python: get the last directory name in a path

If I want to pull just the parent directory name out of a long path to a file:

>>> import os
>>> pth="/abeja/blanca/zumbas/ebria/de/miel/en/mi/alma/y.te.tuerces"
>>> os.path.split(os.path.dirname(pth))[1]
'alma'
>>>


python: create a dummy file in one line

open('onestep.txt','w').close()

from a comment on http://rogermcd.wordpress.com/2008/07/20/creating-a-file-with-python/

Linux: grep for $

This was harder than grep for tab.
I found instructions to escape the special character, but none of these
grep $ *
grep \$ *
grep "$" *
grep "\$" *
grep - $ *
grep "$" *
grep '$' *
egrep '$' *

worked until I read "Note that a $ sign loses its meaning if characters follow it" and remembered I was looking for a $ followed by a capital letter. So this works:

grep '$[A-Z]' *

but I still don't know how to find a $ by itself.

Python: os.path.lexists() vs os.path.exists() before os.symlink()

We were having intermittent failures with shutil.copy(src,dest) and decided to replace it with just a symbolic link, using os.symlink(src,dest). This triggered an error when we tried to create a symbolic link with the name of an existing file. So the code checks first. It uses os.path.lexists(), since that will return True for broken as well as valid links. os.path.exists() returns True for valid links, False for broken ones.
The os.remove() works whether or not the link is broken or valid or an actual file.

if os.path.lexists(dest):
    os.remove(dest)
os.symlink(src,dest)

(For more detail, see also June2013)

Linux: paste and diff -b

tidbits recently of use to me:

• paste to put two files together by merging their columns, without the complications of join
• diff -b to ignore blank lines etc, when comparing how my new code is different from the old code.

Rohit Bhardwaj's talk on Android programming last night at NE-JUG

(from NEJUG talk last night: http://www.nejug.org/events/show/128)

http://dl.dropbox.com/u/8106990/Android_Cool_apps_built_by_developers_like_you_1UP.pdf

http://dl.dropbox.com/u/8106990/Android_Cool_apps_built_by_developers_like_you_2UP_BW.pdf

Just text notes here:

http://dl.dropbox.com/u/8106990/AndroidNotes.txt

Listing other dl.dropbox URL's for a zip of samples source code.

His twitter:

http://twitter.com/#!/rbhardwaj1

My notes:

• Each App has its own 'Linux User' so it can't poach security info.
• All the data locked in for that user.
• There are ways to share data between apps.
• APK 
• instead of EXE
• can be stored in SD-card
• testing: need to test with at least 50 devices
• use ADBridge
• autogenerated R.java has all References. 
• OnCreate(Bundle savedInstance)
• Honeycomb/Android 3 like iPad, multitasking
• Android marketplace $25 to sell your apps.

See also http://code.google.com/edu/android/index.html

Python: appending column to end of tab-separated csv

Given this input file:

$ cat list.csv
a       b       c
d       e       f
g       h       i

Using this code:

import os,csv

def test_fcn() :
    try:
        tabReader = csv.reader( open("list.csv", "rb"), delimiter='\t', quoting=csv.QUOTE_NONE )
        fd = open("out.csv", "wb+")
        tabWriter = csv.writer(fd, delimiter='\t', quoting=csv.QUOTE_NONE)
        if tabReader != None :
            for record in tabReader:
                record.append("new_thing")
                tabWriter.writerow(record)
        fd.close()
        return

    except Exception,e :
        print "Exception : " + str(e)
        
def main():
    test_fcn()

if __name__ == '__main__':
    import sys
    main(*sys.argv[1:])

Here's the output:

$ cat out.csv
a       b       c       new_thing
d       e       f       new_thing
g       h       i       new_thing

Linux: Using whereis to find ifconfig

If something isn't in default path you still may be able to find it with whereis:

$ ifconfig
bash: ifconfig: command not found
$ whereis ifconfig
ifconfig: /sbin/ifconfig /usr/share/man/man8/ifconfig.8.gz
$ /sbin/ifconfig
eth2 Link encap:Ethernet HWaddr ...



Linux: to grep for dash, use grep --

For example, to search for python code containing the string "-D":

grep -- -D *.py

From http://stackoverflow.com/questions/2427913/grepping-for-string-containing-dash

Linux: Getting a list of known hosts without all the key stuff


cut -d ' ' -f 1 ~/.ssh/known_hosts