, in kilobytes.
MemFree — The amount of physical RAM, in kilobytes, left unused by the system.
MemShared — Unused with 2.4 and higher kernels but left in for compatibility with earlier kernel versions.
Buffers — The amount of physical RAM, in kilobytes, used for file buffers.
Cached — The amount of physical RAM, in kilobytes, used as cache memory.
SwapCached — The amount of swap, in kilobytes, used as cache memory.
Active — The total amount of buffer or page cache memory, in kilobytes, that is in active use.
Inact_dirty — The total amount of buffer or cache pages, in kilobytes, estimated to be free and available.
Inact_laundry — The total amount of buffer or cache pages, in kilobytes, that are about to become free and available, possibly after disk IO for these pages has finished.
Inact_clean — The total amount of buffer or cache pages, in kilobytes, that are free and available.
Inact_target — The net amount of allocations per second, in kilobytes, averaged over one minute. This is a deprecated statistic which has little meaning on modern systems.
HighTotal and HighFree — The total and free amount of memory, in kilobytes, that is not directly mapped into kernel space. The HighTotal value can vary based on the type of kernel used.
LowTotal and LowFree — The total and free amount of memory, in kilobytes, that is directly mapped into kernel space. The LowTotal value can vary based on the type of kernel used.
SwapTotal — The total amount of swap available, in kilobytes.
SwapFree — The total amount of swap free, in kilobytes.
HugePages_Total — The total number of hugepages for the system. The number is derived by dividing Hugepagesize by the megabytes set aside for hugepages specified in /proc/sys/vm/hugetlb_pool. This statistic only appears on x86, Itanium, AMD64, and Intel® EM64T.
HugePages_Free — The total number of hugepages available for the system. This statistic only appears on x86, Itanium, AMD64, and Intel® EM64T.
Hugepagesize — The size for each hugepages unit in kilobytes. By default, the value is 4096 KB on uniprocessor kernels for 32 bit architectures. For SMP and hugemem kernels, the default is 2048 KB. For 64 bit architectures, the default is 262144 KB. This statistic only appears on x86, Itanium, AMD64, and Intel® EM64T.
This file lists miscellaneous drivers registered on the miscellaneous major device, which is device number 10:
135 rtc 1 psaux 134 apm_bios
The first column is the minor number of each device, while the second column shows the driver in use.
This file displays a list of all modules loaded into the kernel. Its contents vary based on the configuration and use of your system, but it should be organized in a similar manner to this sample /proc/modules file output:
ide-cd 27008 0 (autoclean) cdrom 28960 0 (autoclean) [ide-cd] soundcore 4100 0 (autoclean) agpgart 31072 0 (unused) binfmt_misc 5956 1 iscsi 32672 0 (unused) scsi_mod 94424 1 [iscsi] autofs 10628 0 (autoclean) (unused) tulip 48608 1 ext3 60352 2 jbd 39192 2 [ext3]
The first column contains the name of the module. The second column refers to the memory size of the module, in bytes. The third column lists whether the module is currently loaded (1) or unloaded (0). The final column states if the module can unload itself automatically after a period without use (autoclean) or if it is not being utilized (unused). A module with a line containing a name listed in brackets ([ or ]) indicates that the module depends upon another module to be present in order to function.
This information can also be viewed via the /sbin/lsmod command.
This file provides a list of all mounts in use by the system:
rootfs / rootfs rw 0 0 /dev/hda2 / ext3 rw 0 0 /proc /proc proc rw 0 0 /dev/hda1 /boot ext3 rw 0 0 none /dev/pts devpts rw 0 0 none /dev/shm tmpfs rw 0 0 none /proc/sys/fs/binfmt_misc binfmt_misc rw 0 0
The output found here is similar to contents of /etc/mtab, except that /proc/mount is more up-to-date.
The first column specifies the device that is mounted, the second column reveals the mount point, and the third column tells the file system type, and the fourth column tells you if it is mounted read-only (ro) or read-write (rw). The fifth and sixth columns are dummy values designed to match the format used in /etc/mtab.
This file refers to the current Memory Type Range Registers (MTRRs) in use with the system. If the system architecture supports MTRRs, then the /proc/mtrr file may look similar to the following:
reg00: base=0x00000000 ( 0MB), size= 64MB: write-back, count=1
MTRRs are used with the Intel P6 family of processors (Pentium II and higher) and control processor access to memory ranges. When using a video card on a PCI or AGP bus, a properly configured /proc/mtrr file can increase performance more than 150%.
Most of the time, this value is properly configured by default. More information on manually configuring this file, can be found online at the following URL:
Most of the information here is of little importance to the user, except for the following columns:
major — The major number of the device with this partition. The major number in our example (3) corresponds with the block device ide0 in /proc/devices.
minor — The minor number of the device with this partition. This serves to separate the partitions into different physical devices and relates to the number at the end of the name of the partition.
#blocks — Lists the number of physical disk blocks contained in a particular partition.
name — The name of the partition.
