如何在linux中grep帮助输出？

Question

当我使用的是不存在的命令，如开关lsof和ps有返回一个漫长的帮助菜单。而不是阅读整个事情，我希望能够为我正在寻找的开关grep它。我宁愿不使用两个命令。

这是我尝试 grep 时得到的结果：

ubuntu@ubuntu:~$ lsof -h | grep "\-U"
lsof 4.81
 latest revision: ftp://lsof.itap.purdue.edu/pub/tools/unix/lsof/
 latest FAQ: ftp://lsof.itap.purdue.edu/pub/tools/unix/lsof/FAQ
 latest man page: ftp://lsof.itap.purdue.edu/pub/tools/unix/lsof/lsof_man
 usage: [-?abhlnNoOPRtUvVX] [+|-c c] [+|-d s] [+D D] [+|-f[gG]]
 [-F [f]] [-g [s]] [-i [i]] [+|-L [l]] [+m [m]] [+|-M] [-o [o]] [-p s]
[+|-r [t]] [-s [p:s]] [-S [t]] [-T [t]] [-u s] [+|-w] [-x [fl]] [--] [names]
Defaults in parentheses; comma-separated set (s) items; dash-separated ranges.
  -?|-h list help          -a AND selections (OR)     -b avoid kernel blocks
  -c c  cmd c ^c /c/[bix]  +c w  COMMAND width (9)     
  +d s  dir s files        -d s  select by FD set     +D D  dir D tree *SLOW?*
                           -i select IPv[46] files    -l list UID numbers
  -n no host names         -N select NFS files        -o list file offset
  -O avoid overhead *RISK  -P no port names           -R list paRent PID
  -s list file size        -t terse listing           -T disable TCP/TPI info
  -U select Unix socket    -v list version info       -V verbose search
  +|-w  Warnings (+)       -X skip TCP&UDP* files     -- end option scan
  +f|-f  +filesystem or -file names     +|-f[gG] flaGs 
  -F [f] select fields; -F? for help  
  +|-L [l] list (+) suppress (-) link counts < l (0 = all; default = 0)
                                        +m [m] use|create mount supplement
  +|-M   portMap registration (-)       -o o   o 0t offset digits (8)
  -p s   exclude(^)|select PIDs         -S [t] t second stat timeout (15)
  -T qs TCP/TPI Q,St (s) info
  -g [s] exclude(^)|select and print process group IDs
  -i i   select by IPv[46] address: [46][proto][@host|addr][:svc_list|port_list]
  +|-r [t[m<fmt>]] repeat every t seconds (15);  + until no files, - forever.
       An optional suffix to t is m<fmt>; m must separate t from <fmt> and
      <fmt> is an strftime(3) format for the marker line.
  -s p:s  exclude(^)|select protocol (p = TCP|UDP) states by name(s).
  -u s   exclude(^)|select login|UID set s
  -x [fl] cross over +d|+D File systems or symbolic Links
  names  select named files or files on named file systems
Anyone can list all files; /dev warnings disabled; kernel ID check disabled.

Answer 1

您的 grep 不起作用，因为在大多数情况下，帮助输出在标准错误输出 (stderr) 上刷新。

这个流的文件描述符是 2 而不是 1，管道命令重定向文件描述符 1。解决这个问题的方法是将错误输出重定向到标准输出：

lsof -h 2>&1 | grep "\-U"