From 2f94d498ef6492d50fadc8919c97677a7a6e8a25 Mon Sep 17 00:00:00 2001
From: "B. Watson" <yalhcru@gmail.com>
Date: Wed, 22 Aug 2018 07:07:30 -0400
Subject: gammatrip.sh: a bit of error checking. Probably never touch this
 again.

---
 gammatrip.sh | 124 +++++++++++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 103 insertions(+), 21 deletions(-)

diff --git a/gammatrip.sh b/gammatrip.sh
index 2b81a09..ef17f54 100755
--- a/gammatrip.sh
+++ b/gammatrip.sh
@@ -1,61 +1,143 @@
 #!/bin/bash
 
-# gammatrip.sh by B. Watson, licensed under the WTFPL.
+# gammatrip.sh by B. Watson <yalhcru@gmail.com>.
+# Licensed under the WTFPL. See http://www.wtfpl.net/txt/copying/ for details.
+# Developed on Slackware 14.2, should work on most modern Linux distros.
+
 # Randomly change gamma in X, crude simulation of an acid trip.
-# It's also similar to the Atari 400/800/XL/XE "Atrract Mode"
+# It's also similar to the Atari 400/800/XL/XE "Attract Mode".
+# Runs until killed via ^C or closing its terminal window, at which time
+# it restores the original gamma setting, then exits.
+
+#### User-tweakable knobs. If you don't feel like editing the script,
+#    just put them on the command line, in front of the command:
+#    $ DELAY=0.5 MAX=150 MIN=0 gammatrip.sh
 
-#### user-tweakable knobs:
+# XRandR output to use. Leaving this blank means to use the
+# first output listed by the xrandr command. Usual values are
+# e.g. VGA-1 or DVI-1.
+OUTPUT=
 
-# seconds to sleep between gamma changes
+# Seconds to sleep between gamma changes. Decimal point allowed, so
+# long as your system's sleep command allows it.
 DELAY=${DELAY:-3}
 
-# max gamma for each individual color. lower number here means darker
+# Max gamma for each individual color. Lower number here means darker
 # colors on average. Units are 1/100ths of xrandr's --gamma floats,
 # MAX=100 means 1.0. Maximum value for MAX is 999.
 MAX=${MAX:-100}
 
-# minimum gamma for each color. don't set higher than MAX!
-# same units as MAX.
+# Minimum gamma for each color. Don't set higher than MAX!
+# Same units as MAX.
 MIN=${MIN:-30}
 
-#### end of user-tweakable knobs.
+# VERBOSE=y to see the commands as they're executed, anything else
+# means don't display them.
+VERBOSE=y
+
+#### End of user-tweakable knobs.
 
+printmsg() {
+	[ "$VERBOSE" = "y" ] && echo "$@"
+}
+
+errmsg() {
+	echo "$SELF:" "$@" 1>&2
+}
+
+# Check knobs for sane(ish) settings, set up signal handlers,
+# guess the active XRandR output if not provided. Get original
+# gamma setting, so we can restore it later.
 init() {
-	trap cleanup SIGINT SIGABRT SIGHUP SIGTERM
+	SELF="$( basename $0 )"
+	err=0
+
+	if [ "$MAX" -ge 1000 ]; then
+		errmsg "You can't set MAX >= 1000"
+		err=1
+	fi
+
+	if [ "$MIN" -ge "$MAX" ]; then
+		errmsg "You can't set MIN >= MAX"
+		err=1
+	fi
+
+	if [ "$#" != "0" ]; then
+		errmsg "No arguments accepted."
+		errmsg "Edit script or set MIN/MAX/OUTPUT/DELAY/VERBOSE in environment."
+		err=1
+	fi
+
+	if [ -z "$DISPLAY" ]; then
+		errmsg "DISPLAY not set (are you running X?)"
+		err=1
+	fi
+
+	[ "$err" -gt 0 ] && exit "$err"
+
+	# SIGINT is ^C, SIGHUP is what we get when our xterm is closed.
+	# SIGABRT is ^\, I hardly ever remember it exists. Trapping it here
+	# doesn't work: ^\ sends the abort signal to the active child
+	# process (usually sleep). Oh well. The other signals are 'just in case'.
+	trap cleanup SIGINT SIGABRT SIGHUP SIGTERM SIGHUP
+
 	OUTPUT="${OUTPUT:-$( xrandr | grep ' connected' | cut -d' ' -f1 )}"
-	echo "Using output '$OUTPUT'"
+
+	if [ -z "$OUTPUT" ]; then
+		errmsg "Invalid OUTPUT in environment, or XRandR not supported."
+		exit 1
+	fi
+
+	# Try to save the old gamma, for cleanup() to restore. Not thoroughly
+	# tested (works on my system).
+	OLDGAMMA="$( xrandr --verbose | sed -n '/^'$OUTPUT' /,/^[A-Z]/s/.*Gamma: *//p' )"
+
+	# If the above command fails:
+	OLDGAMMA="${OLDGAMMA:-1.0:1.0:1.0}"
+
+	printmsg "Using output '$OUTPUT', old gamma '$OLDGAMMA'. Have a nice trip!"
 }
 
+# Called when we get a signal we trapped in init().
 cleanup() {
-	echo "Restoring default gamma"
-	xrandr --output "$OUTPUT" --gamma "1.0:1.0:1.0"
+	echo
+	printmsg "Restoring old gamma"
+	xrandr --output "$OUTPUT" --gamma "$OLDGAMMA"
 	exit 0
 }
 
+# Takes 2 agruments: min and max. Returns a random number between
+# min and max (actually >= min and < max).
 getrand() {
-	range=$(( MAX - MIN ))
-	echo $(( ( RANDOM % range ) + MIN ))
+	min="$1"
+	max="$2"
+	range=$(( max - min ))
+	echo $(( ( RANDOM % range ) + min ))
 }
 
+# Pick random RBG values between MIN and MAX, use them to set the gamma.
 cycle() {
-	r=0; g=0; b=0
-	r="$( getrand )"
-	g="$( getrand )"
-	b="$( getrand )"
+	r="$( getrand $MIN $MAX )"
+	g="$( getrand $MIN $MAX )"
+	b="$( getrand $MIN $MAX )"
+
 	cmd="xrandr --output $OUTPUT --gamma "
+
 	for i in $r $g $b; do
 		cmd="$cmd$( printf "%03d" $i | sed 's,^.,&.,' ):"
 	done
 
-	# remove trailing colon. actually, xrandr doesn't care about it,
+	# Remove trailing colon. Actually, xrandr doesn't care about it,
 	# but someday it might.
 	cmd="$( echo $cmd | sed 's,:$,,' )"
 
-	echo $cmd
+	printmsg $cmd
 	$cmd
 }
 
-init
+### main()
+init "$@"
+
 while true; do
 	cycle
 	sleep "$DELAY"
-- 
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