重定向的内核实现

whpu000625

回复 10# Godbach

看了一下strace的参数，没有能够显示更详细内容的。
我现在就是想知道执行"echo 0 > file"后，是如何将0写入到file文件中的。
至少应该有open该文件的系统调用吧，但是用strace却没看到？那么系统是如何做到的呢？

sditmaner

回复 5# whpu000625

whpu000625

回复 12# sditmaner

说什么了？

咸鱼1988

这个"echo 0 > file"语句是shell执行的，尝试下写个test.sh文件

1. #!/bin/sh
   
2. echo 0 > file

复制代码

然后用strace跟踪这个shell文件的执行

whpu000625

回复 14# 咸鱼1988

1. execve("./test.sh", ["./test.sh"], [/* 21 vars */]) = 0
   
2. brk(0)                                  = 0x1815000
   
3. mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f65f2392000
   
4. access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
   
5. open("/etc/ld.so.cache", O_RDONLY)      = 3
   
6. fstat(3, {st_mode=S_IFREG|0644, st_size=17666, ...}) = 0
   
7. mmap(NULL, 17666, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f65f238d000
   
8. close(3)                                = 0
   
9. open("/lib64/libtinfo.so.5", O_RDONLY)  = 3
   
10. read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0@\310\0\0\0\0\0\0"..., 832) = 832
    
11. fstat(3, {st_mode=S_IFREG|0755, st_size=135896, ...}) = 0
    
12. mmap(NULL, 2232320, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f65f1f53000
    
13. mprotect(0x7f65f1f70000, 2097152, PROT_NONE) = 0
    
14. mmap(0x7f65f2170000, 16384, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1d000) = 0x7f65f2170000
    
15. close(3)                                = 0
    
16. open("/lib64/libdl.so.2", O_RDONLY)     = 3
    
17. read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\340\r\0\0\0\0\0\0"..., 832) = 832
    
18. fstat(3, {st_mode=S_IFREG|0755, st_size=19536, ...}) = 0
    
19. mmap(NULL, 2109696, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f65f1d4f000
    
20. mprotect(0x7f65f1d51000, 2097152, PROT_NONE) = 0
    
21. mmap(0x7f65f1f51000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x2000) = 0x7f65f1f51000
    
22. close(3)                                = 0
    
23. open("/lib64/libc.so.6", O_RDONLY)      = 3
    
24. read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0000\356\1\0\0\0\0\0"..., 832) = 832
    
25. fstat(3, {st_mode=S_IFREG|0755, st_size=1921216, ...}) = 0
    
26. mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f65f238c000
    
27. mmap(NULL, 3750152, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f65f19bb000
    
28. mprotect(0x7f65f1b46000, 2093056, PROT_NONE) = 0
    
29. mmap(0x7f65f1d45000, 20480, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x18a000) = 0x7f65f1d45000
    
30. mmap(0x7f65f1d4a000, 18696, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f65f1d4a000
    
31. close(3)                                = 0
    
32. mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f65f238b000
    
33. mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f65f238a000
    
34. arch_prctl(ARCH_SET_FS, 0x7f65f238b700) = 0
    
35. mprotect(0x7f65f1d45000, 16384, PROT_READ) = 0
    
36. mprotect(0x7f65f1f51000, 4096, PROT_READ) = 0
    
37. mprotect(0x7f65f2393000, 4096, PROT_READ) = 0
    
38. munmap(0x7f65f238d000, 17666)           = 0
    
39. rt_sigprocmask(SIG_BLOCK, NULL, [], 8)  = 0
    
40. open("/dev/tty", O_RDWR|O_NONBLOCK)     = 3
    
41. close(3)                                = 0
    
42. brk(0)                                  = 0x1815000
    
43. brk(0x1836000)                          = 0x1836000
    
44. open("/usr/lib/locale/locale-archive", O_RDONLY) = 3
    
45. fstat(3, {st_mode=S_IFREG|0644, st_size=99158576, ...}) = 0
    
46. mmap(NULL, 99158576, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f65ebb2a000
    
47. close(3)                                = 0
    
48. getuid()                                = 0
    
49. getgid()                                = 0
    
50. geteuid()                               = 0
    
51. getegid()                               = 0
    
52. rt_sigprocmask(SIG_BLOCK, NULL, [], 8)  = 0
    
53. gettimeofday({1487153274, 647747}, NULL) = 0
    
54. open("/proc/meminfo", O_RDONLY|O_CLOEXEC) = 3
    
55. fstat(3, {st_mode=S_IFREG|0444, st_size=0, ...}) = 0
    
56. mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f65f2391000
    
57. read(3, "MemTotal:        1020076 kB\nMemF"..., 1024) = 1024
    
58. close(3)                                = 0
    
59. munmap(0x7f65f2391000, 4096)            = 0
    
60. rt_sigaction(SIGCHLD, {SIG_DFL, [], SA_RESTORER, 0x7f65f19ed9a0}, {SIG_DFL, [], 0}, 8) = 0
    
