debug a shell, 查看shell执行过程中每一步的结果, set -x

我们最开始写脚本的时候总是比较简单的，慢慢的脚本写的越来越复杂，这个时候我们在写的过程中就会遇到些问题，最开始的时候我总是不停的试，不停的改，最后还是可以折腾出来的，不过效率比较低。

后来我无意中看到一遍文章讲Linux的一些技巧的，里面提到在脚本的开始添加 set -x 这么一句，它可以让我们看到脚本执行的时候每一句的输出结果，有了这个在写脚本的时候就比较快了，它怎么用呢？

1. 我们可以在脚本开始的地方加入这个命令
set -x

2. 也可以向下面这样直接加在后面
#!/bin/ksh -x

3. 可以在执行脚本时这样做
#ksh -x /locale/shell.sh

怎么感觉跟孔乙己教鲁迅“回”字的四种写法:)


关于set 有一篇man page 英文的

set

Manipulate shell variables and functions.

Syntax
      set [--abBCefhHkmnpPtuvx] [-o option] [argument ...]

If no options or arguments are supplied, set displays the names and values of all shell variables and functions, sorted according to the current locale, in a format that may be reused as input. When options are supplied, they set or unset shell attributes.

Options

Using `+' rather than `-' will cause the option to be turned off. 

-a   allexport
          Mark variables which are modified or created for export.      

-b   notify
      
          Cause the status of terminated background jobs to be 
          reported immediately, rather than before printing the
          next primary prompt. 
    
-B   braceexpand 
          The shell will perform brace expansion. 
          This option is on by default. 

-C   noclobber
          Prevent output redirection using `>', `>&',
          and `<>' from overwriting existing files. 

-e   errexit
          Exit immediately if a simple command exits with a non-zero
          status, unless the command that fails is part of an until or
          while loop, part of an if statement, part of a && or || list,
          or if the command's return status is being inverted using !. 

-f   noglob
          Disable file name generation (globbing). 

-h   hashall
          Locate and remember (hash) commands as they are looked
          up for execution. This option is enabled by default.

-H   histexpand 
          Enable `!' style history substitution 
          This option is on by default for interactive shells. 

-k   keyword
          All arguments in the form of assignment statements are
          placed in the environment for a command, not just those that
          precede the command name.
 
-m   monitor
          Job control is enabled. 

-n   noexec
          Read commands but do not execute them; this may be used
          to check a script for syntax errors. 
          This option is ignored by interactive shells. 

-o option-name
          Set the option corresponding to `option-name'
          The `option-names' are listed above and below (in ITALICS)      

          emacs     : Use an emacs-style line editing interface . 
          history   : Enable command history,
                      this option is on by default in interactive shells. 
          ignoreeof : An interactive shell will not exit upon reading EOF. 
          posix     : Change the behavior of Bash to match the POSIX 1003.2 standard. 
          vi        : Use a vi-style line editing interface. 

-p    privileged
          Turn on privileged mode. In this mode,
          the $BASH_ENV and $ENV files are not processed,
          shell functions are not inherited from the environment, 
          and the SHELLOPTS variable, if it appears in the environment, 
          is ignored. 
          If the shell is started with the effective user (group) id 
          not equal to the real user (group) id, and the -p option 
          is not supplied, these actions are taken and the effective 
          user id is set to the real user id. 
          If the -p option is supplied at startup, the effective user 
          id is not reset. Turning this option off causes the effective 
          user and group ids to be set to the real user and group ids. 

-P   physical
          If set, do not follow symbolic links when performing 
          commands. The physical directory is used instead. 

-t   onecmd 
          Exit after reading and executing one command. 

-u   nounset
          Treat unset variables as an error when performing 
          parameter expansion. An error message will be written 
          to the standard error, and a non-interactive shell will exit.

-v   verbose
          Print shell input lines as they are read. 

-x   xtrace
          Print a trace of simple commands and their arguments
          after they are expanded and before they are executed. 
  
--
    If no arguments follow this option, then the positional parameters are unset. 
    Otherwise, the positional parameters are set to the arguments, 
    even if some of them begin with a `-'. 

-
    Signal the end of options, cause all remaining arguments to be 
    assigned to the positional parameters. The `-x' and `-v' 
    options are turned off. If there are no arguments, the positional parameters 
    remain unchanged.

Examples

Set the variable 'mydept' equal to 'Sales' :

 mydept=Sales

To make the change permanent:

 export mydept

Symbolic Links

By default, Bash follows the logical chain of directories when performing commands which change the current directory. e.g.

If `/usr/sys' is a symbolic link to `/usr/local/sys' then:

$ cd /usr/sys; echo $PWD
/usr/sys
$ cd ..; pwd
/usr

    If set -P is on (do not follow symbolic links), then: 

$ cd /usr/sys; echo $PWD
/usr/local/sys
$ cd ..; pwd
/usr/local

Debugging part of a script:

set -x			# activate debugging 
# your commands go here...
set +x			# stop debugging 

The options can also be used upon invocation of the shell. The current set of options may be found in $-. The remaining N arguments are positional parameters and are assigned, in order, to $1, $2, ... $N. The special parameter # is set to N.

set is a POSIX `special' builtin. The return status is always zero unless an invalid option is supplied.

"Never memorize what you can look up in books" - Einstein

一本非常好的Linux+Unix shell编程教材：