[linux device driver]Chapter 03:scull_load中涉及到的bash命令 exit

用途说明

exit命令用于退出当前shell，在shell脚本中可以终止当前脚本执行。

 

常用参数

格式：exit n

退出。设置退出码为n。（Cause the shell to exit with a status of n.）

 

格式：exit

退出。退出码不变，即为最后一个命令的退出码。（If n is omitted, the exit status is that of the  last  command executed. ）

 

格式：$?

上一个命令的退出码。

 

格式：trap "commands" EXIT

退出时执行commands指定的命令。（ A trap on EXIT is executed before the shell terminates.）

 

退出码（exit status，或exit code）的约定：

0表示成功（Zero - Success）

非0表示失败（Non-Zero  - Failure）

2表示用法不当（Incorrect Usage）

127表示命令没有找到（Command Not Found）

126表示不是可执行的（Not an executable）

>=128 信号产生

 

man 3 exit 写道

The C standard specifies two constants, EXIT_SUCCESS and EXIT_FAILURE, that may be passed to exit() to indicate
successful or unsuccessful termination, respectively.

 

以下摘自/usr/include/stdlib.h

C代码  

1. #define EXIT_FAILURE    1       /* Failing exit status.  */  
2. #define EXIT_SUCCESS    0       /* Successful exit status.  */  

 

BSD试图对退出码标准化。

man 3 exit 写道

BSD has attempted to standardize exit codes; see the file <sysexits.h>.

 

 

以下摘自/usr/include/sysexits.h

C代码  

1. #define EX_OK           0       /* successful termination */  
2.   
3. #define EX__BASE        64      /* base value for error messages */  
4.   
5. #define EX_USAGE        64      /* command line usage error */  
6. #define EX_DATAERR      65      /* data format error */  
7. #define EX_NOINPUT      66      /* cannot open input */  
8. #define EX_NOUSER       67      /* addressee unknown */  
9. #define EX_NOHOST       68      /* host name unknown */  
10. #define EX_UNAVAILABLE  69      /* service unavailable */  
11. #define EX_SOFTWARE     70      /* internal software error */  
12. #define EX_OSERR        71      /* system error (e.g., can't fork) */  
13. #define EX_OSFILE       72      /* critical OS file missing */  
14. #define EX_CANTCREAT    73      /* can't create (user) output file */  
15. #define EX_IOERR        74      /* input/output error */  
16. #define EX_TEMPFAIL     75      /* temp failure; user is invited to retry */  
17. #define EX_PROTOCOL     76      /* remote error in protocol */  
18. #define EX_NOPERM       77      /* permission denied */  
19. #define EX_CONFIG       78      /* configuration error */  
20.   
21. #define EX__MAX 78      /* maximum listed value */  

 

使用示例

示例一 退出当前shell

[root@new55 ~]# 
[root@new55 ~]# exit 
logout

 

示例二 在脚本中，进入脚本所在目录，否则退出

Bash代码  

1. cd $(dirname $0) || exit 1  

 

示例三 在脚本中，判断参数数量，不匹配就打印使用方式，退出

Bash代码  

1. if [ "$#" -ne "2" ]; then  
2.     echo "usage: $0 <area> <hours>"  
3.     exit 2  
4. fi  

 

示例四 在脚本中，退出时删除临时文件

Bash代码  

1. trap "rm -f tmpfile; echo Bye." EXIT  

 

示例五 检查上一命令的退出码

Bash代码  

1. ./mycommand.sh  
2. EXCODE=$?  
3. if [ "$EXCODE" == "0" ]; then  
4.     echo "O.K"  
5. fi  

 --------------------------------------------英文解释---------------------------

The exit command terminates a script, just as in a C program. It can also return a value, which is available to the script's parent process.

Every command returns an exit status (sometimes referred to as a return status or exit code). A successful command returns a 0, while an unsuccessful one returns a non-zerovalue that usually can be interpreted as an error code. Well-behaved UNIX commands, programs, and utilities return a 0 exit code upon successful completion, though there are some exceptions.

Likewise, functions within a script and the script itself return an exit status. The last command executed in the function or script determines the exit status. Within a script, anexit nnn command may be used to deliver an nnn exit status to the shell (nnn must be an integer in the 0 - 255 range).

When a script ends with an exit that has no parameter, the exit status of the script is the exit status of the last command executed in the script (previous to theexit). #!/bin/bash COMMAND_1 . . . COMMAND_LAST # Will exit with status of last command. exit The equivalent of a bare exit is exit $? or even just omitting the exit. #!/bin/bash COMMAND_1 . . . COMMAND_LAST # Will exit with status of last command. exit $? #!/bin/bash COMMAND1 . . . COMMAND_LAST # Will exit with status of last command.

$? reads the exit status of the last command executed. After a function returns, $? gives the exit status of the last command executed in the function. This is Bash's way of giving functions a "return value." [1]

Following the execution of a pipe, a $? gives the exit status of the last command executed.

After a script terminates, a $? from the command-line gives the exit status of the script, that is, the last command executed in the script, which is, by convention, 0 on success or an integer in the range 1 - 255 on error.

Example 6-1. exit / exit status

#!/bin/bash echo hello echo $? # Exit status 0 returned because command executed successfully. lskdf # Unrecognized command. echo $? # Non-zero exit status returned -- command failed to execute. echo exit 113 # Will return 113 to shell. # To verify this, type "echo $?" after script terminates. # By convention, an 'exit 0' indicates success, #+ while a non-zero exit value means an error or anomalous condition. # See the "Exit Codes With Special Meanings" appendix.

$? is especially useful for testing the result of a command in a script (see Example 16-35and Example 16-20).

The !, the logical not qualifier, reverses the outcome of a test or command, and this affects its exit status. Example 6-2. Negating a condition using ! true # The "true" builtin. echo "exit status of \"true\" = $?" # 0 ! true echo "exit status of \"! true\" = $?" # 1 # Note that the "!" needs a space between it and the command. # !true leads to a "command not found" error # # The '!' operator prefixing a command invokes the Bash history mechanism. true !true # No error this time, but no negation either. # It just repeats the previous command (true). # =========================================================== # # Preceding a _pipe_ with ! inverts the exit status returned. ls | bogus_command # bash: bogus_command: command not found echo $? # 127 ! ls | bogus_command # bash: bogus_command: command not found echo $? # 0 # Note that the ! does not change the execution of the pipe. # Only the exit status changes. # =========================================================== # # Thanks, Stéphane Chazelas and Kristopher Newsome.

Certain exit status codes have reserved meanings and should not be user-specified in a script.