ps

# To list every process on the system:
ps aux

# To list a process tree
ps axjf

# To list every process owned by foouser:
ps -aufoouser

# To list every process with a user-defined format:
ps -eo pid,user,command

# List the processes being run by a particular set of usernames
ps -f -u username1, username2, .... ,usernameN 

# Display a list of processes with a particular parent ID (5589)
# Note that when a process is launched it may spawn several other sub processes which all share a common parent process ID
ps -f -ppid 5589

# List processes with given PIDs
ps -f -p 25001, 4567, 789

# Display all processes owned by the current user
ps -U $USER

# Sort processes based on CPU and memory usage (useful for finding memory leaks)
ps aux --sort pmem

# Show environment variable settings for each PID
COLUMNS=10240 ps axel