BASH: Basic if then and variable assignment

Question

I'm used to csh, so this is kinda irritating having to use bash. What is wrong with this code?

if[$time > 0300] && [$time < 0900]
then
$mod=2
else
$mod=0
fi

That's not valid csh, either.

chepner
– chepner

2013-09-17 18:11:58 +00:00
Commented Sep 17, 2013 at 18:11 — chepner
– chepner, Commented Sep 17, 2013 at 18:11

konsolebox · Accepted Answer · 2013-09-17 17:44:18Z

39

By standard it should be

if [ "$time" -gt 300 ] && [ "$time" -lt 900 ]
then
   mod=2
else
   mod=0
fi

In normal shell scripts you use [ and ] to test values. There are no arithmetic-like comparison operators like > and < in [ ], only -lt, -le, -gt, -ge, -eq and -ne.

When you're in bash, [[ ]] is preferred since variables are not subject to splitting and pathname expansion. You also don't need to expand your variables with $ for arithmetic comparisons.

if [[ time -gt 300 && time -lt 900 ]]
then
   mod=2
else
   mod=0
fi

Also, using (( )) for arithmetic comparisons could be best for your preference:

if (( time > 300 && time < 900 ))
then
   mod=2
else
   mod=0
fi

answered Sep 17, 2013 at 17:44

konsolebox

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5 Comments

Corepuncher Over a year ago

Thanks. Crazy sensitive this bash is. I typed what you had, and got erors like ./read.sh: line 14: =0: command not found But when I copy and pasted your code directly, it worked. Do you have to always indent 3 spaces before "mod=2" ?

konsolebox Over a year ago

@Corepuncher It's not necessary. Indent style is anyone's option in shell scripting. I think you tried to add $ to mod when assigning a value to it? $mod=0. In bash it would be interpreted as an expansion of $mod which is an empty string plus =0.

chepner Over a year ago

Bash needs to be sensitive about whitespace because something like FOO= bar is interpreted as running the command bar in an environment where FOO is set to the empty string.

Mehdi Over a year ago

will mod exist outside the scope of the if , if it hasn't been declared before?

konsolebox Over a year ago

@Mehdi Yes it will.

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