New command: prepend

Brock Noland — Mon, 07 Apr 2008 02:44:37 +0000

I am utilizing Google’s project hosting to host software which I create and feel is useful or want to keep track of. I called the project Brock’s Tools. The code that led me to create this project was a command I am calling prepend 1.1. (UPDATE: See this post on sponge as its a better general case tool.)

prepend, prepend’s files or standard input to a file. For example, you have three files:

$ echo BROCK > a
$ echo DAVID > b
$ echo NOLAND > c

And you want to combine them into one file:

$ echo "My name is:" | prepend - a b c
$ cat c
My name is:
BROCK
DAVID
NOLAND

Or lets say you just want to append a file to itself:

$ cat a
BROCK
$ cat a >> a
cat: a: input file is output file

prepend does this:

$ prepend a
$ cat a
BROCK
BROCK

I come across the a situation where this would be useful quite often. Of course prepend’ing can be done in the shell:

$ { echo "My name is:"; cat a b c; } > tmp && mv -f tmp c
$ cat c
My name is:
BROCK
DAVID
NOLAND

However, that is unsafe and I have lost data that way. I perform this operation most often when dealing with XML. In this example, its trivial to open the file in an editor, but with a large file, its quite nasty to do so:

$ cat something.xml
stuff 1
stuff 2
stuff 3 
stuff 4
$ echo "" >> something.xml
$ cat something.xml
stuff 1
stuff 2
stuff 3 
stuff 4

$ echo "" | prepend - something.xml
$ cat something.xml

stuff 1
stuff 2
stuff 3 
stuff 4

Wrapping dynamic languages in shell without an extra script

Brock Noland — Tue, 25 Mar 2008 23:16:30 +0000

There are situations where, if you want a Python, PERL, PHP, etc script to be portable among a few different servers, it makes sense to wrap the script in shell. A few years ago I was trying to use the Python cx_Oracle module. This module is a wrapper for the native Oracle database driver. However, it requires the driver library directory be in the LD_LIBRARY_PATH environment variable.

No problem I thought. I’ll use the os.environ dict to set the variable. Example script:

$ cat python-only.sh
#!/usr/bin/python
import sys, os
sys.path.append("/usr/local/lib/python2.4/site-packages/")
if not os.environ.has_key('LD_LIBRARY_PATH'):
        os.environ['LD_LIBRARY_PATH'] = "/home/noland/oracle-lib"
else:
        os.environ['LD_LIBRARY_PATH'] = "/home/noland/oracle-lib:" + os.environ['LD_LIBRARY_PATH']
print "LD_LIBRARY_PATH looks OK in Python: LD_LIBRARY_PATH = ", os.environ['LD_LIBRARY_PATH']
os.system('echo LD_LIBRARY_PATH looks OK via os.system: LD_LIBRARY_PATH = $LD_LIBRARY_PATH')
try:
        import cx_Oracle
        print "Imported cx_Oracle! LD_LIBRARY_PATH was set correctly."
except ImportError, e:
        print "Woops, LD_LIBRARY_PATH was not set correctly: ", e

This method does not work:

$ ./python-only.sh
LD_LIBRARY_PATH looks OK in Python: LD_LIBRARY_PATH =  /home/noland/oracle-lib
LD_LIBRARY_PATH looks OK via os.system: LD_LIBRARY_PATH = /home/noland/oracle-lib
Woops, LD_LIBRARY_PATH was not set correctly:  libclntsh.so.10.1: cannot open shared object file: No such file or directory

This seems to be a common problem. However, when I was dealing with this a few years ago, I could not find a good resource on Google. I bite the bullet and wrote a separate shell script wrapper – hating invocation of the shell script. However, there is absolutely no reason I needed a separate shell script. I could have embedded the Python within a shell script. Example:

$ cat python-and-bash.sh
#/bin/bash
export LD_LIBRARY_PATH=/home/noland/oracle-lib:$LD_LIBRARY_PATH
/usr/bin/python<
Ahh, much better:
$ ./python-and-bash.sh
Imported cx_Oracle! LD_LIBRARY_PATH was set correctly.
Of course I could have just set this variable in my profile. However, this creates an additional external dependency – which is what I was trying to avoid.

BASH Cures Cancer » python

New command: prepend

Wrapping dynamic languages in shell without an extra script