scsi_ch_swp man page

scsi_ch_swp — change SCSI Software WRITE Protect and block device settings


scsi_ch_swp [--help] [--set 0|1] [--verbose] [--wr] BLK_DEVICE


This bash shell script attempts to read or change the Software Write Protect (SWP) bit in the SCSI Control mode page using the sdparm utility. If that change is successful then it uses the blockdev utility to change BLK_DEVICE's read-write or read-only setting in sympathy. This script is only available in Linux since the blockdev utility is Linux specific.

If changing the SWP bit is supported, setting the SWP bit makes BLK_DEVICE read-only at the device (disk) itself. Clearing the SWP bit makes BLK_DEVICE read-write which is usually the default setting at device (disk) power up.

If no options are given then the current state of the SWP bit is printed out (using sdparm) together with blockdev's RO bit. If they are both 1 then BLK_DEVICE is in read-only mode; if they are both 0 then BLK_DEVICE is in read-write mode.


Arguments to long options are mandatory for short options as well.

-h, --help

print out the usage message then exit.

-s, --set 0|1

when 0 is given then the SWP bit in the Control mode page is cleared (i.e. set that bit to 0) using sdparm; and blockdev is used to set BLK_DEVICE to read-write mode. When 1 is given then the SWP bit in the Control mode page is set using sdparm; and blockdev is used to set BLK_DEVICE to read-only mode. The default (i.e. when this option is not given) is to read the state of the SWP bit and BLK_DEVICE's read-only mode and print those values out.

-v, --verbose

increase level or verbosity.

-w, --wr

when the BLK_DEVICE is in read-only mode, Linux will not typically allow BLK_DEVICE to be opened read-write by sdparm, so the default action of this script is to use the sdparm --readonly option when --set 0 has been invoked. It is possible that BLK_DEVICE could be a generic device (e.g. /dev/sg3) which has a different interpretation of the read-only open flag and may fail. For such rare cases the --wr option makes sdparm open BLK_DEVICE read-write when --set 0 is invoked.

Exit Status

The exit status of this script is 0 when it is successful. Otherwise the exit status is that of the sdparm or the blockdev utility, whichever is called last. See the sdparm(8) man page.


Written by D. Gilbert

See Also

sdparm(sdparm), blockdev(8)


May 2013 sdparm-1.08 SDPARM