linux: check memory usage by 'free'

free -m -t

in MB. Note: since the memory is utilized by kernal than it is wasted. If you are curious how much memory you can use for your applications, you should check the 'free' column in the row of '-/+ buffers/cache'.

linux: sort

Use the command

sort -n -t. -k4

to sort strings like:
------------------------------
str1.str2.str3.1.txt
str1.str2.str3.10.txt
str1.str2.str3.2.txt
str1.str2.str3.21.txt
------------------------------

Use '-t' to define delimiter, '-n' for number sorting, '-k' for column number

linux: how to generate an animated gif picture

gifsicle --loopcount --colors 256 --delay=50 1.gif 2.gif 3.gif > output.gif

'delay' is a parameter similar as 'frame rate', 50 means 50/100 seconds between frames.

linux: convert pictures (jpg) to avi movie

If you have pic files like:
drilldata00001.jpg
drilldata00002.jpg
drilldata00003.jpg

then you can use:

mencoder -speed 0.1 "mf://drilldata%05d.jpg" -o movie_drilldata.avi -ovc lavc -lavcopts vcodec=mjpeg

to generate an avi movie. The speed '0.1' is slower, '1' used the default frame rate. '1o' goes faster...