07-08-2009
Help with Bash Shell Script
I am fairly new to Unix scripting. A need has come where I need to to read the first column of a tab delimited file. The first column will be an id number and so I would need to read the first column one by one and create another file with only unique id numbers. So for example, if I have the following in my input file
12345 ABC
12345 XYZ
45678 ABC
12345 BVC
78906 ABC
12345 ABC
Here I need to create a file that should have the following:
12345
45678
78906
Thanks for the help in advance.
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LEARN ABOUT OSX
perldbmfilter
PERLDBMFILTER(1) Perl Programmers Reference Guide PERLDBMFILTER(1)
NAME
perldbmfilter - Perl DBM Filters
SYNOPSIS
$db = tie %hash, 'DBM', ...
$old_filter = $db->filter_store_key ( sub { ... } );
$old_filter = $db->filter_store_value( sub { ... } );
$old_filter = $db->filter_fetch_key ( sub { ... } );
$old_filter = $db->filter_fetch_value( sub { ... } );
DESCRIPTION
The four "filter_*" methods shown above are available in all the DBM modules that ship with Perl, namely DB_File, GDBM_File, NDBM_File,
ODBM_File and SDBM_File.
Each of the methods works identically, and is used to install (or uninstall) a single DBM Filter. The only difference between them is the
place that the filter is installed.
To summarise:
filter_store_key
If a filter has been installed with this method, it will be invoked every time you write a key to a DBM database.
filter_store_value
If a filter has been installed with this method, it will be invoked every time you write a value to a DBM database.
filter_fetch_key
If a filter has been installed with this method, it will be invoked every time you read a key from a DBM database.
filter_fetch_value
If a filter has been installed with this method, it will be invoked every time you read a value from a DBM database.
You can use any combination of the methods from none to all four.
All filter methods return the existing filter, if present, or "undef" if not.
To delete a filter pass "undef" to it.
The Filter
When each filter is called by Perl, a local copy of $_ will contain the key or value to be filtered. Filtering is achieved by modifying the
contents of $_. The return code from the filter is ignored.
An Example: the NULL termination problem.
DBM Filters are useful for a class of problems where you always want to make the same transformation to all keys, all values or both.
For example, consider the following scenario. You have a DBM database that you need to share with a third-party C application. The C
application assumes that all keys and values are NULL terminated. Unfortunately when Perl writes to DBM databases it doesn't use NULL
termination, so your Perl application will have to manage NULL termination itself. When you write to the database you will have to use
something like this:
$hash{"$key "} = "$value ";
Similarly the NULL needs to be taken into account when you are considering the length of existing keys/values.
It would be much better if you could ignore the NULL terminations issue in the main application code and have a mechanism that
automatically added the terminating NULL to all keys and values whenever you write to the database and have them removed when you read from
the database. As I'm sure you have already guessed, this is a problem that DBM Filters can fix very easily.
use strict;
use warnings;
use SDBM_File;
use Fcntl;
my %hash;
my $filename = "filt";
unlink $filename;
my $db = tie(%hash, 'SDBM_File', $filename, O_RDWR|O_CREAT, 0640)
or die "Cannot open $filename: $!
";
# Install DBM Filters
$db->filter_fetch_key ( sub { s/ $// } );
$db->filter_store_key ( sub { $_ .= "