XXX 9.2. Typing variables: declare or typeset¶

The declare or typeset builtins , which are exact synonyms, permit modifying the properties of variables. This is a very weak form of the typing ` [1]

<declareref.html#FTN.AEN5685>`__ available in certain programming

languages. The declare command is specific to version 2 or later of Bash. The typeset command also works in ksh scripts.

** declare/typeset options**

-r `` readonly ``

( `` declare -r var1 `` works the same as `` readonly var1 `` )

This is the rough equivalent of the C const type qualifier. An attempt to change the value of a readonly variable fails with an error message.

declare -r var1=1
echo "var1 = $var1"   # var1 = 1

(( var1++ ))          # x.sh: line 4: var1: readonly variable

-i `` integer ``

declare -i number
# The script will treat subsequent occurrences of "number" as an integer.

number=3
echo "Number = $number"     # Number = 3

number=three
echo "Number = $number"     # Number = 0
# Tries to evaluate the string "three" as an integer.

Certain arithmetic operations are permitted for declared integer variables without the need for expr or let .

n=6/3
echo "n = $n"       # n = 6/3

declare -i n
n=6/3
echo "n = $n"       # n = 2

-a `` array ``

declare -a indices

The variable `` indices `` will be treated as an array .

-f `` function(s) ``

declare -f

A `` declare -f `` line with no arguments in a script causes a listing of all the functions previously defined in that script.

declare -f function_name

A `` declare -f function_name `` in a script lists just the function named.

-x export

declare -x var3

This declares a variable as available for exporting outside the environment of the script itself.

-x var=$value

declare -x var3=373

The declare command permits assigning a value to a variable in the same statement as setting its properties.

#!/bin/bash

func1 ()
{
  echo This is a function.
}

declare -f        # Lists the function above.

echo

declare -i var1   # var1 is an integer.
var1=2367
echo "var1 declared as $var1"
var1=var1+1       # Integer declaration eliminates the need for 'let'.
echo "var1 incremented by 1 is $var1."
# Attempt to change variable declared as integer.
echo "Attempting to change var1 to floating point value, 2367.1."
var1=2367.1       # Results in error message, with no change to variable.
echo "var1 is still $var1"

echo

declare -r var2=13.36         # 'declare' permits setting a variable property
                              #+ and simultaneously assigning it a value.
echo "var2 declared as $var2" # Attempt to change readonly variable.
var2=13.37                    # Generates error message, and exit from script.

echo "var2 is still $var2"    # This line will not execute.

exit 0                        # Script will not exit here.

foo ()
{
FOO="bar"
}

bar ()
{
foo
echo $FOO
}

bar   # Prints bar.

foo (){
declare FOO="bar"
}

bar ()
{
foo
echo $FOO
}

bar  # Prints nothin

foo ()
{
FOO="bar"
}

bar ()
{
foo
echo $FOO
}

bar   # Prints bar.

foo (){
declare FOO="bar"
}

bar ()
{
foo
echo $FOO
}

bar  # Prints nothing.


# Thank you, Michael Iatrou, for pointing this out.

foo ()
{
FOO="bar"
}

bar ()
{
foo
echo $FOO
}

bar   # Prints bar.

  foo (){
  declare FOO="bar"
  }

  bar ()
  {
  foo
  echo $FOO
  }

  bar  # Prints nothing.


  # Thank you, Michael Iatrou, for pointing this out.





9.2.1. Another use for *declare*

bash$ declaregrep HOME
HOME=/home/bozo


bash$ zzy=68
bash$ declaregrep zzy
zzy=68


bash$ Colors=([0]="purple" [1]="reddish-orange" [2]="light green")
bash$ echo ${Colors[@]}
purple reddish-orange light green
bash$ declaregrep Colors
Colors=([0]="purple" [1]="reddish-orange" [2]="light green")

declare -i intvar

intvar=23
echo "$intvar"   # 2

declare -i intvar

intvar=23
echo "$intvar"   # 23
intvar=stringval
echo "$intvar"   # 0

declare -i intvar

intvar=23
echo "$intvar"   # 23
intvar=stringval
echo "$intvar"   # 0