How to findout whether app is installed or not

[AC Question]

Lets say we have 2 apps named A and B. I installed app A in my device . When i am installing app B in the same device i have to find out whether app A is installed and perform some action while installing the app B or i should merge app B inside A.

 

[belodion]

I would check app C first.

What are you trying to accomplish?
You can leave B downloading and installing while you open app B and use it.

Don't you mean 'while you open app A and use it'?

 

[Rukbat]

An app can look for another app. Shell ls if you don't want anything more elegant than Linux 101.