Diff for "ArithmeticExpression"

Arithmetic in BASH is integer math only. You can't do floating point math in Bash; if you need that capability, see Bash FAQ #22.

The $[ ] syntax is deprecated. Please use $(( )) instead.

Arithmetic Expansion

POSIX sh (and all shells based on it, including Bash and ksh) uses the $(( )) syntax to do arithmetic, using the same syntax as C. (See the Bash hackers article for the full syntax.) Bash calls this an "Arithmetic Expansion", and it obeys the same basic rules as all other $... substitutions.

$(( )) is the first example of a math context, meaning a context where the syntax and semantics of C's integer arithmetic are used. This will be discussed in more detail below.

Here are a few examples using $(( )):

# POSIX sh
i=$((j + 3))
lvcreate -L "$((24 * 1024))" -n lv99 vg99
q=$((29 / 6)) r=$((29 % 6))
if test "$((a%4))" = 0; then ...

Notes:

• Arithmetic operators include all of the C operators (arithmetic, bit shifting/masking, ternary ?:), plus ** for exponentiation.

• You may use commas to separate multiple expressions within a single math context. The final value of the arithmetic expression is that of the last comma-delimited expression.

• All arithmetic is done with 64-bit signed integers (as of Bash 2.05b -- may be different in other shells).
• Variable names in a math context are substituted with their values, as long as those values are valid integers. In Bash, this substitution continues recursively until an integer or an invalid variable name is generated. Unset or empty variables are evaluated as 0.

• Numbers without leading 0 are treated as base 10. Numbers with a leading 0x are treated as base 16. Numbers with a leading 0 (not followed by x) are treated as base 8. You may put 10# or 16# (etc.) in front of a number to force it to be interpreted in a given base -- more on this later.

Arithmetic Commands

Bash also offers two forms of commands that use a math context. The expressions within the math context follow the same rules as the expressions inside $(( )), but since we have a command, we also get an exit status, and side effects.

The exit status is based on the value of the (last) expression. If the expression evaluates to 0, the command is considered "false" and returns 1. Otherwise, the command is considered "true" and returns 0. See below for examples of this.

The first arithmetic command is let:

let a=17+23
echo "a = $a"      # Prints a = 40

Note that each arithmetic expression has to be passed as a single argument to the let command, so you need quotes if there are spaces or globbing characters. Thus:

let a=17 + 23      # WRONG
let a="17 + 23"    # Right
let 'a = 17 + 23'  # Right
let a=17 a+=23     # Right (2 arithmetic expressions)

let a[1]=1+1       # Wrong (try after touch a1=1+1 or with shopt -s failglob)
let 'a[1]=1+1'     # Right
let a\[1\]=1+1     # Right, but very odd.

The second command is (( )). It works identically to let, but you don't need to quote the expressions because they are delimited by the (( and )) syntax. For example:

((a=$a+7))         # Add 7 to a
((a = a + 7))      # Add 7 to a.  Identical to the previous command.
((a += 7))         # Add 7 to a.  Identical to the previous command.

((a = RANDOM % 10 + 1))     # Choose a random number from 1 to 10.
                            # % is modulus, as in C.

> or < inside (( )) means greater/less than, not output/input redirection. An expression containing one of these evalutes to either 1 (if the comparison is true) or 0 (if false).

if ((a > 5)); then echo "a is more than 5"; fi

(( )) is used more widely than let, because it fits so well into an if or while command.

Finally, a note on the exit status of commands, and the notions of "true" and "false", is in order. When Bash runs a command, that command will return an exit status from 0 to 255. 0 is considered "success" (which is "true" when used in the context of an if or while command). However, when evaluating an arithmetic expression, C language rules (0 is false, anything else is true) apply.

Some examples:

true; echo "$?"       # Writes 0, because a successful command returns 0.
((10 > 6)); echo "$?" # Also 0.  An arithmetic command returns 0 for true.
echo "$((10 > 6))"    # Writes 1.  An arithmetic expression evaluates to 1 for true.

In addition to a comparison returning 1 for true, an arithmetic command that evaluates to any non-zero value returns true as an exit status.

if ((1)); then echo true; fi     # Writes true.

This also lets you use "flag" variables, just like in a C program:

found=0
while ...; do
  ...
  if something; then found=1; fi    # Found one!  Keep going.
  ...
done
if ((found)); then ...

This also means that every arithmetic command we run is returning an exit status. Most scripts ignore these, but if you are using set -e you may be unpleasantly surprised when your program aborts because you assigned 0 to a number.

Math Contexts

As we've seen, the insides of $(( )) and (( )), and the arguments of let, are math contexts.

Also, (non-associative) array indices are a math context:

n=0
while read line; do
   array[n++]=$line      # array[] forces a numeric context
done

In case it wasn't obvious, the (( )) in a C-style for command are a math context. Or three separate math contexts, depending on your point of view.

for ((i=0, j=0; i<100; i++, j+=5)); do ...

Finally, the start and length arguments of Bash's ${var:start:length} parameter expansion are math contexts.

echo "${string:i+2:len-2}"

Leading Zeros and Base Selection

There is one common pitfall with arithmetic expressions in Bash: numbers with leading zeroes are treated as octal. For example,

# Suppose today is September 19th.
month=$(date +%m)
next_month=$(( (month == 12) ? 1 : month+1 ))
# bash: 09: value too great for base (error token is "09")

This causes great confusion among people who are extracting zero-padded numbers from various sources (dates and times are by far the most common) and then doing math on them without sanitizing them first. It's especially bad if you write a program like this in March, test it, roll it out... and then it doesn't blow up until August 1.

There are two possible solutions. The first is, obviously, to remove the leading zeroes from the numbers before doing math with them. This is not trivial in Bash, unfortunately, because Bash has no ability to perform substitutions on a variable using regular expressions (it can only do it with "glob" patterns). But you could use a loop:

# This removes leading zeroes from a, one at a time.
while [[ $a = 0* ]]; do a=${a#0}; done

You can do the above without using a loop, by using extended globs; see FAQ #67 for more information. Or, you could use sed; that may be more efficient if you're reading many numbers from a stream, and can arrange to sanitize them all in one command, rather than one by one.

With an extended glob:

shopt -s extglob

# This removes leading zeroes from a, all at once.
a=${a##+(0)}

The second solution is to force Bash to treat all numbers as base 10 by prefixing them with 10#. This might be more efficient, but also may be less elegant to read.

a=008
let b=a+1       # Generates an error because 008 is not valid in octal.
let b=10#$a+1   # Force a to be treated as base 10.  Note: the $ is required.

Here is a function to convert numbers in other bases to decimal (base 10):

frombase() {
    echo "$(( $1#$2 ))"
}

# Examples:

frombase 16 ffe    # prints 4094
frombase 2 100100  # prints 36

Integer Declaration

Integer delcaration is considered harmful. Don't do this. It makes your code difficult to read, because you need to know whether a given variable is declared as an integer or not, to know what a command does.

Variables may be declared as integers so that any subsequent assignments to them will always assume a numeric context. Essentially any variable that's declared as an integer acts as if you had a let command in front of it when you assign to it. For example:

unset b             # Forget any previous declarations
b=7+5; echo "$b"    # Prints 7+5
declare -i b        # Declare b as an integer
b=7+5; echo "$b"    # Prints 12

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