Linux bash shell-declare

DeclareOrTypesetBuilt-in commands (they are identical) can be used to restrict attributes of variables. This is a method that is not strictly defined in some programming languages. CommandDeclareIt is only available after bash version 2. CommandTypesetYou can also run it in the ksh script.

Declare/typeset options

-R Read-Only

1 declare -r var1

(Declare-r var1AndReadonly var1Same role)

This is similar to C'sConstThe qualifier is the same. An operation that tries to change the value of the read-only variable will cause an error message and fail.

-I Integer

1 declare-I number 2 # the remaining part of the script treats "number" as an integer. 3 4 number = 3 5 echo "Number = $ number" # Number = 3 6 7 number = three 8 echo "Number = $ number" # Number = 0 9 # The Script tries to put the string "three" is used as an integer to evaluate the value: of course it will fail, so the value is 0 ).

Some arithmetic calculations can be completed in a variable declared as an integer without expr or let.

1 n=6/3 2 echo "n = $n" # n = 6/3 3  4 declare -i n 5 n=6/3 6 echo "n = $n" # n = 2

- Array

1 declare -a indices

VariableIndicesWill be treated as an array.

-F Function

1 declare -f

The script does not contain any parametersDeclare-fAll functions defined before this script are listed.

1 declare -f function_name

WhileDeclare-f function_nameOnly the specified function is listed.

-X export

1 declare -x var3

In this way, a variable is declared as the environment variable of the script and exported.

-X var = $ value

1 declare -x var3=373

DeclareThe command allows you to assign values to a variable while declaring the variable type.

Example 9-21. UseDeclare to declare the variable type

1 #! /Bin/bash 2 3 func1 () 4 {5 echo This is a function. 6} 7 8 declare-f # list the above functions. 9 10 echo 11 12 declare-I var1 # var1 is an integer. 13 var1 = 2367 14 echo "var1 declared as $ var1" 15 var1 = var1 + 1 # After the integer declaration, you do not need to use 'let '. 16 echo "var1 incremented by 1 is $ var1." 17 # try to change the value of a variable declared as an integer to a floating point value. 18 echo "Attempting to change var1 to floating point value, 2367.1. "19 var1 = 2367.1 # cause an error message. The value of this variable remains unchanged. 20 echo "var1 is still $ var1" 21 22 echo 23 24 declare-r var2 = 13.36 # 'desc' allows you to set attributes of a variable. 25 # + also assigns values to the variable. 26 echo "var2 declared as $ var2" # try to change the value of the read-only variable. 27 var2 = 13.37 # cause an error and exit from the script. 28 29 echo "var2 is still $ var2" # this line will not be executed. 30 31 exit 0 # The script will not exit from here.

Use built-inDeclareVariable range. 1 foo () 2 {3 FOO = "bar" 4} 5 6 bar () 7 {8 foo 9 echo $ FOO 10} 11 12 bar # print bar. However... 1 foo () {2 declare FOO = "bar" 3} 4 5 bar () 6 {7 foo 8 echo $ FOO 9} 10 11 bar # print nothing. 12 13 14 # thanks to Michael Iatrou for pointing out this point.