Difference:
1, kill -9 id: generally, kill without parameters is to kill with 15, which is equivalent to stopping the process normally, which will release the resources occupied by the process; The difference between them is just like soft shutdown (choose "shutdown" through the "Start" menu) and hard shutdown (directly cut off the power supply) in computer shutdown. Although both of them can be shut down, the procedures are different.
2, kill-9 means forcibly killing the process; Kill, on the other hand, has limitations, such as background process, daemon process, etc.
3. execute the kill command, and the system will send a SIGTERM signal to the corresponding program. SIGTERM will probably be blocked. Kill -9 command, the signal sent by the system to the corresponding program is SIGKILL, that is, exit. The exit signal will not be blocked by the system, so kill -9 can kill the process smoothly.
Extended information:
UNIX operating system (Eunice) is a powerful multi-user and multi-task operating system, which supports a variety of processor architectures. According to the classification of operating systems, it belongs to time-sharing operating system. It was first developed by KenThompson, Dennis Ritchie and Douglas McIlroy in 1969. T's bell lab development.
At present, its trademark right is owned by the International Open Standards Organization. Only UNIX systems that conform to a single UNIX specification can use the name UNIX, otherwise it can only be called UNIX(UNIX-like.
UNIX users' association first started standardization work in 198s, and issued trial standards in 1984. Later, IEEE formulated POSIX standard (IEEE13 standard) for this purpose. The international standard name is ISO/IEC9945. It defines the language interface compatible between UNIX operating system and application programs through a set of minimal functions.
POSIX is an easy-to-remember name proposed by Richard Stallman at the request of IEEE, which means portable operating system interface, and X indicates the inheritance of its API.
Resources: unix_ Baidu Encyclopedia.