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Full Discussion: Bash: How to remove the last character of a string?
Top Forums Shell Programming and Scripting Bash: How to remove the last character of a string? Post 302599766 by agama on Saturday 18th of February 2012 08:42:57 AM
Old 02-18-2012
Quote:
Originally Posted by balajesuri
Code:
echo $x | sed s/.$//

Yes, this works, but it is horribly inefficient. Consider these two scripts which both chop the last character from the string stored in 'foo' and assign it to 'x':

Code:
#!/usr/bin/env bash
foo="hello world"
for (( i = 0; i < 1000; i++ ))
do
    x="${foo%?}"
done

and

Code:
#!/usr/bin/env bash
foo="hello world"
for (( i = 0; i < 1000; i++ ))
do
    x=$(echo $foo | sed s/.$// )
done

The first, using shell string manipulation, takes about 2/100ths of a second to execute:

Code:
time script1

real    0m0.02s
user    0m0.01s
sys     0m0.01s

while the second script takes more than 5 seconds:

Code:
time script2

real    0m5.03s
user    0m1.05s
sys     0m3.80s

If I've done my math correctly, that's about 2500 times slower, just to do this one assignment. Imagine if the script needed to chop 100 strings in its loop and did them all this way?

You'll note that the majority of the time in the second script is system time -- that's the time that bash had to wait on the system to start each sed process and deal with the I/O from sed back to bash.
Last edited by agama; 02-18-2012 at 09:46 AM.. Reason: clarification in text
 

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LEARN ABOUT OPENSOLARIS

libbash

LIBBASH(7)							  libbash Manual							LIBBASH(7)

NAME

     libbash -- A bash shared libraries package.

DESCRIPTION

     libbash  is  a package that enables bash dynamic-like shared libraries. Actually its a tool for managing bash scripts whose functions you may
     want to load and use in scripts of your own.

     It contains a 'dynamic loader' for the shared libraries ( ldbash(1)), a configuration tool (ldbashconfig(8)), and some libraries.

     Using ldbash(1) you are able to load loadable bash libraries, such as getopts(1) and hashstash(1).  A bash shared library that can be  loaded
     using
     ldbash(1) must answer 4 requirments:

     1.     It must be installed in $LIBBASH_PREFIX/lib/bash (default is /usr/lib/bash).

     2.     It must contain a line that begins with '#EXPORT='.  That line will contain (after the '=') a list of functions that the library
	    exports.  I.e. all the function that will be usable after loading that library will be listed in that line.

     3.     It must contain a line that begins with '#REQUIRE='.  That line will contain (after the '=') a list of bash libraries that are
	    required for our library.  I.e. every bash library that is in use in our bash library must be listed there.

     4.     The library must be listed (For more information, see ldbashconfig(8)).

   Basic guidelines for writing library of your own:
     1.     Be aware, that your library will be actually sourced. So, basically, it should contain (i.e define) only functions.

     2.     Try to declare all variables intended for internal use as local.

     3.     Global variables and functions that are intended for internal use (i.e are not defined in '#EXPORT=') should begin with:
		  __<library_name>_
	    For example, internal function myfoosort of hashstash library should be named as
		  __hashstash_myfoosort
	    This helps to avoid conflicts in global name space when using libraries that come from different vendors.

     4.     See html manual for full version of this guide.

AUTHORS

     Hai Zaar <haizaar@haizaar.com>
     Gil Ran <ril@ran4.net>

SEE ALSO

     ldbash(1), ldbashconfig(8), getopts(1), hashstash(1) colors(1) messages(1) urlcoding(1) locks(1)

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