Understanding Exec

At times, you have noticed that there is a usage of exec in shell scripts. What it actually does, and where to use it exactly?

The command works on the theory of, is "never coming back"

exec command is shell builtin, and other members of the commands are execve, execvp, execv.

1. exec command starts the new process, without forking, so once it starts the new process, the old process get finished.

Case 1:
bash:~$ ps -ef | grep -i bash
csblog  13062 12371  1 15:19 pts/5    00:00:00 bash
ksh$ ksh
ksh$ ps -ef | grep -i ksh
csblog  13093 13062  0 15:19 pts/5    00:00:00 ksh
ksh$ ps -ef | grep -i bash
csblog  13062 12371  0 15:19 pts/5    00:00:00 bash

Case 2:
bash:~$ exec ksh
ksh$ ps -ef | grep -i bash
ksh$ ps -ef | grep -i ksh
csblog  13062 12371  0 15:19 pts/5    00:00:00 ksh

You can see, in case 1, i have started the process, without using exec, so the new shell, is subshell for the old one. But in case of case2, a totally new process is started. In this manner, your resources are saved.

2. Sometimes, you don't need user to access the shell, you can directly change in /etc/passwd, but you need to load the environment, so you can add the exec command, at the end of .profile.

3. exec, also used with file descriptor. To open, close, read and write.

exec 3< thisfile          # open "thisfile" for reading on file descriptor 3
exec 4> thatfile          # open "thatfile" for writing on file descriptor 4
exec 8<> tother           # open "tother" for reading and writing on fd 8
exec 6>> other            # open "other" for appending on file descriptor 6
exec 5<&0                 # copy read file descriptor 0 onto file descriptor 5
exec 7>&4                 # copy write file descriptor 4 onto 7
exec 3<&-                 # close the read file descriptor 3
exec 6>&-                 # close the write file descriptor 6

read <&3
echo stuff >&4


Source: Stackoverflow.com

How to log your mysql queries in Ubuntu 16.04

At times, you need to log your mysql queries.

The first you need to locate your configuration file of mysql. You can use either locate command or the find command for it. The name of the configuration file for mysql is "my.cnf", and in ubuntu its usually present in your /etc directory.

$ find /etc -name "my.cnf"

or

$ locate my.cnf

In my system, it's located in /etc/mysql/my.cnf

Once the file is located, open it using your favorite text editor.
There are chances that the file is not writable by your user, so you need to use sudo to switch your user to root to edit the file.

Open the file and add the lines,

[mysqld]
general_log = on
general_log_file = /var/log/mysql/mysql_query.log

It will add the group as mysqld, and start the logging. The logs will be save in file:

/var/log/mysql/mysql_query.log

Save it and close it.

Then you need to restart your mysql server, to enable the option. Once the mysql service is started, the logging will start.

$ sudo service mysqld restart

Sample log file:

$ cat /var/log/mysql/mysql_query.log
/usr/sbin/mysqld, Version: 5.7.15-0ubuntu0.16.04.1-log ((Ubuntu)). started with:
Tcp port: 3306  Unix socket: /var/run/mysqld/mysqld.sock
Time                                                  Id Command    Argument
2016-10-13T14:02:57.300324Z    3 Connect myuser@localhost on  using Socket
2016-10-13T14:02:57.300563Z    3 Query select @@version_comment limit 1
2016-10-13T14:03:02.895987Z    3 Query SELECT DATABASE()
2016-10-13T14:03:02.896302Z    3 Init DB test
2016-10-13T14:03:02.898415Z    3 Query show databases
2016-10-13T14:03:02.902113Z    3 Query show tables