Shared memory and Semaphores are types of IPC. IPC stands for Inter Process Communication. There are couple more IPCs, namely Pipes & Message Queue. I am not going to go deep into the details of these IPCs because its not in the context here. Lets directly go to the commands under discussion:
ipcs: To display the shared memory and semaphores.
Example: ipcs command with -a option lists all the IPC facilities which can be read by the current process. It provides details about message queue, semaphore and shared memory.
# ipcs -a ------ Shared Memory Segments -------- key shmid owner perms bytes nattch status 0xc616cc44 1056800768 oracle 660 4096 0 0x0103f577 323158020 root 664 966 1 0x0000270f 325713925 root 666 1 2 ------ Semaphore Arrays -------- key semid owner perms nsems 0x0103eefd 0 root 664 1 0x0103eefe 32769 root 664 1 0x4b0d4514 1094844418 oracle 660 204 ------ Message Queues -------- key msqid owner perms used-bytes messages 0x000005a4 32768 root 644 0 0
Command to list all Message Queues, -q.
$ ipcs -q ------ Message Queues -------- key msqid owner perms used-bytes messages 0x000005a4 32768 root 644 0 0
List all the Semaphores, -s.
# ipcs -s ------ Semaphore Arrays -------- key semid owner perms nsems 0x0103eefd 0 root 664 1 0x0103eefe 32769 root 664 1 0x4b0d4514 1094844418 oracle 660 204
List all Shared Memories, -m.
# ipcs -m ------ Shared Memory Segments -------- key shmid owner perms bytes nattch status 0xc616cc44 1056800768 oracle 660 4096 0 0x0103f577 323158020 root 664 966 1 0x0000270f 325713925 root 666 1 2
ipcrm: To remove the shared memory and semaphores present in the memory.
The following options shall be supported:
-q msgid
Remove the message queue identifier msgid from the system and destroy the message queue and data structure associated with it.
-m shmid
Remove the shared memory identifier shmid from the system. The shared memory segment and data structure associated with it shall be destroyed after the last detach.
-s semid
Remove the semaphore identifier semid from the system and destroy the set of semaphores and data structure associated with it.
-Q msgkey
Remove the message queue identifier, created with key msgkey, from the system and destroy the message queue and data structure associated with it.
-M shmkey
Remove the shared memory identifier, created with key shmkey, from the system. The shared memory segment and data structure associated with it shall be destroyed after the last detach.
-S semkey
Remove the semaphore identifier, created with key semkey, from the system and destroy the set of semaphores and data structure associated with it.
Note: Please refer manual pages for more information on these commands.
Background: In any application using large database, there is a tendency to use shared memory concept to speed up the response time. In such applications, after updating the database, if the new change didn’t reflect in the list or GUI, try clearing the shared memory/semaphores by using the above mentioned commands.
I faced such an issue, that’s why I have posted this. 