61. rt_sigaction(SIGCHLD, {SIG_DFL, [], SA_RESTORER, 0x7f65f19ed9a0}, {SIG_DFL, [], SA_RESTORER, 0x7f65f19ed9a0}, 8) = 0
    
62. rt_sigaction(SIGINT, {SIG_DFL, [], SA_RESTORER, 0x7f65f19ed9a0}, {SIG_DFL, [], 0}, 8) = 0
    
63. rt_sigaction(SIGINT, {SIG_DFL, [], SA_RESTORER, 0x7f65f19ed9a0}, {SIG_DFL, [], SA_RESTORER, 0x7f65f19ed9a0}, 8) = 0
    
64. rt_sigaction(SIGQUIT, {SIG_DFL, [], SA_RESTORER, 0x7f65f19ed9a0}, {SIG_DFL, [], 0}, 8) = 0
    
65. rt_sigaction(SIGQUIT, {SIG_DFL, [], SA_RESTORER, 0x7f65f19ed9a0}, {SIG_DFL, [], SA_RESTORER, 0x7f65f19ed9a0}, 8) = 0
    
66. rt_sigprocmask(SIG_BLOCK, NULL, [], 8)  = 0
    
67. rt_sigaction(SIGQUIT, {SIG_IGN, [], SA_RESTORER, 0x7f65f19ed9a0}, {SIG_DFL, [], SA_RESTORER, 0x7f65f19ed9a0}, 8) = 0
    
68. uname({sys="Linux", node="centos6.5-make", ...}) = 0
    
69. stat("/root", {st_mode=S_IFDIR|0550, st_size=4096, ...}) = 0
    
70. stat(".", {st_mode=S_IFDIR|0550, st_size=4096, ...}) = 0
    
71. getpid()                                = 31049
    
72. open("/usr/lib64/gconv/gconv-modules.cache", O_RDONLY) = 3
    
73. fstat(3, {st_mode=S_IFREG|0644, st_size=26060, ...}) = 0
    
74. mmap(NULL, 26060, PROT_READ, MAP_SHARED, 3, 0) = 0x7f65f2383000
    
75. close(3)                                = 0
    
76. getppid()                               = 31048
    
77. gettimeofday({1487153274, 649657}, NULL) = 0
    
78. getpgrp()                               = 31048
    
79. rt_sigaction(SIGCHLD, {0x43f2b0, [], SA_RESTORER, 0x7f65f19ed9a0}, {SIG_DFL, [], SA_RESTORER, 0x7f65f19ed9a0}, 8) = 0
    
80. getrlimit(RLIMIT_NPROC, {rlim_cur=7838, rlim_max=7838}) = 0
    
81. rt_sigprocmask(SIG_BLOCK, NULL, [], 8)  = 0
    
82. open("./test.sh", O_RDONLY)             = 3
    
83. ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, 0x7fff1d061f40) = -1 ENOTTY (Inappropriate ioctl for device)
    
84. lseek(3, 0, SEEK_CUR)                   = 0
    
85. read(3, "#!/bin/sh\necho 0 > file\n", 80) = 24
    
86. lseek(3, 0, SEEK_SET)                   = 0
    
87. getrlimit(RLIMIT_NOFILE, {rlim_cur=1024, rlim_max=4*1024}) = 0
    
88. fcntl(255, F_GETFD)                     = -1 EBADF (Bad file descriptor)
    
89. dup2(3, 255)                            = 255
    
90. close(3)                                = 0
    
91. fcntl(255, F_SETFD, FD_CLOEXEC)         = 0
    
92. fcntl(255, F_GETFL)                     = 0x8000 (flags O_RDONLY|O_LARGEFILE)
    
93. fstat(255, {st_mode=S_IFREG|0755, st_size=24, ...}) = 0
    
94. lseek(255, 0, SEEK_CUR)                 = 0
    
95. rt_sigprocmask(SIG_BLOCK, NULL, [], 8)  = 0
    
96. read(255, "#!/bin/sh\necho 0 > file\n", 24) = 24
    
97. rt_sigprocmask(SIG_BLOCK, NULL, [], 8)  = 0
    
98. open("file", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
    
99. fcntl(1, F_GETFD)                       = 0
    
100. fcntl(1, F_DUPFD, 10)                   = 10
     
101. fcntl(1, F_GETFD)                       = 0
     
102. fcntl(10, F_SETFD, FD_CLOEXEC)          = 0
     
103. dup2(3, 1)                              = 1
     
104. close(3)                                = 0
     
105. fstat(1, {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
     
106. mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f65f2391000
     
107. write(1, "0\n", 2)                      = 2
     
108. dup2(10, 1)                             = 1
     
109. fcntl(10, F_GETFD)                      = 0x1 (flags FD_CLOEXEC)
     
110. close(10)                               = 0
     
111. rt_sigprocmask(SIG_BLOCK, NULL, [], 8)  = 0
     
112. read(255, "", 24)                       = 0
     
113. exit_group(0)                           = ?

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nswcfd

> 发生在bash层面，在strace执行之前就已经发生了。
strace -e open,dup,dup2,write -f bash -c 'echo > xxx'

有人已经回答过了，请忽略。

nswcfd

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