This file contains a full listing of every PCI device on the system. Depending on the number of PCI devices, /proc/pci can be rather long. A sampling of this file from a basic system looks similar to the following:
Bus 0, device 0, function 0: Host bridge: Intel Corporation 440BX/ZX - 82443BX/ZX Host bridge (rev 3). Master Capable. Latency=64. Prefetchable 32 bit memory at 0xe4000000 [0xe7ffffff]. Bus 0, device 1, function 0: PCI bridge: Intel Corporation 440BX/ZX - 82443BX/ZX AGP bridge (rev 3). Master Capable. Latency=64. Min Gnt=128. Bus 0, device 4, function 0: ISA bridge: Intel Corporation 82371AB PIIX4 ISA (rev 2). Bus 0, device 4, function 1: IDE interface: Intel Corporation 82371AB PIIX4 IDE (rev 1). Master Capable. Latency=32. I/O at 0xd800 [0xd80f]. Bus 0, device 4, function 2: USB Controller: Intel Corporation 82371AB PIIX4 USB (rev 1). IRQ 5. Master Capable. Latency=32. I/O at 0xd400 [0xd41f]. Bus 0, device 4, function 3: Bridge: Intel Corporation 82371AB PIIX4 ACPI (rev 2). IRQ 9. Bus 0, device 9, function 0: Ethernet controller: Lite-On Communications Inc LNE100TX (rev 33). IRQ 5. Master Capable. Latency=32. I/O at 0xd000 [0xd0ff]. Non-prefetchable 32 bit memory at 0xe3000000 [0xe30000ff]. Bus 0, device 12, function 0: VGA compatible controller: S3 Inc. ViRGE/DX or /GX (rev 1). IRQ 11. Master Capable. Latency=32. Min Gnt=4.Max Lat=255. Non-prefetchable 32 bit memory at 0xdc000000 [0xdfffffff].
This output shows a list of all PCI devices, sorted in the order of bus, device, and function. Beyond providing the name and version of the device, this list also gives detailed IRQ information so an administrator can quickly look for conflicts.
To get a more readable version of this information, type:
This file gives information about memory usage on the slab level. Linux kernels greater than version 2.2 use slab pools to manage memory above the page level. Commonly used objects have their own slab pools. The following is a portion of a typical /proc/slabinfo virtual file:
slabinfo - version: 1.1 (statistics) kmem_cache 64 68 112 2 2 1 nfs_write_data 0 0 384 0 0 1 nfs_read_data 0 160 384 0 16 1 nfs_page 0 200 96 0 5 1 ip_fib_hash 10 113 32 1 1 1 journal_head 51 7020 48 2 90 1 revoke_table 2 253 12 1 1 1 revoke_record 0 0 32 0 0 1 clip_arp_cache 0 0 128 0 0 1 ip_mrt_cache 0 0 96 0 0 1
The values in this file occur in the following order: cache name, number of active objects, number of total objects, size of the object, number of active slabs (blocks) of the objects, total number of slabs of the objects, and the number of pages per slab.
Note that active in this case means an object is in use.
This file keeps track of a variety of different statistics about the system since it was last restarted. The contents of /proc/stat, which can be quite long, usually begins like the following example:
cpu 1139111 3689 234449 84378914 cpu0 1139111 3689 234449 84378914 page 2675248 8567956 swap 10022 19226 intr 93326523 85756163 174412 0 3 3 0 6 0 1 0 428620 0 60330 0 1368304 5538681 disk_io: (3,0):(1408049,445601,5349480,962448,17135856) ctxt 27269477 btime 886490134 processes 206458
Some of the more commonly used statistics include:
cpu — Measures the number of jiffies (1/100 of a second) that the system has been in user mode, user mode with low priority (nice), system mode, and in idle task, respectively. The total for all CPUs is given at the top, while each individual CPU is listed below with its own statistics.
page — The number of memory pages the system has written in and out to disk.
swap — The number of swap pages the system has brought in and out.
intr — The number of interrupts the system has experienced.
btime — The boot time, measured in the number of seconds since January 1, 1970, otherwise known as the epoch.
Using the echo command to wrote to this file, a remote root user can execute most System Request Key commands remotely as if at the local terminal. To echo values to this file, the /proc/sys/kernel/sysrq must be set to a value other than 0. For more information about the System Request Key, refer to Section 22.214.171.124 /proc/sys/kernel/.
Although it is possible to write to this file, it cannot be read, even by the root user.
This file measures swap space and its utilization. For a system with only one swap partition, the output of /proc/swap may look similar to the following:
Filename Type Size Used Priority /dev/hda6 partition 136512 20024 -1
While some of this information can be found in other files in the /proc/ directory, /proc/swap provides a snapshot of every swap file name, the type of swap space, the total size, and the amount of space is in use (in kilobytes). The priority column is useful when multiple swap files are in use. The lower the priority, the more likely the swap file is to be used.
This file contains information detailing how long the system has been on since its last restart. The output of /proc/uptime is quite minimal:
The first number is the total number of seconds the system has been up. The second number is how much of that time the machine has spent idle, in seconds.
This file specifies the version of the Linux kernel and gcc in use, as well as the version of Red Hat Enterprise Linux installed on the system:
Linux version 2.4.20-1.19126.96.36.199.12.ent (email@example.com) (gcc version 3.2.3 20030422 (Red Hat Enterprise Linux 3.2.3-7)) #1 Thu Jun 19 14:57:04 EDT 2003
This information is used for a variety of purposes, including the version data presented when a user logs in.