Shell: readonly介绍

红太狼 2021-12-02 00:02 257阅读 0赞

# 1. 概念 #

`readonly`在shell脚本中，用来标记变量是只读的，后续执行过程中就不能对其值进行改变，这个用来定义一些常量类的变量. 如果对其进行重新赋值，会提示错误

[root@localhost etc]# hours_per_day=24
    [root@localhost etc]# echo $hours_per_day 
    24
    [root@localhost etc]# readonly hours_per_day 
    [root@localhost etc]# hours_per_day=0
    -bash: hours_per_day: readonly variable

# 2. 重置readonly变量 #

如何将变量调整为`"非只读"`状态呢？我们可以试试`unset`命令.

[root@localhost etc]# unset hours_per_day 
    -bash: unset: hours_per_day: cannot unset: readonly variable
    [root@localhost etc]#

从上面可以得知unset是不能够将readonly的变量重置的.

下面是一个非常规的操作（注意:不推荐使用)

[root@localhost etc]# unset hours_per_day 
    -bash: unset: hours_per_day: cannot unset: readonly variable
    [root@localhost etc]# 
    [root@localhost etc]# cat << EOF| sudo gdb
    attach $$
    call unbind_variable("hours_per_day")
    detach
    EOF
    GNU gdb (GDB) Red Hat Enterprise Linux 7.6.1-110.el7
    Copyright (C) 2013 Free Software Foundation, Inc.
    License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
    This is free software: you are free to change and redistribute it.
    There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
    and "show warranty" for details.
    This GDB was configured as "x86_64-redhat-linux-gnu".
    For bug reporting instructions, please see:
    <http://www.gnu.org/software/gdb/bugs/>.
    (gdb) Attaching to process 8704
    Reading symbols from /usr/bin/bash...Reading symbols from /usr/bin/bash...(no debugging symbols found)...done.
    (no debugging symbols found)...done.
    Reading symbols from /lib64/libtinfo.so.5...Reading symbols from /lib64/libtinfo.so.5...(no debugging symbols found)...done.
    (no debugging symbols found)...done.
    Loaded symbols for /lib64/libtinfo.so.5
    Reading symbols from /lib64/libdl.so.2...(no debugging symbols found)...done.
    Loaded symbols for /lib64/libdl.so.2
    Reading symbols from /lib64/libc.so.6...(no debugging symbols found)...done.
    Loaded symbols for /lib64/libc.so.6
    Reading symbols from /lib64/ld-linux-x86-64.so.2...(no debugging symbols found)...done.
    Loaded symbols for /lib64/ld-linux-x86-64.so.2
    Reading symbols from /lib64/libnss_files.so.2...(no debugging symbols found)...done.
    Loaded symbols for /lib64/libnss_files.so.2
    0x00007f91ec13f17c in waitpid () from /lib64/libc.so.6
    Missing separate debuginfos, use: debuginfo-install bash-4.2.46-30.el7.x86_64
    (gdb) $1 = 0
    (gdb) Detaching from program: /usr/bin/bash, process 8704
    (gdb) quit
    [root@localhost etc]# echo $hours_per_day
    
    [root@localhost etc]# hours_per_day=2
    [root@localhost etc]# echo $hours_per_day
    2
    [root@localhost etc]#