How to use a variable in if-else in bash?

Question

I installed git bash on windows and try to write a few simple script. One thing I could not figure out so far is simple math and comparison. What I need is a random boolean currently:

#!/usr/bin/env bash

condition=$(($(($RANDOM%2)) == 1))

if $condition; then
    echo "a"
else
    echo "b"
fi

I got command not found. It looks like git bash is missing comparison operators, <,>,==,!=,-eq,-lt,-gt does not work no matter what I try. I found those in all of the examples. Any idea?

The operators aren't missing. The problem is just that the content of your condition variable is not a command, and if takes a command as the thing to check. (anubhava's answer enters an arithmetic context, in which you can use an arithmetic operation as a command; you could also do something like, say, if [ "$condition" -eq 1 ]; then, as [ is also a command). — Charles Duffy
– Charles Duffy, Commented Apr 1, 2020 at 22:31
There is no such thing as "git bash". Its name is Git for Windows and there is nothing special about it; it is a package that includes Git, Bash and a bunch of other standard Unix/Linux tools ported to Windows (on top of MinGW). — axiac
– axiac, Commented Apr 1, 2020 at 22:40

anubhava · Accepted Answer · 2020-04-01 22:32:58Z

2

You may use it like this:

#!/usr/bin/env bash

condition=$(((RANDOM % 2) == 1))

if ((condition)); then
    echo "a"
else
    echo "b"
fi

Note that you may also shorten variable assignment to this:

condition=$((RANDOM % 2))

edited Apr 1, 2020 at 22:32

answered Apr 1, 2020 at 22:30

anubhava

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4 Comments

inf3rno Over a year ago

fantastic, thanks. this happens when you are too lazy to read the manual and try to learn it from tutorials :D

inf3rno Over a year ago

y, I figured meanwhile I can shorten it, thanks. I was just not sure how automatic the type conversion is

Charles Duffy Over a year ago

There is no type conversion. You may do math, but the result is stored as a string; refer to it in an arithmetic context again, and it converts back from string to integer again each time. (There are exceptions to that; one can use declare -i to make a variable's native form arithmetic, but that's more a side note than anything)

Gordon Davisson Over a year ago

@inf3rno Shell syntax and semantics are a lot more context-dependent than in most languages, so this may be throwing you. For example, depending on context a = b might mean "run the command 'a' with arguments '=' and 'b'; or it might mean "compare 'a' and 'b' as strings"; or it might mean "figure out what number b represents (probably by treating it as a variable name), and set the variable a to it". Another thing that throws people is that in shell syntax matters; adding or removing them can change the meaning of something completely.

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