Routines and Expressions

Logical expression blocks

for name in [wordlist]
do
  command list
done

for (( expr1; expr2; ... ))
do
   command list
done

while [ logical expr ]
do
  command list
done

until [ logical expr ]
do
  command list
done

case "$variable" in
  value1)
    command list ;;
  value2)
    command list ;;
esac

if [ logical expr ]
then
  command list
elif [ logical expr ]
then
  command list
else
  command list
fi

Arithmetic Evaluation

Under certain circumstances, the shell evaluates arithmetic expressions in fixed-width integers with no check for overflow, though division by 0 is trapped and flagged as an error. The operators and their precedence, associativity, and values are the same as in the C language. Arithmetic operators and arithmetic assignment expressions are listed separately.

Shell variables are allowed operands in arithmetic expressions, and parameter expansion is performed before the expression is evaluated. Within an expression, shell variables may be referenced by name without using the parameter expansion syntax. A shell variable that is null or unset evaluates to 0 when referenced by name without using the parameter expansion syntax. The value of a variable is evaluated as an arithmetic expression when it is referenced, or when a variable which has been given the integer attribute using declare -i is assigned a value. A shell variable need not have its integer attribute turned on to be used in an expression.

Constants with a leading 0 are interpreted as octal numbers. A leading 0x or 0X denotes hexadecimal. Otherwise, numbers take the form [base#]n, where the optional base is a decimal number between 2 and 64 representing the arithmetic base, and n is a number in that base. If base# is omitted, then base 10 is used. When specifying n, the digits greater than 9 are represented by the lowercase letters, the uppercase letters, @, and _, in that order. If base is less than or equal to 36, lowercase and uppercase letters may be used interchangeably to represent numbers between 10 and 35.

Operators are evaluated in order of precedence. Sub-expressions in parentheses are evaluated first and may override the precedence rules.