手表仅监视可见输出吗？

难道watch只监视命令的可见的输出？假设我位于一个包含以下内容的目录中：

$ ls
a  b  c  d  e  f  g  h  i  j  k  l  m  n

如果我运行，watch -g ls -1我希望它在添加或删除文件后退出。实际发生的情况是，只有当相关文件在以下终端输出中可见时，它才会退出watch：

$ watch -g ls -1
Every 2.0s: ls -1                   Wed Nov 13 16:35:03 2013

a
b
c
d
e
f

删除文件m（由于终端的大小而看不到）不会执行任何操作。删除可见文件（例如d）会导致watch按预期退出。

-g因此，在我的man页面中解释了该标志：

   -g, --chgexit
          Exit when the output of command changes.

这是怎么回事？这正常吗？如何使用watch长输出的命令？我正在使用watch from procps-ng 3.3.4从Debian仓库安装的软件。

我发现此线程的标题为：Bug＃225549：具有watch monitor stderr。该线程来自2008年，但看起来旧版本不支持观看STDOUT以外的任何内容。

因此，我们仅限于STDOUT。至于可见的语言info watch，man watch这让我觉得您的观察/假设是正确的。

摘抄

   watch runs command repeatedly, displaying its output (the first screen‐
   full).   This  allows you to watch the program output change over time.
   By default, the program is run every 2 seconds; use -n or --interval to
   specify a different interval.

同样在BUGS下的这一位：

BUGS
       Upon  terminal resize, the screen will not be correctly repainted until
       the next scheduled update.  All --differences highlighting is  lost  on
       that update as well.

如果我不得不猜测，我认为它们是在运行之间将可见位存储在缓冲区中，然后仅分析那些字符。

编辑＃1

我使用了进一步的调试功能strace，您可以看到watch从ls命令中读取了输出，因此它在内部删除了更改。

在我删除m文件之前

$ strace -o w.log watch -g 'ls -1'
read(3, "a\nb\nc\nd\ne\nf\ng\nh\ni\nj\nk\nl\nm\nn\nw.lo"..., 4096) = 34
close(3)                                = 0
--- SIGCHLD (Child exited) @ 0 (0) ---
munmap(0x7f4da83af000, 4096)            = 0
wait4(31011, [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], 0, NULL) = 31011
rt_sigaction(SIGTSTP, {SIG_IGN, [], SA_RESTORER|SA_RESTART, 0x7f4da79b94a0}, {0x7f4da7f81ee0, [], SA_RESTORER|SA_RESTART, 0x7f4da79b94a0}, 8) = 0
write(1, "\33[H\33[2JEvery 2.0s: ls -1\33[1;140H"..., 119) = 119
rt_sigaction(SIGTSTP, {0x7f4da7f81ee0, [], SA_RESTORER|SA_RESTART, 0x7f4da79b94a0}, NULL, 8) = 0
nanosleep({2, 0}, NULL)                 = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=3519, ...}) = 0
pipe([3, 4])                            = 0
clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f4da839f9d0) = 31014
close(4)                                = 0
fcntl(3, F_GETFL)                       = 0 (flags O_RDONLY)
fstat(3, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f4da83af000
lseek(3, 0, SEEK_CUR)                   = -1 ESPIPE (Illegal seek)
read(3, "a\nb\nc\nd\ne\nf\ng\nh\ni\nj\nk\nl\nm\nn\nw.lo"..., 4096) = 34
close(3)                                = 0
munmap(0x7f4da83af000, 4096)            = 0
--- SIGCHLD (Child exited) @ 0 (0) ---

m删除文件后

--- SIGCHLD (Child exited) @ 0 (0) ---
rt_sigaction(SIGTSTP, {SIG_IGN, [], SA_RESTORER|SA_RESTART, 0x7f4da79b94a0}, {0x7f4da7f81ee0, [], SA_RESTORER|SA_RESTART, 0x7f4da79b94a0}, 8) = 0
poll([{fd=0, events=POLLIN}], 1, 0)     = 0 (Timeout)
poll([{fd=0, events=POLLIN}], 1, 0)     = 0 (Timeout)
write(1, "\33[1;158H8\33[11;163H", 18)  = 18
rt_sigaction(SIGTSTP, {0x7f4da7f81ee0, [], SA_RESTORER|SA_RESTART, 0x7f4da79b94a0}, NULL, 8) = 0
nanosleep({2, 0}, NULL)                 = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=3519, ...}) = 0
pipe([3, 4])                            = 0
clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f4da839f9d0) = 31028
close(4)                                = 0
fcntl(3, F_GETFL)                       = 0 (flags O_RDONLY)
fstat(3, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f4da83af000
lseek(3, 0, SEEK_CUR)                   = -1 ESPIPE (Illegal seek)
read(3, "a\nb\nc\nd\ne\nf\ng\nh\ni\nj\nk\nl\nn\nw.log\n", 4096) = 32
close(3)                                = 0
--- SIGCHLD (Child exited) @ 0 (0) ---
munmap(0x7f4da83af000, 4096)            = 0
wait4(31028, [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], 0, NULL) = 31028

如果添加或删除文件，我希望它会退出

我很确定您正在使用inotify-tools。

我来自procps-ng的观看手册，说

watch反复运行命令，显示其输出和错误（第一个全屏显示）。