what are AT commands

i'v bought a bluetooth module and all the manual is talking about is AT commands,bought GPS module and AT commands again...what are they?What are AT commands?..where to find information about them?what books can you recommend for me to read?...I have tried searching online but cannot find anything useful

AT commands are command sequences starting with the ASCII characters 'A' 'T' respectively 'a' 't'. See https://en.wikipedia.org/wiki/AT_command
To know the commands available for a specific device, you need to consult the manual. In so far, "searching online" is not necessarily required, but might be instructive, though.

AT commands are instructions used to control a modem. AT is the abbreviation of ATtention. Every command line starts with "AT" or "at". That's why modem commands are called AT commands. Many of the commands that are used to control wired dial-up modems, such as ATD (Dial), ATA (Answer), ATH (Hook control) and ATO (Return to online data state), are also supported by GSM/GPRS modems and mobile phones. Besides this common AT command set, GSM/GPRS modems and mobile phones support an AT command set that is specific to the GSM technology, which includes SMS-related commands like AT+CMGS (Send SMS message), AT+CMSS (Send SMS message from storage), AT+CMGL (List SMS messages) and AT+CMGR (Read SMS messages).

https://www.developershome.com/sms/atCommandsIntro.asp
https://oldlight.wordpress.com/2009/06/16/tutorial-using-at-commands-to-send-and-receive-sms/
https://tutorial-on.blogspot.com/2008/01/introduction-to-at-commands.html


List of AT Commands:
https://www.engineersgarage.com/tutorials/at-commands?page=4

Types of AT Commands:
https://www.engineersgarage.com/tutorials/at-commands?page=2

@tpetar thank you very much for that information

You can test AT commands with your GSM modems with the online AT command tester tool. You need to have you modem connected to your PC through a serial cable or USB interface.