and now for something completely different...
sometimes in a shell script you need to do stuff that modifies
the positional parameters (
$@,
$1,
$2, etc) from the cmdline, usually via the builtin
shift, maybe from using
getopt(1) or similar.
for example, it's quite common to see the following in a shell
script that parses cmdline flags with getopt
# parse cmdline options with getopt
TEMP= getopt -o h123 -n $0 -- "$@"
if [ $? != 0 ] ; then usage >&2 ; exit 1 ; fi
# overwrites $@ with getopt-generated/sanitized arguments
eval set -- "$TEMP"
# destructively iterates across $@
while true; do
case "$1" in
-h)
# handle option h
shift
;;
...
--)
shift
break
;;
esac
...
done
but sometimes you want to be able to restore the original
positional parameters later, or otherwise quote them in a manner
where they can be passed along to another script/function, log
them, embed them in a string/file, etc. you can't always just stash
them in another variable to use that later, because
"$@" has a special meaning to the shell that can't
otherwise be reproduced with another variable.
one way you could try to store/save them would be to save
the original copy of
$@ and use
eval set
-- later on to change it back to the saved variable. using a
slightly simplified version of the above:
#!/bin/sh
# save the cmdline
saved="$@"
echo original command line, $# arguments: "$@"
while test -n "$*"; do echo arg: "$1"; shift; done
echo mangled command line, $# arguments: "$@"
# restore the cmdline
eval set -- "$saved"
echo restored command line, $# arguments: "$@"
while test -n "$*"; do echo arg: "$1"; shift; done
and at first glance, it seems to work:
rangda[/home/sean] ./bad.sh one two three
original command line, 3 arguments: one two three
arg: one
arg: two
arg: three
mangled command line, 0 arguments:
restored command line, 3 arguments: one two three
arg: one
arg: two
arg: three
but you may run into problems when some parameters have spaces,
or quotes:
rangda[/home/sean] ./bad.sh "1: one" "2: two"
original command line, 2 arguments: 1: one 2: two
arg: 1: one
arg: 2: two
mangled command line, 0 arguments:
restored command line, 4 arguments: 1: one 2: two
arg: 1:
arg: one
arg: 2:
arg: two
rangda[/home/sean] ./bad.sh "one's one"
original command line, 1 arguments: one's one
arg: one's one
mangled command line, 0 arguments:
./bad.sh: eval: line 12: unexpected EOF while looking for matching ''
or worse, shell meta characters:
rangda[/home/sean] ./bad.sh 'i wonder what pwd would do'
original command line, 1 arguments: i wonder what pwd would do
arg: i wonder what pwd would do
mangled command line, 0 arguments:
restored command line, 6 arguments: i wonder what /home/sean would do
arg: i
arg: wonder
arg: what
arg: /home/sean
arg: would
arg: do
the solution is to build up the saved value a bit more manually,
single-quoting each parameter (which should prevent any unintended
side effects of space/quotes/metacharacters), and escaping existing
single quotes inside of parameters.
# escape any single quotes in an argument
quote()
echo "$1" sed -e "s,','\\\\'',g"
# save up a properly quoted/escaped version of "$@"
for arg in "$@"; do
saved="$ saved:+$saved '$(quote "$arg")'"
done
echo original command line, $# arguments: "$@"
while test -n "$*"; do echo arg: "$1"; shift; done
echo mangled command line, $# arguments: "$@"
# restore the cmdline
eval set -- "$saved"
echo restored command line, $# arguments: "$@"
while test -n "$*"; do echo arg: "$1"; shift; done
let's see how it behaves:
rangda[/home/sean] ./good.sh "1 one" "2: two"
original command line, 2 arguments: 1 one 2: two
arg: 1 one
arg: 2: two
mangled command line, 0 arguments:
restored command line, 2 arguments: 1 one 2: two
arg: 1 one
arg: 2: two
rangda[/home/sean] ./good.sh "one's one"
original command line, 1 arguments: one's one
arg: one's one
mangled command line, 0 arguments:
restored command line, 1 arguments: one's one
arg: one's one
rangda[/home/sean] ./good.sh 'i wonder what pwd would do'
original command line, 1 arguments: i wonder what pwd would do
arg: i wonder what pwd would do
mangled command line, 0 arguments:
restored command line, 1 arguments: i wonder what pwd would do
arg: i wonder what pwd would do
while not entirely pretty, it does to do the trick. i'd be
interested to know if anyone has a better way of doing this (or if
there are hidden problems in what i've posted above).
UNIX filesystems generally store three pieces of timing information about files - ctime (when the file was changed in any way), mtime (when the file contents, as opposed to its metadata, was last changed) and atime (when the file was last accessed by any process). This is a usefully flexible system, but the semantics of atime can be troublesome. atime must be updated every time a file is read, causing a read operation to instead become a read/write operation. This results in a surprising amount of io being generated in normal filesystem use, slowing the more relevant io and causing disks to spin up due to atime updates being required even if the file was read out of cache. It also results in a lot of unnecessary activity on flash media which may reduce their lifetime.
Two days ago I read about the 
...
If you ever have the problem described below, you have to realize that Apache HTTPD is selecting the SSL certificate (and/or key) not based on you VirtualHost definition, but based on the ServerName in the current context. This means that you have two virtual hosts with the same ServerName, but on two different IPs, and are wondering why the wrong key (and/or certificate) is used, it might be because of this. Assume we have the following (simplified) configuration:
This is my recipe for easily running 
OK, it's gone 2am and I'm still battling with what was meant to be a simple task - check the latest (unreleased) changes in tslib on my touchscreen device.
As of this morning,
I generally try not to get caught up in writing auxiliary tools for my
free software projects. My experience in the past has been that given
how little spare time I have each week, a tool that takes more than an
hour or two to write quickly ends up becoming a time sink that
prevents me from making progress on the main project. But
occasionally it's worth the effort.
