Extracting substring Post: 302524039

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Top Forums Shell Programming and Scripting Extracting substring Post 302524039 by agama on Friday 20th of May 2011 08:03:42 PM

05-20-2011

Registered User

Quote:

Originally Posted by mira

Hi Franklin,

I am curious to know what does this '##*' mean in this regular expression?

Thanks!

This isn't a regex, it's a shell (kshell or bash) variable substitution syntax. The ##pattern and #pattern match the pattern at the beginning of the value contained by the variable and deletes it before the value is used. (The contents of the variable are unchanged)

The difference between the two is that ## matches the longest pattern and # matches the shortest. For example, if you had a filename /home/scooter/lib/cal.rb in variable fname and want the basename, coding ${fname##*/} matches all charcters (*) from the front of the string up to the right most slant. The result would be cal.rb

If you had coded [${fname#*/} then all characters up to the first slant would be removed and the result would have been home/scooter/lib/cal.rb.

You will also note that the "pattern" isn't a regular expression pattern, but a file globbing pattern and thus its ##*/ rather than ##.*/. This can be confusing.

The advantage to using these, as opposed to $(basename $fname), is that it is much more efficient to let the shell do the string manipulation than to invoke a separate process just to chop a string up.

Have a look at the Kshell man page as there are a lot of variable substitution tricks that can be used.
man/man1/ksh.html man page

agama

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LEARN ABOUT LINUX

lsusb

lsusb(8)							Linux USB Utilities							  lsusb(8)

NAME

       lsusb - list USB devices

SYNOPSIS

       lsusb [ options ]

DESCRIPTION

       lsusb is a utility for displaying information about USB buses in the system and the devices connected to them.

OPTIONS

       -v, --verbose
	      Tells lsusb to be verbose and display detailed information about the devices shown.  This includes configuration descriptors for the
	      device's current speed.  Class descriptors will be shown, when available, for USB device classes including hub, audio, HID, communi-
	      cations, and chipcard.

       -s [[bus]:][devnum]
	      Show only devices in specified bus and/or devnum.  Both ID's are given in decimal and may be omitted.

       -d [vendor]:[product]
	      Show only devices with the specified vendor and product ID.  Both ID's are given in hexadecimal.

       -D device
	      Do  not  scan  the /dev/bus/usb directory, instead display only information about the device whose device file is given.	The device
	      file should be something like /dev/bus/usb/001/001.  This option displays detailed information like the v option; you must  be  root
	      to do this.

       -t     Tells lsusb to dump the physical USB device hierarchy as a tree. This overrides the v option.

       -V, --version
	      Print  version information on standard output, then exit successfully.

RETURN VALUE

       If the specified device is not found, a non-zero exit code is returned.

FILES

       /var/lib/usbutils/usb.ids
	      A list of all known USB ID's (vendors, products, classes, subclasses and protocols).

SEE ALSO

       lspci(8), usbview(8).

AUTHOR

       Thomas Sailer, <sailer@ife.ee.ethz.ch>.

usbutils-001							    6 May 2009								  lsusb(8)