How did the Linux operating system display and update progress bars?-CodePudding

For instance, when you pull images from Docker Hub, progress bars (downloading, extracting) in terminal show the progress and are refreshed in accordance with the progress:

9b5a7fa51869: Downloading [==============>                                    ]  8.485MB/29.15MB
9b5a7fa51869: Pull complete
9a72a43d6e84: Extracting [=>                                                 ]  2.785MB/89.96MB

I think if you want to print something to the command line, you put content to some channel or stdout, and it will append to the stream, not update.

I looked up on Internet but I was unable to get any helpful information.

Can you explain how it worked?

CodePudding user response：

If you look at the so-called escape sequences, it is possible to send character sequences which will be interpreted by the terminal as explained

The below script increases the length of an arrow every seconds:

#!/bin/sh

[ $# -ne 1 ] && echo "Usage: `basename $0` arrow_length" >&2 && exit 1

length=$1

while [ $length -gt 0 ]
do
  # Move the cursor one step backward to overwrite the preceding arrow's end
  echo -ne "\033[1D=>"
  sleep 1
  length=`expr $length - 1`
done

echo

Example of execution: