Extra output on input redirection

Those "strange characters" are escape sequences for coloring the output.

This will print that number in blue color:

echo -e '\033[01;34m1010729039145\033[0m'

See man console_codes for more details.

You can tell ls in which cases it shall output colors:

   --color[=WHEN]
          colorize the output; WHEN can be 'never', 'auto', or 'always' (the default); more info below

Looks like your ls is actually an alias for ls --color=always. (Type type ls to verify.)

Usually ls --color=auto will do the right thing: It will print colors on screen but no into files.

If you really want the color in your file, your have to decide, if you want to see the actual escape sequences or the interpreted colors.

For example the less command will default to printing the actual sequences, but you can tell it to show the colors instead with the -R option:

   -R or --RAW-CONTROL-CHARS
          Like -r, but only ANSI "color" escape sequences are output in "raw" form.

Try less textfile vs. less -R textfile.

The program ag (I'm not familiar with it), could use the system call

stdout_is_tty = isatty(1);

The isatty() function tests whether fd is an open file descriptor referring to a terminal.

This allows it to modify the output depending on where it is writing to.

I also think I found the relevant source code section

/* If we're not outputting to a terminal. change output to:
* turn off colors
* print filenames on every line
*/
if (!isatty(fileno(stdout))) {
  opts.color = 0;
  group = 0;
....

See also man isatty.