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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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LOGPROF.CONF(5) 						     AppArmor							   LOGPROF.CONF(5)

NAME
logprof.conf - configuration file for expert options that modify the behavior of the AppArmor aa-logprof(1) program. DESCRIPTION
The aa-logprof(1) program can be configured to have certain default behavior by the contents of logprof.conf. The [qualifiers] section lists specific programs that should have a subset of the full ix/px/ux list when asking what mode to execute it using. Since creating a separate profile for /bin/bash is dangerous, we can specify that for /bin/bash, only (I)nherit, (U)nconstrained, and (D)eny should be allowed options and only those will show up in the prompt when we're asking about adding that to a profile. Likewise, if someone currently exec's /bin/mount in ix or px mode, things won't work, so we can provide only (U)nconstrained and (D)eny as options. And certain apps like grep, awk, sed, cp, and mkdir should always inherit the parent profile rather than having their own profile or running unconfined, so for them we can specify that only (I)nherit and (D)eny are the allowed options. Any programs that are not listed in the qualifiers section get the full (I)nherit / (P)rofile / (U)nconstrained / (D)eny option set. If the user is doing something tricky and wants different behavior, they can tweak or remove the corresponding line in the conf file. The [defaulthat] section lists changehat-aware programs and what hat aa-logprof(1) will collapse the entries to for that program if the user specifies that the access should be allowed, but should not have it's own hat. The [globs] section allows modification of the logprof rule engine with respect to globbing suggestions that the user will be prompted with. The format of each line is-- "<perl glob> = <apparmor glob>". When aa-logprof(1) asks about a specific path, if the perl glob matches the path, it replaces the part of the path that matched with the corresponding apparmor glob and adds it to the list of globbing suggestions. Lines starting with # are comments and are ignored. EXAMPLE
[qualifiers] # things will very likely be painfully broken if bash has it's own profile /bin/bash = iu # mount doesn't work if it's confined /bin/mount = u # these helper utilities should inherit the parent profile and # shouldn't have their own profiles /bin/awk = i /bin/grep = i /bin/sed = i [defaulthat] /usr/sbin/sshd = EXEC /usr/sbin/httpd2 = DEFAULT_URI /usr/sbin/httpd2-prefork = DEFAULT_URI [globs] # /foo/bar/lib/libbaz.so -> /foo/bar/lib/lib* /lib/lib[^/]+so[^/]*$ = /lib/lib*so* # strip kernel version numbers from kernel module accesses ^/lib/modules/[^/]+/ = /lib/modules/*/ # strip pid numbers from /proc accesses ^/proc/d+/ = /proc/*/ BUGS
If you find any bugs, please report them at <http://https://bugs.launchpad.net/apparmor/+filebug>. SEE ALSO
apparmor(7), apparmor.d(5), aa-enforce(1), aa-complain(1), aa-disable(1), aa_change_hat(2), aa-logprof(1), aa-genprof(1), and <http://wiki.apparmor.net>. AppArmor 2.7.103 2012-06-28 LOGPROF.CONF(5)
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