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Shell Programming: Batch Files, continued...

        What happens when the script is called? The if construct checks whether there is a argument given to the script.  If it is missing, it prompts you a little help and terminates gracefully. 

        If there is an argument, it takes it as the name or a part of the name of a library, which will be stored in $1. "ldconfig -p" reports ALL shared libraries, which are known by the system along with the paths, where they are stored. This output, if piped though grep, looks for all lines containing the given name of that library.  The "-i" switches grep into "ignore case" mode.

        This is more convenient for all those X-related libraries, because they use also uppercase letters in their names...

*************************************************

Newbie note: "X-related libraries are not dirty pictures.  They are function libraries for X-windows, which does for Unix systems what Windows does for Unix.  If you want point and click hacking, install a Unix type operating system on your home computer and get X-windows running.

*************************************************

        The output of grep (all lines containing the library name) are piped into gawk, which filters all but the fourth argument (the path to the library and the library name itself) out of the stream. Each line of the finally resulting output is taken -- line by line -- by a for-loop. For each looping $i contains one new line -- a path and the library name -- which is take by "ls". There is a little trick at this line.

        There is not only a         

ls "$i"

 

        There is also a 

ls "$i"*

        This is because they may be different versions of that library, which can be listed by using ls "$i"*.

        Finally the output of the loop is piped through "sort -u" which sorts the output alphabetically and removes all identically lines. Therefore each line is only reported once. The "-u" switch stands for "unique".

Next one!

------------------------------------------------------------------

(store this script as "ldprint"

#!/bin/sh
#
# prints all libraries used by a 
# certain program
#
############################################
if [ -z $1 ]
then
    echo "usage: ldprint <program to examine>"
    exit
fi

ldd $(which $1)

        This one is simple. It runs with the same versions of tools mentioned above. Additionally: ldd is of version 1.9.6.. The version of "which" is unknown to me, but this program is so "unspecial", that I think, all versions should work.... :-)

        What happens here? ldd reports all libraries, which are used by a certain program. But! if you type 

ldd emacs

        This will not work in most cases, cause emacs is not in the current directory. "ldd" will only work if the full path to the program (emacs) is specified.  

        If you type

which emacs

        It will print (for example)

/usr/X11R6/bin/emacs

        Now! If you type 

ldd which emacs

        The shell cannot decide correctly what to call first: ldd, which, or emacs. It decides: First is the tool, all further things are arguments to ldd.

        Wrong!!!

        This is why we have to give the shell a hint, a tip. Are we friendly hackers?  YES! OK, 

        The $(which $1) replace "which $1" with the result of the run of which, removes the $( and the closing ) and THEN it will execute ldd.  Now ldd "sees" the full path, the call of which left behind. 

        But this is a more complicate thing. It is called "argument substitution" and you should read more about it in the man pages of your shell.

More on shell batch files --->>


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