Shell Scripting: Expert Recipes for Linux, Bash, and More

echo

Most people are reasonably familiar with the echo command. It can be used for more than just displaying sequences of text to the terminal. These dial1 and dial2 scripts show you how to keep an interactive user updated and assured that something is still happening during long or slow periods when the script may be doing something complicated but cannot know how long it may take.

dial1

This first implementation of a “dial” actually just displays the current time, once per second, but without filling the screen with sequences of timestamps. It does this by sending a Control-M (^M) character to the terminal before the date itself, and disabling echo’s default behavior of adding a \r\n sequence to the end of each line it displays, so that subsequent output will be at the start of a new line on the screen.

Control-M is the Carriage Return character (also commonly referred to as CR or \r), which is normally followed by the \n (New Line) character. Just sending Control-M by itself means that the cursor is brought back to the beginning of the line, then the date is displayed, and finally, the cursor stays where it is instead of going to the start of the next line, shifting up the current line if at the bottom of the terminal.

By default, the bash builtin echo treats a backslash as a regular character. The -e switch to echo tells it to interpret backslash sequences as marking special ...