I'm trying to write a script that get the actual brightness and decrease the string number by 0.1. I've tried numerous syntax but since I'm very new to bash this doesn't work.
#!/bin/sh
actualBrightness=$(xrandr --verbose | grep -i brightness | cut -f2 -d ' ')
echo ${actualBrightness} # 0.5 for ex
if [[ $actualBrightness < 1 ]]
then
newBrightness=$(($actualBrightness-0.1))
echo $newBrightness # must be 0.4
fi
Suppose you have:
$ echo "$b"
0.5
The 0.5 only has meaning to Bash as a string. You need to use bc or awk to interpret that as a floating point value change or change its value:
$ echo "$b-.1" | bc
.4
Bash does not do floating point math -- only integer math and string manipulation.
So to change the value of b you would do:
$ b=$(echo "$b-.1" | bc)
$ echo "$b"
.4
You can also use bc for testing:
$ echo "0.55 > 7" | bc
0
$ echo "55.2 > 7" | bc
1
1 is 'true' and then use that in your Bash [[ test ]] like so:
[[ $(echo "5 > 1" | bc) -eq 1 ]] # sets $? to 0 for true or 1 for false
# BACKWARDS from bc's exit code
Or, single statement:
$ [[ $(echo "5 > 1" | bc) -eq "1" ]] && echo "true" || echo "false"
true
$ [[ $(echo ".5 > 1" | bc) -eq "1" ]] && echo "true" || echo "false"
false
bash by having #!/bin/bash as the shebang -- NOT #!/bin/sh or the [[ test ]] won't work.You can use awk to facilitate the floating-point arithmetic:
actualBrightness='0.5'
awk '{if ($1 < 1) { new=($1 - 0.1); printf new; }}' <<<"$actualBrightness"
# output: 0.4
Awk operates on fields separated by a field-separator, which defaults to whitespace; the above statement checks to see:
new to ($1 - 0.1) and print new to STDOUT
#!/bin/shand[[ ]], but[[ ]]is a bash feature.[[ $actualBrightness < 1 ]]is doing a string comparison. Even for integers, you want to use actual integer math -- as a string, for instance,20is greater than100, since comparison is character-by-character left-to-right.