difference between "readonly" and "declare -r"

[Cristian Zoicas]

---- BEGIN CONFIGURATION 1: Debian GNU/Linux 5.0 ----

Configuration Information [Automatically generated, do not change]:
Machine: i486
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='i486'
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='i486-pc-linux-gnu'
-DCONF_VENDOR='pc' -DLOC$
uname output: Linux griffo.sissa.it 2.6.26-2-686 #1 SMP Wed Aug 19
06:06:52 UTC 2009 i686 GNU/Linux
Machine Type: i486-pc-linux-gnu

Bash Version: 3.2
Patch Level: 39
Release Status: release

---- END CONFIGURATION 1 Debian GNU/Linux 5.0 ----




---- BEGIN CONFIGURATION 2: ./configure --prefix=<dir> under Debian
GNU/Linux 5.0  ----

Configuration Information [Automatically generated, do not change]:
Machine: i686
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='i686'
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='i686-pc-linux-gnu'
-DCONF_VENDOR='pc' -DLOC$
uname output: Linux griffo.sissa.it 2.6.26-2-686 #1 SMP Wed Aug 19
06:06:52 UTC 2009 i686 GNU/Linux
Machine Type: i686-pc-linux-gnu

Bash Version: 4.1
Patch Level: 9
Release Status: release

---- END  CONFIGURATION 2: ./configure --prefix=<dir> under Debian
GNU/Linux 5.0  ----



Description:

        CONFIGURATION 1: this is bash 3.2.39 you can find on Debian GNU/Linux 
5.0
        CONFIGURATION 2: this is bash 4.1.9 downloaded on a Debian GNU/Linux 5.0
                         machine and configured with
                              ./configure --prefix=<dir>

        I have tested the script below under both configurations above
        and I found differences between "readonly" and "declare -r".

        I was not able to  find something justifying this behaviour in
        the bash documentation.

        The bug which manifests on the line 77.

        Read the notes at the end.

        ---- begin script ----

           1    function tst_readonly()
           2    {
           3        # - Declare a global variable since it is not
declared "local".
           4        #
           5        RO_VAR=f
           6
           7        # - The global variable defined above becomes readonly.
           8        #
           9        readonly RO_VAR
          10    }
          11
          12    # - Call function tst_readonly to create global
variable and make it readonly
          13    #
          14    tst_readonly
          15
          16    # - Print the readonly variable contents;
          17    # - The contents is ok.
          18    #
          19    echo "RO_VAR print 1: $RO_VAR"
          20
          21
          22    # !!!!!!!!!!!!!!! Trying to assign a readonly variable
          23    #
          24    # - bash behaves correctly and prints an error message
and do not assigns
          25    #   any value to the readonly variable
          26    #
          27    RO_VAR=m
          28
          29
          30    # - The readonly variable contents is still "f" since
the above assignment failed.
          31    # - This is good.
          32    #
          33    echo "RO_VAR print 2: $RO_VAR"
          34
          35    echo
"-------------------------------------------------------------"
          36
          37    function tst_D_VAR_1()
          38    {
          39        # - Declare a global variable since it is not
declared "local".
          40        #
          41        D_VAR_1=d_var_1_first_assignment
          42
          43        # - The global variable defined above becomes readonly.
          44        #
          45        declare -r D_VAR_1
          46
          47
          48        # !!!!!!!!!!!!!!! Trying to assign a readonly variable
          49        #
          50        # - bash behaves correctly and prints an error
message and do not assigns
          51        #   any value to the readonly variable
          52        #
          53        D_VAR_1=d_var_1_second_assignment
          54
          55
          56        # - The readonly variable contents is still
"d_var_1_first_assignment" since the above assignment failed.
          57        # - This is good.
          58        #
          59        # This following statement is not executed  (isn't
this another bug?)
          60        #
          61        echo "D_VAR_1 print 1:  $D_VAR_1"
          62
          63    }
          64
          65    # - Call function to create a readonly global variable
          66    #
          67    tst_D_VAR_1
          68
          69    # - print variable contents
          70    # - The contents is ok; the second assignment in the
function did not work
          71    #
          72    echo "D_VAR_1 print 2:  $D_VAR_1"
          73
          74    # - assign the global readonly variable
          75    # - BUG : the assignment succeeds !!!!!!
          76    #
          77    D_VAR_1=d_var_1_third_assignment
          78
          79    echo "D_VAR_1 print 3:  $D_VAR_1"

        ---- end script ----

        a) "readonly" and "declare -r" do not behave the same way;

        b) "declare -r" makes the  variable readonly but only in a
           function and this is not correct.


Repeat-By:
        [Describe the sequence of events that causes the problem to occur.]

Fix:

        1) In  order to  solve point  b the  bash behaviour  should be
           corrected  since "declare  -r"  is not  able  to provide  a
           variable with the "readonly" attribute.

        2) I find the behaviour of "readonly" correct since it is able
           to give the readonly attribute to an existing variable. Nothing
           to be done here.



regards
cristian zoicas