Bash – Variables of integer type in Bash

arithmeticbashshellvariable

There are two ways to define and use variables of integer type in Bash

declare -i a new variable
use a variable in an arithmetic expression, without declaring it.

My questions:

What are the differences between the variables created by the two ways? Especially differences in their purposes, and when to use which?

Best Answer

The fact the variable is typed gives it some properties a generic variable won't have:

f() {
  v=0xff
  echo $v
  v=hello
  echo $v
  v=123a
  echo $v
}

f
declare -i v
f

will print

0xff
hello
123a

255
0
bash: 123a: value too great for base (error token is "123a")

If you are sure your variable will only contain integer values, typing it will give you some flexibility and error checking.

Related Solutions

Bash Variables – Difference Between Exported and Non-Exported Shell Variables

Exported variables are carried into the environment of commands executed by the shell that exported them, while non-exported variables are local to the current shell invocation. From the export man page:

The shell shall give the export attribute to the variables corresponding to the specified names, which shall cause them to be in the environment of subsequently executed commands.

set outputs the current environment, which includes any local non-exported variables. env is used to launch programs in a new environment, and with no arguments will output what that new environment would be. Since env is creating a new environment, only exported variables are brought through, as is the case for any program launched from that shell. For example, spawning a second shell within the first (I used $$ to represent prompts in the inner shell):

$ FOO=BAR
$ bash
$$ echo $FOO             # Note the empty line

$$ exit
$ export FOO
$ bash
$$ echo $FOO
BAR
$$

Note that it's the variable that's exported, not just its value. This means that once you export FOO, FOO becomes a global variable and shows up in subsequent environments, even if changed later:

$ export FOO
$ FOO=BAR
$ bash
$$ echo $FOO
BAR
$$

Bash: integer expression expected, using read/test

Actually you can set some attributes on variables using the declare (or the old typeset) builtin. declare -i var1 var2 will set integer attribute on those variables. After that assignments which attempt to set non-integer values to those variables will raise error.

But your problem is with the syntax. When using a variable's value you have to prefix its name with $:

if [ "$var1" -lt "$var2" ]; then
    echo "$var1 is lt $var2"
else
    echo "$var2 is lt $var1"
fi

Exceptions are the arithmetic evaluations, where is no need for the $:

if ((var1<var2)); then
    echo "$var1 is lt $var2"
else
    echo "$var2 is lt $var1"
fi

As a word of warning, inside [..] always double quote your variables to avoid word expansion messing up your expression's syntax. (I mean, you will have problems with unset variables, variables containing empty string and variables containing IFS characters.) Or you can use the newer and better [[..]] instead which handles such cases correctly:

if [[ $var1 -lt $var2 ]]; then
    echo "$var1 is lt $var2"
else
    echo "$var2 is lt $var1"
fi

Best Answer

Related Solutions

Bash Variables – Difference Between Exported and Non-Exported Shell Variables

Bash: integer expression expected, using read/test

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