head and tail Command Related

commandline session

following session display may not be accurate

$sudo head dmesg
[    0.000000] Initializing cgroup subsys cpuset
[    0.000000] Initializing cgroup subsys cpu
[    0.000000] Linux version 3.2.0-4-amd64 (debian-kernel@lists.debian.org) (gcc version 4.6.3 (Debian 4.6.3-14) ) #1 SMP Debian 3.2.46-1+deb7u1
[    0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-3.2.0-4-amd64 root=UUID=3219e911-81fb-4d52-ba7c-117a58d09312 ro quiet
[    0.000000] BIOS-provided physical RAM map:
[    0.000000]  BIOS-e820: 0000000000000000 - 000000000009e800 (usable)
[    0.000000]  BIOS-e820: 000000000009e800 - 00000000000a0000 (reserved)
[    0.000000]  BIOS-e820: 00000000000e0000 - 0000000000100000 (reserved)
[    0.000000]  BIOS-e820: 0000000000100000 - 000000009e5e7000 (usable)
[    0.000000]  BIOS-e820: 000000009e5e7000 - 000000009e619000 (reserved)
$sudo head ls
head: cannot open `ls' for reading: No such file or directory
$sudo head -6 dmesg
[    0.000000] Initializing cgroup subsys cpuset
[    0.000000] Initializing cgroup subsys cpu
[    0.000000] Linux version 3.2.0-4-amd64 (debian-kernel@lists.debian.org) (gcc version 4.6.3 (Debian 4.6.3-14) ) #1 SMP Debian 3.2.46-1+deb7u1
[    0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-3.2.0-4-amd64 root=UUID=3219e911-81fb-4d52-ba7c-117a58d09312 ro quiet
[    0.000000] BIOS-provided physical RAM map:
[    0.000000]  BIOS-e820: 0000000000000000 - 000000000009e800 (usable)
$sudo head -3 auth.log daemon.log
==> auth.log  daemon.log <==
Nov 10 20:11:07 debian acpid: starting up with netlink and the input layer
Nov 10 20:11:07 debian acpid: 1 rule loaded
Nov 10 20:11:07 debian acpid: waiting for events: event logging is off
$tail -f iostat
tail: cannot open `iostat' for reading: No such file or directory
$tail -f dmesg
tail: cannot open `dmesg' for reading: Permission denied
$sudo tail -f dmesg
[   14.815581] NET: Registered protocol family 31
[   14.815587] Bluetooth: HCI device and connection manager initialized
[   14.815593] Bluetooth: HCI socket layer initialized
[   14.815603] Bluetooth: L2CAP socket layer initialized
[   14.815824] Bluetooth: SCO socket layer initialized
[   14.847521] Bluetooth: RFCOMM TTY layer initialized
[   14.847531] Bluetooth: RFCOMM socket layer initialized
[   14.847535] Bluetooth: RFCOMM ver 1.11
[   14.924529] Bluetooth: BNEP (Ethernet Emulation) ver 1.3
[   14.924536] Bluetooth: BNEP filters: protocol multicast
^C$

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