batch renaming files with spaces

Why do people keep giving me tons of files with spaces in the filename?

Anyway, here's a good way to get rid of those pesky spaces:

ls * | while read file; do mv "$file" ${file// /_}; done

First I tried using "for file in `ls *`" but of course the whitespaces came back to bite me... This was also true for the mv command. You have to quote "$file" in order for the whitespace ridden filename to be recognised as a unit rather than multiple file descriptors.

bash array size

Strangely this caught me off guard. If you use ${#array[@]} to get the 'size' of an array, it actually only returns the number of assigned elements in the array.

e.g.

> array[23]=123
> echo ${#array[@]}
1

Hmm... only 1? Not 24?

As far as I can tell, there's no way around this. Just don't expect this behaviour and fill your arrays wisely.

While we're on bash arrays, remember that you can change the 'join' character for naive printing of arrays by manipulating the IFS (Internal Field Separator) variable. Below I also show that the quotation context is important for this:

> array[0]=1;array[1]=2;array[2]=3;
> echo ${array[*]}
1 2 3
> echo "${array[*]}"
1 2 3
> IFS=","
> echo ${array[*]}
1 2 3
> echo "${array[*]}"
1,2,3
> 

Note: It's best practice to store and then restore the original IFS variable.

> ORIG_IFS=IFS
... do stuff
> IFS=ORIG_IFS

Java: Heap Dump on OutOfMemoryError

You can request the jvm create a heap dump when an OutOfMemoryError is thrown. This is handy if you have a process that consumes a ton of RAM and you don't know why. Set the max heap size to something around 500M (or less. It needs to be fairly small if you're going to inspect the heap with 'jhat'). Use the -XX:+HeapDumpOnOutOfMemoryError flag to request the heap dump. This will output to java_pid.hprof by default. You can set the output filename manually using -XX:HeapDumpPath=<filename>.

e.g.

> java -Xmx100m -XX:+HeapDumpOnOutOfMemoryError -XX:HeadDumpPath=/tmp/dump.hprof com.geekbraindump.MyMemoryHoggingClass