scroll

Copy STDIN to STDOUT unchanged while showing the last N lines of the stream in a live, in-place display on STDERR. It is a pipeline filter — like an interactive tail, but the stream keeps flowing through to the next command.

long-running-build | scroll -20 | tee build.log

Two guarantees drive the design:

• The STDOUT copy is never slowed by the display. STDOUT is written on a tight path that hands buffers to a separate render fiber. When the terminal can't keep up, the display drops intermediate states; the STDOUT copy runs at full speed.
• The display never shows non-contiguous output. Each frame is a contiguous run of the most recent complete lines. When lines are skipped (because the display fell behind), the window resets to the newest contiguous segment rather than splicing a pre-gap line onto a post-gap one. Control and escape bytes are stripped from the display so a hostile stream cannot corrupt the terminal — the bytes on STDOUT are always untouched.

The display is only drawn when STDERR is a terminal; when STDERR is redirected, scroll is a plain cat (use --force to draw anyway).

Installation

shards build --release
# copies to ./bin/scroll

Usage

Usage: scroll [options]

Options:
    -n, --lines INT      Lines to show (default: 10)
        --interval MS    Minimum ms between redraws (default: 40)
        --force          Draw the display even when STDERR is not a TTY
        --no-sanitize    Do not strip control/escape bytes from the display
        --final          On EOF, also show a trailing line that has no newline
        --null           Consume STDIN without copying it to STDOUT
        --no-null        Force copying STDIN to STDOUT, even in modes that imply --null (e.g. --file)
        --version        Show version and exit
    -h, --help           Show this help and exit

A bare -N is shorthand for --lines N (e.g. -20 means --lines 20).

tail -f access.log | scroll | grep -v healthcheck > filtered.log

Development

shards build --no-debug --error-trace   # dev build
crystal spec --error-trace              # run tests
crystal tool format                     # format
ameba                                   # lint

Contributing

1. Fork it (https://github.com/plambert/scroll.cr/fork)
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create a new Pull Request

Contributors

• Paul M. Lambert - creator and maintainer