[Unix] a potential usage of script(1) command - test script for progress bar

script(1) command

script(1) command on Unix or Linux aims to make typescript of terminal session (Reference: Unix Help or man 1 script ). Its implementation is based on pseudoterminal. Here’s a diagram that shows how script is implemented.

For example, $script -c "echo helloworld" will run echo helloword to print helloword on terminal (stdout). In addition, it will produce a typescript file that is a copy of everything printed on terminal of the command. In this case, typescript likes below

    Script started on Tue 19 May 2015 10:34:26 BST
    helloworld

    Script done on Tue 19 May 2015 10:34:26 BST

one use case

Scenario

If we have a command that will show a progress bar (like scp) and we need to write a test case for this command to test if the progress bar works as expected, we can not easily capture its output by redirecting its stdout to a file because usually this command will check if the file descriptor for output is a terminal or not by isatty function.

if(isatty(fd))
{
// show a progress bar
}

With redirection like $cmd > file, stdout points to a file. Therefore, isatty function will return 0. Finally progress bar is hidden. That’s not what we expect.

Solution

script -c "cmd" | sed "s/\\r/\\n/g" > prog.txt

With script, the output of cmd is still a terminal (stdout) and so we can capture its output. Besides, through sed, we replace carriage return (\r) by line feed (\n), and then every print by cmd can be captured.

Now prog.txt will record the output of progress bar in this cmd command. That’s what we expected.

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