How to add a progress bar to a shell script?

Question

When scripting in bash or any other shell in *NIX, while running a command that will take more than a few seconds, a progress bar is needed.

For example, copying a b

Answer 1

Most unix commands will not give you the sort of direct feedback from which you can do this. Some will give you output on stdout or stderr that you can use.

For something like tar you could use the -v switch and pipe the output to a program that updates a small animation for each line it reads. As tar writes out a list of files it's unravelled the program can update the animation. To do a percent complete you would have to know the number of files and count the lines.

cp doesn't give this sort of output as far as I know. To monitor the progress of cp you would have to monitor the source and destination files and watch the size of the destination. You could write a small c program using the stat (2) system call to get the file size. This would read the size of the source then poll the destination file and update a % complete bar based on the size of the file written to date.