How To Create A Wrapper Script With Customized Environment

When working in a team, everyone will have to conform and work in a same environment. But there are times that, a certain script of yours needs a different environment in order to run. An example would be, everyone in the team has their python version set to 2.7.13. But for some reason, you have a python script that will only work with python 3. Having a wrapper script that customize the environment before running your script would solve this issue. Here's how the wrapper script will look like

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#!/bin/tcsh -f setenv BNR_PATH /some/special/path/ setenv BNR_BIN /another/special/path/bin setenv BNR_ROOT /special/root/path /path/to/your/script.py $argv:q

How To Run A Script As The Owner Of The Script

For the explanation below, just keep in mind that these assumption is true:-
- owner of file == yltan(uid=742)
- user that executes the script == icetools(uid=48102)

How to run a python script as the owner of the script.

1. Create any simple python script, eg a.py:-

#!/usr/bin/env python
import os

print "Before swap::" + str(os.getresuid())
os.system("whoami")

os.setreuid(os.geteuid(),os.getuid())

print "After Swap:" + str(os.getresuid())
os.system("whoami")

2. This is the magic wrapper script, setuid_swap.c. :-

/* This binary is intended to be a setuid script wrapper for dbRsync.pl.
  
   Example compilation on an hp machine:  gcc dbRsync.c -o dbRsync
   This compilation creates an executable called dbRsync. After compiling,
   chmod +s dbRsync (or whatever the executable is) to set the sticky bit or
   it won't work.
*/

int main(int ac, char **av) {
    execv( "/path/to/your/a.py", av );
}

3. Compile the setuid_wrap.c to wrapper_a :-

gcc setuid_swap.c -o wrapper_a

4. Change the setuid bit:-

chmod 4755 wrapper_a

5. Run wrapper_a as someone else other than the owner of the file:-

sudo su - icetools

./wrapper_a

6. ... and this is the output that you get (Running as icetools):-

Before swap::(48102, 742, 742)
icetools
After Swap:(742, 48102, 48102)
yltan

For a detail explanation of how setuid work,

https://drive.google.com/open?id=0B_HHt58thGk_MTVwQkhleWNmZE0



Alternatively, you could swap the users in the compiled code itself by doing this:-

/* This binary is intended to be a setuid script wrapper for dbRsync.pl.
  
   Example compilation on an hp machine:  gcc dbRsync.c -o dbRsync
   This compilation creates an executable called dbRsync. After compiling,
   chmod +s dbRsync (or whatever the executable is) to set the sticky bit or
   it won't work.
*/

int main(int ac, char **av) {
    int uid;
    uid = geteuid();
    setreuid(uid, uid);
    execv( "/path/to/your/a.py", av );
}

How To Diff A Directory Efficiently

diff -qr -x '*~' -x '*.swp' -x '*.pyc'  -I '$Revision:'  -I '$File:'  -I '$Header:' -I '$Change:' -I '$DateTime:' -I '$Id:' -I '$Date:' -I '$Change:' -I '$Author:' directory1 directory2

Debugging Flow For A Process

I've been stucked with a job which took a lot longer than what it is suppose to take to complete. A friend of mine shared his gems of tricks on how he troubleshoot it, and with a little bit of tweak to my taste, I've came up with a flow(well, almost all of it still from him :p) which I'm logging it here so that I won't forget it.


#1 Find the offending process id (_pid_)

ps -aux | grep job_name

#2 Look at the entire hierarchy of the pid and look at where it stops

pstree -pulna _pid_

#3 Look at the trace of the running program.

strace -t -s 22222 -p _pid_

#4 Look at the read/write IO activities 

cat /proc/_pid_/io

Useful Links

http://www.linux-tutorial.info/modules.php?name=MContent&pageid=84
http://chadfowler.com/blog/2014/01/26/the-magic-of-strace/

Quotes Within Quotes In Linux

I keep forgetting this every time even though I have stumbled across this so many times.

I'm now gonna stick this here so that I can refer it back and hopefully remember this for good.

http://stackoverflow.com/questions/1250079/how-to-escape-single-quotes-within-single-quoted-strings

Basically, the idea is to ....

always just replace each embedded single quote with the sequence: '\'' (that is: quote backslash quote quote) or '"'"' , which closes the string, appends an escaped single quote and reopens the string. 

 https://stackoverflow.com/a/1315213/335181


Putting the above into a perl script works wonder:

#!/usr/bin/perl -pl
s/'/'\\''/g;    ### or s/'/'"'"'/g;
$_ = qq['$_'];