Bash local AND readonly variable

Question

I like to organize my code with functions, as I am using function I like to use local variables also. As much as possible I use read-only variables, so if I make any mistake inside my code it will be easily noticed.

But I have been using it wrong for a long time. So I decided to write it to share my experience.

How to define local AND read-only inside bash function? (See the answer and explanation below).

Answer 1

First attempt: local readonly var1

That is the way I used to define it. It is wrong. I will define my variable var1 as local, but it will not be readonly, as you can see on example below, I can change the value of var1, and I don't want that!

:~$ ( >     myfunction() >     { >         # Define variable >         local readonly var1="val1" >          >         echo "Readonly output:" >         readonly | grep -E 'readonly|local|var1' >         echo "" >  >         echo "Local output:" >         local | grep -E 'readonly|local|var1' >         echo "" >  >         var1="val2" >         echo "VAR1 INSIDE: ${var1}" >     } >     myfunction >     echo "VAR1 OUTSIDE: ${var1}" > ) Readonly output:  Local output: var1=val1  VAR1 INSIDE: val2 VAR1 OUTSIDE: 

Second attempt: readonly local var1

This time it will define var1 as readonly, but it will also define a variable called local, so using this way it will not handle local as keyword, it will be a variable name.

Check also that the scope of var1 is not local, in fact it is global, we can see the value of var1 outside the function.

:~$ ( >     myfunction() >     { >         # Define variable >         readonly local var1="val1" >          >         echo "Readonly output:" >         readonly | grep -E 'readonly|local|var1' >         echo "" >  >         echo "Local output:" >         local | grep -E 'readonly|local|var1' >         echo "" >  >         echo "VAR1 INSIDE: ${var1}" >     } >     myfunction >     echo "VAR1 OUTSIDE: ${var1}" > ) Readonly output: declare -r local declare -r var1="val1"  Local output:  VAR1 INSIDE: val1 VAR1 OUTSIDE: val1 

---

As it should be: local -r var1

This way it will do exactly what I want, it will define var1 as scope local AND readonly.

:~$ ( >     myfunction() >     { >         # Define variable >         local -r var1="val1" >          >         echo "Readonly output:" >         readonly | grep -E 'readonly|local|var1' >         echo "" >  >         echo "Local output:" >         local | grep -E 'readonly|local|var1' >         echo "" >  >         #var1="val2" >         echo "VAR1 INSIDE: ${var1}" >     } >     myfunction >     echo "VAR1 OUTSIDE: ${var1}" > ) Readonly output: declare -r var1="val1"  Local output: var1=val1  VAR1 INSIDE: val1 VAR1 OUTSIDE:  

We can define it as below also, but one line is better than two!

local var1="val1" readonly var1 

Answer 2

The bash man page summarizes things thusly for the declare command:

    -r   Make names readonly.  These names cannot then be assigned values          by subsequent assignment statements or unset. 

and

   When used in a function, declare and typeset make each name local, as with    the local command, unless the -g option is supplied. 

So if you declare within a function, the variable you declare will be local by default. And if you use the -r option, it will be read-only.

$ cat testv #!/usr/bin/env bash  test1() {   declare -r var="$1"   var="bacon" }  s=foo test1 bar echo "$s - $var"  $ bash testv testv: line 5: var: readonly variable foo -