pigeon

#!/bin/sh
#
# Usage: pigeon COMMAND [ARGUMENTS...]
# Usage: pigeon SHELL_EVAL_EXPRESSION
#
# Silences the given command (both stdout and stderr) if its exit status
# is different this time (compared to the last time it was run) and then
# propagates that status back to the caller.  Memory of previous statuses
# is kept in files in the $PIGEON_STATUS_DIR directory, which defaults to
# $XDG_DATA_HOME/pigeon/status or ~/.local/share/pigeon/status otherwise.
#
# Like a carrier pigeon, this program exists to convey changes in status.
# It also prints the given command and its changed exit status to stderr.
#
# Written in 2018 by Suraj N. Kurapati <https://github.com/sunaku>

: ${PIGEON_STATUS_DIR:=${XDG_DATA_HOME:-$HOME/.local/share}/pigeon/status}
mkdir -p "$PIGEON_STATUS_DIR" || exit $?
storage="$PIGEON_STATUS_DIR/$(echo "$*" | sha1sum | cut -f 1 -d ' ')"
previous=$(test -s "$storage" && cat "$storage" || echo undefined)

output=$(
  # capture stderr alongside stdout
  exec 2>&1

  # SHELL_EVAL_EXPRESSION was given
  test $# -eq 1 && set -- eval "$@"

  # execute the given command line
  "$@"
)
status=$?

if [ $status != $previous ]; then
  echo "pigeon: command '$*' exit status changed from $previous to $status" >&2
  echo $status > "$storage" # store exit status for next time
  echo "$output" # propagate output of command back to caller
fi

exit $status