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