How do I compare numbers in bash?

Question

I have got this script, but it is not working. It is because it is failing to evaluate the number comparison in the if statement, I think.

#!/bin/bash

{ read __ WIDTH; read __ HEIGHT; read __ __ BORDER_WIDTH; } < <(xwininfo -id "$(xdotool getactivewindow)" | grep -o -e 'Height:.*' -e 'Width:.*' -e 'Border width:.*')
echo "Height: $HEIGHT, Width: $WIDTH, Border width: $BORDER_WIDTH"

x = 1920
if($WIDTH == x)
then
    wmctrl -r :ACTIVE: -b toggle,maximized_vert,maximized_horz 
else 
    xdotool key Ctrl+F12
fi

How can I fix this?

Answer 1

There are several issues with the script:

• bash tests are either done with test, [ .. ] or [[ .. ]]; ( .. ) means sub-shell

• Assignment are made without spaces, x = 1920 will call the command x with the parameters = and 1920. Use x=1920 instead.

• Variable names need to be prefixed with a dollar sign when you use them. So == x is bad and == $x is good. (Except within arithmetic evaluations or expansions: (( ... )) or $(( ... )), thanks to comment by Kusalananda).

• Numbers should be compared with -eq, = is for string comparison. In your case it should also work since the numbers are likely to be stored identically, but it's better to use the conceptually correct operator. == is a non-standard equivalent to =.

• You should get used to double quoting variables everywhere when possible, which prevents globbing for instance.

I'll just fix the lines starting from x = 1920, the fixed version is:

x=1920
if [ "$WIDTH" -eq "$x" ]
then
    wmctrl -r :ACTIVE: -b toggle,maximized_vert,maximized_horz 
else 
    xdotool key Ctrl+F12
fi

Answer 2

if(a == b) is not the proper syntax for an integer comparison Additionally, you invoke x without indicating it to be a variable, so you are (trying to) compare the value of $WIDTH with the literal string x. Try instead:

if [[ $WIDTH -eq $x ]]

Answer 3

As other answers noted, (...) indicates subshell. Arithmetic expansion is ((...)), so in case of bash you should be using ((...)) or [[, or POSIXly [. The (( can be used as so:

$ if ((1==1)); then echo "YES" ;fi
YES

Note, however, that in case of [[, the == operator indicates pattern matching, i.e. it is not arithmetic comparison.

Alternatively, if you really want to use == operator with POSIX shell, one could do the following:

$ foo=1
$ bar=2
$ if [ $((foo==bar)) -eq 1 ]; then echo "YES"; else echo "NO"; fi
NO
$ bar=1
$ if [ $((foo==bar)) -eq 1 ]; then echo "YES"; else echo "NO"; fi
YES

In case of POSIX shell /bin/sh there's no (( operator, however there is $((, which expands to the result of an arithmetic expression. Comparison operators within it act in a C-like manner, where 1 indicates a true result, and 0 indicates a false result (as opposed to usual shell way, where 0 is success and non-zero is failure).

Answer 4

Try this:

# use xrandr+sed to get current screen width
screenWidth=`xrandr|sed -En '/connected [0-9]+x[0-9]+/{s/^.*connected ([0-9]+)x[0-9]+.*$/\1/;p}'`

# use xdotool getwindowgeometry --shell to get size/pos of window
eval `xdotool getactivewindow getwindowgeometry --shell`

echo $screenWidth $WIDTH # debug, to see values got from xrandr and xdotool

if [[ $WIDTH = $screenWidth ]]; then
  wmctrl -r :ACTIVE: -b toggle,maximized_vert,maximized_horz 
else 
  xdotool key Ctrl+F12
fi

Explanation:

1. In bash, do not put space(s) arround = when assigning values; example x=123, not x = 123.
2. To compare things in bash, use [[ .. ]].
3. xdotool already has getwindowgeometry --shell for bash programming.