Linux – Redirecting output twice

Use tee:

command | tee -a mylog.txt

will append the output of the command to the file and also show it on screen.

Edit: I cannot be certain this is how mysql does it, but it could be using isatty(3) to determine whether STDOUT is a terminal or not, and modifying the output accordingly.

Edit 2: The mysql command line tool definitely uses isatty(). You can read the source code.

There are some good examples of this (although not in C) over at Stack Overflow:

• PHP: How can I detect STDOUT redirection in PHP CLI?
• Python: How do I detect whether sys.stdout is attached to terminal or not?

To answer your question, "Why?": Because that is what the documentation says. See the reference manual:

When used interactively, query results are presented in an ASCII-table format. When used noninteractively (for example, as a filter), the result is presented in tab-separated format.

I suspect this decision was made for readability. When used interactively, mysql can assume a human is reading the output, and it is generally easier for us humans to read data that is delimited by those lines. When used non-interactively, the assumption is that another program is going to consume the output, and that tab-delimited output is easier to programmatically consume.

If you are interested in overriding this default and getting ASCII-table format when redirecting the output, you can use the --table (-t) command line option:

mysql -t -e 'select a,b from tablefoo' databasename > file