Linux declare command

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The Linux declare command is used to declare shell variables.

declare is a shell command that can be used to declare variables and set their attributes (e.g., [rix] is an attribute of the variable) in the first syntax, and to display shell functions in the second syntax. Without any parameters, it will display all shell variables and functions (the same effect as executing the set command).

Syntax

declare [+/-[rxi][variable name = set value] or declare -f

Parameter Description:

• +/- 　"-" can be used to specify the attributes of the variable, "+" is used to cancel the attributes set for the variable.
• -f 　Only display functions.
• r 　Set the variable as read-only.
• x 　The specified variable will become an environment variable, which can be used by programs outside of shell.
• i 　[Value] can be a number, string, or expression.

Online Examples

Declare Integer Variable

# declare -i ab //Declare Integer Variable
# ab=56 //Change Variable Content
# echo $ab //Display Variable Content
56

Change Variable Attribute

# declare -i ef //Declare Integer Variable
# ef=1  //Variable Assignment (Integer Value)
# echo $ef //Display Variable Content
1
# ef="wer" //Variable Assignment (Text Value)
# echo $ef 
0
# declare +i ef //Cancel Variable Attribute
# ef="wer"
# echo $ef
wer

Set Variable Read-only

# declare -r ab //Set Variable as Read-only
# ab=88 //Change Variable Content
-bash: ab: Read-only variable
# echo $ab //Display Variable Content
56

Declare Array Variable

# declare -a cd='([0]="a" [1]}="b" [2]}="c")' //Declare Array Variable
# echo ${cd[1]}
b //Display Variable Content
# echo ${cd[@]} //Display the entire array variable content
a b c

Display Function

# declare -f
command_not_found_handle () 
{ 
  if [ -x /usr/lib/command-not-found ]; then
    /usr/bin/python /usr/lib/command-not-found -- $1;
    return $?;
  else
    if [ -x /usr/share/command-not-found ]; then
      /usr/bin/python /usr/share/command-not-found -- $1;
      return $?;
    else
      return 127;
    fi;
  fi
}

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