seems pretty easy to get hooked.
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Sort HashMap by Values
Let’s say you have a hash map that stores the frequencies of words. So you are collecting (by incrementing the counters for) word frequency while you see them. After you are done, you want get the list of words ordered by frequency – essentially you want to sort the hashmap by values.
Some code here:
List keys = new ArrayList();
keys.addAll(map.keySet());
Collections.sort(keys, new Comparator() {
public int compare(Object obj1, Object obj2) {
Integer v1 = map.get(obj1);
Integer v2 = map.get(obj2);
return v2.compareTo(v1);
}
});
Iterator words = keys.iterator();
while (words.hasNext())
{
String word = (String) words.next();
int freq = map.get(word).intValue();
System.out.println("Word " + word + " : " + freq);
}
We do
return v2.compareTo(v1);
since we want to sort the list in descending order. If you need sort it in ascending order, you would swap v1 and v2.
Run remote command through ssh and pass in local input
There are times that you want to run some command/script on a remote machine. It’s pretty straightforward, you can just do:
ssh remote_host remote_command
But there are also times you have some input that’s only available on local machine that you want to pass it as input to the remote command. In that case, you can do this:
ssh remote_host “remote_command ‘$(cat local_file.txt)’”
Several things to notice:
- you need the double quote since the entire command has arguments
- you need the dollar sign and parentheses to enclose the local command to pass the value of the output of it
- you need th single quote to enclose the output above
Alternatively, you could also do
cat local_file.txt | ssh remote_host “remote_command”
bash for loop
Here’s a simple shell trick that does a loop through some numbers:
for i in {0..10}; do echo $i; done
For example, sometimes you want to test some concurrent read/write performance on database or whatever, you can do:
for i in {1..10}; do (echo $i; ./read_db.sh $i &); done
This will generate 10 processes that execute the read_db.sh script concurrently.
Simple examples of using screen command
To start a named screen session:
screen -S screen_name
To detach from current screen:
ctrl-a d
To list screens:
screen -ls
To reattach to a screen:
screen -r screen_name
To reattch to a screen and detach the existing attached screen:
screen -dr screen_name
To create a window inside a screen
ctrl-a c
To go to next or previous window
ctrl-a n ctrl-a p
Some reference:
http://www.soulcast.com/post/show/55079/An-introduction-to-the-linux-screen-command
WordPress pretty permalink not working?
After install wordpress, usually you want to change your default permalink settings so that URL for you post will look like this
http://www.mysite.com/wordpress/fun/my-story-here
instead of this
http://www.mysite.com/wordpress?p=32
But after you change your settings in “Settings” -> “Permalinks”, when you click on the post title, it complains about the page does not exist? That’s what happened to me.
Turns out the reason was the “mod_rewrite” is not enabled on my web server (Apache). mod_rewrite is the thing behind the scene that does this magic of translating the pretty permalink to the ugly “?p=32″ – the one WordPress code can understand. So it has to be enabled.
In my case, my web server is Apache on Ubuntu, the mod_rewrite is installed but not enabled. This is what I did to enable it:
cd /etc/apache2/mods-enabled ln -s ../mods-available/rewrite.load
Hope this is helpful to someone who has the same permalink problem.