print to the end of terminal

问题

Like this picture of apt-get, how can I print something at the end of the line, just like the speed on the screen. Although I can use escape like \e[C to make it move several columns to the right and print, but the speed varies and the string length varies. Can anyone tell me how to do this? I am currently programming using bash. Thanks.

回答1:

Often you will find the width of the console window in the environment variable COLUMNS, but this is not completely reliable. A more reliable solution is to use the tput utility (part of ncurses, but can be used standalone) with the argument cols:

$ echo $COLUMNS
148
$ tput cols
148

A simple way to split text is to use the Posix-standard printf utility to first print the text to be aligned on the right margin padded to the column width, then output a carriage return (\r) to return the cursor to the beginning of the line, and then print the text to be aligned on the left margin. Finally a newline will move to the next line. Or, in code:

# Call it like this:
# print_both_sides "This goes on the left" "and this goes on the right."
print_both_sides() {
  printf "\r%*s\r%s\n" $(tput cols) "$2" "$1";
}

The printf format uses the standard feature of * replacements for field width and precision format modifiers; the * means that the corresponding number should be taken from an argument. In this case, I use $(tput cols) -- i.e. the width of the console window -- as the first argument for the format code %*s, which has the effect of right-padding the next string argument ("$2") in a field whose width is precisely the width of the console.

I added an additional carriage return to the beginning of the output in case the cursor is currently not at the left margin.

回答2:

In a terminal, there is more than one way to obtain the actual screen-width:

if the terminal/connection can successfully negotiate the screen size, that (usually) results in a correct value shown in stty -a (the columns value usually on the first line of the report).
the command-line tput utility consults this information, overriding any constant value for the number of columns from the terminal database, and can print it, e.g, tput cols.
the environment variables LINES and COLUMNS usually override that in full-screen applications (including those using curses: see use_env).
sometimes the information is just wrong (see the xterm FAQ Why is my screensize not set?). For terminals supporting the VT100-style cursor-position report (also in ECMA-48, 8.3.14 CPR - ACTIVE POSITION REPORT), you can use resize to update the stty setting and optionally update LINES and COLUMNS.

When printing using escape sequences, there are a few different ways to use this information about the screen-width, which are widely supported. The command-line tput utility also can retrieve these from the terminal description (which may lack specific features):

absolute cursor positioning, allows you to move the cursor to a specific row and column on the screen: tput cup $row $column
horizontal cursor positioning, allows you to move the cursor to a specific column on the current row: tput hpa $column
relative cursor positioning, allows you to move the cursor left or right (or up or down), a given number of cells, e.g., tput cuf $value to move forward (to the right) by the given number of cells. Regarding the question, using a parameter like this is usually faster than repeating single movements as done with tputcuf1.

The capabilities cup, hpa, cuf correspond generally to names in ECMA-48 and are described in the terminfo(5) manual page.

To print text at the right-side of the screen, you would do this:

obtain the number of columns in the screen,
determine the number of columns which your text will use,
compute the starting column: right-margin minus the number of columns in the text
move the cursor to the starting column
print the text, e.g., using echo or printf. The latter has more flexibility.

If you use the command-line printf utility, you may keep in mind that handling multibyte characters such as UTF-8 is not well-supported when you want to know the width of a given string, so portability may be a concern.