在Bash中使用tput el1时为何需要添加回车符(\r)？

Let's break down exactly what's happening here, and why that \r is crucial:

First, what does tput el1 actually do? #

el1 (short for "clear to beginning of line") sends a terminal control sequence that deletes all characters from the cursor's current position back to the start of the line. The key detail here is: it does NOT move the cursor. After running tput el1, your cursor stays right where it was before the command ran.

Why your modified script had the "tab-like" overlay issue #

Let's walk through the flow of your problematic script step by step:

1. First iteration: printf "Reload | 15" outputs the text, and your cursor ends up at the end of that line (say, position 12 in the row).
2. After sleeping 1 second, the second iteration starts with tput el1: this clears all characters from the cursor's current position (end of the line) back to the start, so the line is emptied. But the cursor is still stuck at the end of the line.
3. Next, printf "Reload | 14" outputs the new text starting from the cursor's current position (the end of the now-empty line). This makes it look like the new text is being appended to the old one (even though the old text was cleared—your terminal just renders the new output right after where the cursor was).

What \r fixes #

The carriage return character (\r, ASCII 0x0D) tells the terminal to move the cursor back to the start of the current line, without advancing to a new line.

With \r added to your printf statement, the flow changes:

1. After running tput el1 (which clears from the cursor's position to the line start), printf "Reload | $n\r" outputs the countdown text, then immediately yanks the cursor back to the first column of the line.
2. When the next loop iteration runs, tput el1 now clears from the start of the line (where the cursor is) back to the start (so no change yet), but then the next printf writes over the existing text from the very beginning of the line—exactly what you want for a countdown that updates in place.

Official context (per POSIX standards) #

• The el1 control sequence is defined in the POSIX terminal interface as CSI 1 K (Control Sequence Introducer followed by 1 and K), which explicitly states it clears characters from the cursor to the line start, with no cursor movement.
• The carriage return (\r) is a standard ASCII control character whose terminal behavior is specified to move the cursor to the first column of the current line.

内容的提问来源于stack exchange，提问作者anon