shell编程

                                             (1)追踪一下  shell  strace    

                                                          ./strace   ./command.sh      

# ./strace ./command.sh
execve("./command.sh", ["./command.sh"], 0x7ff0f82270 /* 13 vars */) = 0
brk(NULL)                               = 0xa2ea000
faccessat(AT_FDCWD, "/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "tls/aarch64/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "tls/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "aarch64/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/data/usrlibs/tls/aarch64/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
newfstatat(AT_FDCWD, "/data/usrlibs/tls/aarch64", 0x7fd3b19fc0, 0) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/data/usrlibs/tls/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
newfstatat(AT_FDCWD, "/data/usrlibs/tls", 0x7fd3b19fc0, 0) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/data/usrlibs/aarch64/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
newfstatat(AT_FDCWD, "/data/usrlibs/aarch64", 0x7fd3b19fc0, 0) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/data/usrlibs/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
newfstatat(AT_FDCWD, "/data/usrlibs", 0x7fd3b19fc0, 0) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/lib64/tls/aarch64/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
newfstatat(AT_FDCWD, "/lib64/tls/aarch64", 0x7fd3b19fc0, 0) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/lib64/tls/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
newfstatat(AT_FDCWD, "/lib64/tls", 0x7fd3b19fc0, 0) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/lib64/aarch64/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
newfstatat(AT_FDCWD, "/lib64/aarch64", 0x7fd3b19fc0, 0) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/lib64/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0\267\0\1\0\0\0\270\374\1\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1279888, ...}) = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f7d5ad000
mmap(NULL, 1352480, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f7d43a000
mprotect(0x7f7d56c000, 61440, PROT_NONE) = 0
mmap(0x7f7d57b000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x131000) = 0x7f7d57b000
mmap(0x7f7d581000, 13088, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f7d581000
close(3)                                = 0
mprotect(0x7f7d57b000, 16384, PROT_READ) = 0
mprotect(0x4b5000, 4096, PROT_READ)     = 0
mprotect(0x7f7d5b1000, 4096, PROT_READ) = 0
getuid()                                = 0
brk(NULL)                               = 0xa2ea000
brk(0xa30b000)                          = 0xa30b000
getpid()                                = 176
rt_sigaction(SIGCHLD, {sa_handler=0x4334a4, sa_mask=~[RTMIN RT_1], sa_flags=0}, NULL, 8) = 0
rt_sigaction(SIGHUP, {sa_handler=SIG_DFL, sa_mask=[HUP], sa_flags=SA_RESTART}, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=0}, 8) = 0
getppid()                               = 173
uname({sysname="Linux", nodename="rockchip", ...}) = 0
newfstatat(AT_FDCWD, "/mnt", {st_mode=S_IFDIR|0775, st_size=4096, ...}, 0) = 0
newfstatat(AT_FDCWD, ".", {st_mode=S_IFDIR|0775, st_size=4096, ...}, 0) = 0
openat(AT_FDCWD, "./command.sh", O_RDONLY) = 3
fcntl(3, F_DUPFD, 10)                   = 10
close(3)                                = 0
fcntl(10, F_SETFD, FD_CLOEXEC)          = 0
rt_sigaction(SIGINT, NULL, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=0}, 8) = 0
rt_sigaction(SIGINT, {sa_handler=0x4334a4, sa_mask=~[RTMIN RT_1], sa_flags=0}, NULL, 8) = 0
rt_sigaction(SIGQUIT, NULL, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=0}, 8) = 0
rt_sigaction(SIGQUIT, {sa_handler=SIG_IGN, sa_mask=~[RTMIN RT_1], sa_flags=0}, NULL, 8) = 0
rt_sigaction(SIGTERM, NULL, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=0}, 8) = 0
rt_sigaction(SIGTERM, {sa_handler=SIG_DFL, sa_mask=~[RTMIN RT_1], sa_flags=0}, NULL, 8) = 0
read(10, "#! /bin/sh\n\n\nsleep 10s\nprintf \"h"..., 1023) = 41
newfstatat(AT_FDCWD, "/bin/sleep", {st_mode=S_IFREG|S_ISUID|0755, st_size=685024, ...}, 0) = 0
clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f7d5adc40) = 177
wait4(-1,

　    调试技术:

  　   shell中打印:

                               echo "xxxxxxxxxxxxxxxx"

                               printf "xxxxxxxxxxxxx\n"

         shell中注释:  

                            方法一:   行注释:  #

#! /bin/sh
:<<eof
sleep 1s
i am is programmer \n
i am is doctor
printf "hello\n"
eof

　　                    方法二: 

                                    :<<!

                                    !

                           方法三:

                                      if  false; then 

                                      xxxxxxxx

                                      xxxxxxxxxx

                                      fi 

                            方法四

        shell 中控制语句：

                             有个坑:  if 后面[ ]  这个符号 

xxflag  = 0
if [ -f "/jffs2/xxx.sh" ]; then 
   /jffs2/xxxx.sh &
   xxxflag=1
fi 

　　            shell中 sleep

       论shell后台运行的重要性:            否则里面有阻塞操作就会导致卡顿

                                                            source     xxx  &  

                  shell中函数调用和函数定义: