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Top Forums UNIX for Beginners Questions & Answers Behaviour of echo commands used by Linux admins Post 303023888 by kraljic on Tuesday 25th of September 2018 07:51:22 AM
Old 09-25-2018
Behaviour of echo commands used by Linux admins

version info : Fedora 28 (Kernel version: 4.16.12-300)
shell : bash

Using echo command , if I redirect a text like "Chocolate" to a file , all the contents in the file are overwritten as shown below.

Code:
[root@john215 ~]# cat /tmp/someTest 
Hello world 
One more Hello world 
myLine3 
 
[root@john215 ~]# echo Chocolate > /tmp/someTest 
[root@john215 ~]# cat /tmp/someTest 
Chocolate 
[root@john215 ~]# file /tmp/someTest 
/tmp/someTest: ASCII text

But, I have seen Linux system admins using echo commands like below to scan for new LUNs.

Why is "/sys/class/scsi_host/host1/scan" file not being overwritten with "- - -" or "1" by the echo commands below ?

Code:
 
# echo "1" > /sys/class/fc_host/host/issue_lip 
# echo "- - -" > /sys/class/scsi_host/host1/scan

Source:

1.8. Scanning Storage Interconnects - Red Hat Customer Portal


Code:
# file /sys/class/scsi_host/host1/scan 
/sys/class/scsi_host/host1/scan: writable, regular file, no read permission

 

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HPSA(4) 						     Linux Programmer's Manual							   HPSA(4)

NAME
hpsa - HP Smart Array SCSI driver SYNOPSIS
modprobe hpsa [ hpsa_allow_any=1 ] DESCRIPTION
hpsa is a SCSI driver for HP Smart Array RAID controllers. Options hpsa_allow_any=1: This option allows the driver to attempt to operate on any HP Smart Array hardware RAID controller, even if it is not explicitly known to the driver. This allows newer hardware to work with older drivers. Typically this is used to allow installation of operating systems from media that predates the RAID controller, though it may also be used to enable hpsa to drive older controllers that would normally be handled by the cciss(4) driver. These older boards have not been tested and are not supported with hpsa, and cciss(4) should still be used for these. Supported hardware The hpsa driver supports the following Smart Array boards: Smart Array P700M Smart Array P212 Smart Array P410 Smart Array P410i Smart Array P411 Smart Array P812 Smart Array P712m Smart Array P711m StorageWorks P1210m Since Linux 4.14, the following Smart Array boards are also supported: Smart Array 5300 Smart Array 5312 Smart Array 532 Smart Array 5i Smart Array 6400 Smart Array 6400 EM Smart Array 641 Smart Array 642 Smart Array 6i Smart Array E200 Smart Array E200i Smart Array E200i Smart Array E200i Smart Array E200i Smart Array E500 Smart Array P400 Smart Array P400i Smart Array P600 Smart Array P700m Smart Array P800 Configuration details To configure HP Smart Array controllers, use the HP Array Configuration Utility (either hpacuxe(8) or hpacucli(8)) or the Offline ROM-based Configuration Utility (ORCA) run from the Smart Array's option ROM at boot time. FILES
Device nodes Logical drives are accessed via the SCSI disk driver (sd(4)), tape drives via the SCSI tape driver (st(4)), and the RAID controller via the SCSI generic driver (sg(4)), with device nodes named /dev/sd*, /dev/st*, and /dev/sg*, respectively. HPSA-specific host attribute files in /sys /sys/class/scsi_host/host*/rescan This is a write-only attribute. Writing to this attribute will cause the driver to scan for new, changed, or removed devices (e.g., hot-plugged tape drives, or newly configured or deleted logical drives, etc.) and notify the SCSI midlayer of any changes detected. Normally a rescan is triggered automatically by HP's Array Configuration Utility (either the GUI or the command-line variety); thus, for logical drive changes, the user should not normally have to use this attribute. This attribute may be useful when hot plugging devices like tape drives, or entire storage boxes containing preconfigured logical drives. /sys/class/scsi_host/host*/firmware_revision This attribute contains the firmware version of the Smart Array. For example: # cd /sys/class/scsi_host/host4 # cat firmware_revision 7.14 HPSA-specific disk attribute files in /sys /sys/class/scsi_disk/c:b:t:l/device/unique_id This attribute contains a 32 hex-digit unique ID for each logical drive. For example: # cd /sys/class/scsi_disk/4:0:0:0/device # cat unique_id 600508B1001044395355323037570F77 /sys/class/scsi_disk/c:b:t:l/device/raid_level This attribute contains the RAID level of each logical drive. For example: # cd /sys/class/scsi_disk/4:0:0:0/device # cat raid_level RAID 0 /sys/class/scsi_disk/c:b:t:l/device/lunid This attribute contains the 16 hex-digit (8 byte) LUN ID by which a logical drive or physical device can be addressed. c:b:t:l are the controller, bus, target, and lun of the device. For example: # cd /sys/class/scsi_disk/4:0:0:0/device # cat lunid 0x0000004000000000 Supported ioctl() operations For compatibility with applications written for the cciss(4) driver, many, but not all of the ioctls supported by the cciss(4) driver are also supported by the hpsa driver. The data structures used by these ioctls are described in the Linux kernel source file include/linux/cciss_ioctl.h. CCISS_DEREGDISK, CCISS_REGNEWDISK, CCISS_REGNEWD These three ioctls all do exactly the same thing, which is to cause the driver to rescan for new devices. This does exactly the same thing as writing to the hpsa-specific host "rescan" attribute. CCISS_GETPCIINFO Returns PCI domain, bus, device and function and "board ID" (PCI subsystem ID). CCISS_GETDRIVVER Returns driver version in three bytes encoded as: (major_version << 16) | (minor_version << 8) | (subminor_version) CCISS_PASSTHRU, CCISS_BIG_PASSTHRU Allows "BMIC" and "CISS" commands to be passed through to the Smart Array. These are used extensively by the HP Array Configuration Utility, SNMP storage agents, and so on. See cciss_vol_status at <http://cciss.sf.net> for some examples. SEE ALSO
cciss(4), sd(4), st(4), cciss_vol_status(8), hpacucli(8), hpacuxe(8), <http://cciss.sf.net>, and Documentation/scsi/hpsa.txt and Documentation/ABI/testing/sysfs-bus-pci-devices-cciss in the Linux kernel source tree COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/. Linux 2017-09-15 HPSA(4)
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