>>>ps -ef | grep mysqld
mysql 3068 1413 0 May30 ? 02:26:15 /servers/mysql-5.7.22/bin/mysqld --defaults-file=/servers/my3306/my.cnf --basedir=/servers/mysql-5.7.22 --datadir=/servers/my3306/data --socket=/servers/my3306/run/mysqld.sock --port=3306
>>>gdb -p 3068 -ex "set max_connections=1200"
GNU gdb (GDB) Red Hat Enterprise Linux 7.6.1-110.el7
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-redhat-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Attaching to process 3068
Reading symbols from /servers/mysql-5.7.22/bin/mysqld...done.
Reading symbols from /lib64/libpthread.so.0...(no debugging symbols found)...done.
[New LWP 14461]
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[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
Loaded symbols for /lib64/libpthread.so.0
Reading symbols from /lib64/libaio.so.1...Reading symbols from /lib64/libaio.so.1...(no debugging symbols found)...done.
(no debugging symbols found)...done.
Loaded symbols for /lib64/libaio.so.1
Reading symbols from /lib64/libnuma.so.1...Reading symbols from /lib64/libnuma.so.1...(no debugging symbols found)...done.
(no debugging symbols found)...done.
Loaded symbols for /lib64/libnuma.so.1
Reading symbols from /lib64/libcrypt.so.1...(no debugging symbols found)...done.
Loaded symbols for /lib64/libcrypt.so.1
Reading symbols from /lib64/libdl.so.2...(no debugging symbols found)...done.
Loaded symbols for /lib64/libdl.so.2
Reading symbols from /lib64/librt.so.1...(no debugging symbols found)...done.
Loaded symbols for /lib64/librt.so.1
Reading symbols from /lib64/libstdc++.so.6...(no debugging symbols found)...done.
Loaded symbols for /lib64/libstdc++.so.6
Reading symbols from /lib64/libm.so.6...(no debugging symbols found)...done.
Loaded symbols for /lib64/libm.so.6
Reading symbols from /lib64/libgcc_s.so.1...(no debugging symbols found)...done.
Loaded symbols for /lib64/libgcc_s.so.1
Reading symbols from /lib64/libc.so.6...(no debugging symbols found)...done.
Loaded symbols for /lib64/libc.so.6
Reading symbols from /lib64/ld-linux-x86-64.so.2...(no debugging symbols found)...done.
Loaded symbols for /lib64/ld-linux-x86-64.so.2
Reading symbols from /lib64/libfreebl3.so...Reading symbols from /lib64/libfreebl3.so...(no debugging symbols found)...done.
(no debugging symbols found)...done.
Loaded symbols for /lib64/libfreebl3.so
Reading symbols from /lib64/libnss_files.so.2...(no debugging symbols found)...done.
Loaded symbols for /lib64/libnss_files.so.2
Reading symbols from /lib64/libnss_sss.so.2...Reading symbols from /lib64/libnss_sss.so.2...(no debugging symbols found)...done.
(no debugging symbols found)...done.
Loaded symbols for /lib64/libnss_sss.so.2
Reading symbols from /servers/mysql-5.7.22/lib/plugin/semisync_master.so...done.
Loaded symbols for /servers/mysql-5.7.22/lib/plugin/semisync_master.so
Reading symbols from /servers/mysql-5.7.22/lib/plugin/semisync_slave.so...done.
Loaded symbols for /servers/mysql-5.7.22/lib/plugin/semisync_slave.so
0x00007fdc8e18bf0d in poll () from /lib64/libc.so.6
Missing separate debuginfos, use: debuginfo-install glibc-2.17-222.el7.x86_64 libaio-0.3.109-13.el7.x86_64 libgcc-4.8.5-28.el7.x86_64 libstdc++-4.8.5-28.el7.x86_64 nss-softokn-freebl-3.28.3-6.el7.x86_64 numactl-libs-2.0.9-7.el7.x86_64 sssd-client-1.16.0-19.el7.x86_64
(gdb)
(gdb) q
A debugging session is active.
Inferior 1 [process 3068] will be detached.
Quit anyway? (y or n) y
Detaching from program: /servers/mysql-5.7.22/bin/mysqld, process 3068
在linux系统里，可以使用 gdb 工具（调试工具）去改变参数的值。
mysql> show variables like "%max_connections%";
+-----------------+-------+
| Variable_name | Value |
+-----------------+-------+
| max_connections | 1500 |
+-----------------+-------+
1 row in set (0.00 sec)
gdb -p 3068 -ex "set max_connections=1200"
mysql> show variables like "%max_connections%";
+-----------------+-------+
| Variable_name | Value |
+-----------------+-------+
| max_connections | 1200 |
+-----------------+-------+
1 row in set (0.00 sec)
3068 是mysql 进程的端口，“set max_connections=1200” 是设置mysql的变量值的。当然也可以更改其他的参数值或者执行其他的命令去影响当前服务的功能等。
gdb 是个作弊的好工具，在mac系统下也可以使用，大家自行查询安装方法。
大家切勿在正式环境乱测试，会搞坏服务的，切记。