命令列程式

命令列程式是单独存在的执行档，执行前，Shell 会现在 $PATH 中寻找，若有同名但不同路径的档案，先找到先执行
常用的命令列程式：

cp,date,who,w,ls,cat,cut,wc,last,mv,mkdir,rmdir,mv,ps,pgrep,top,df,dd,ln,sort,sed,awk,ifconfig,dmesg,
diff,hostname,dnsdomainname,chmod,chown,chgrp,grep,find,kill,killall,pkill,more,less,mount,nice,paste,
ping,seq,sleep,su,tar,gzip,xargs,touch,uname,basename,dirname,tr,uniq,mail,which,locate,getconf

1. which 由预设的所寻路径 PATH 中，找到命令列程式的档案位置（默认找到第一个）。 -a 找出所有符合的程式路径
2. locate 由档案资料库中，找出包含「关键字」的档案路径

凡是档案路径中，含有此关键字的，都会被列出来

1. date 展示、设定系统的日期和时间

   FORMAT controls the output.  Interpreted sequences are:
   %%   a literal %
   %a   locale's abbreviated weekday name (e.g., Sun)
   %A   locale's full weekday name (e.g., Sunday)
   %b   locale's abbreviated month name (e.g., Jan)
   %B   locale's full month name (e.g., January)
   %c   locale's date and time (e.g., Thu Mar  3 23:05:25 2005)
   %C   century; like %Y, except omit last two digits (e.g., 20)
   %d   day of month (e.g., 01)
   %D   date; same as %m/%d/%y
   %e   day of month, space padded; same as %_d
   %F   full date; same as %Y-%m-%d
   %g   last two digits of year of ISO week number (see %G)
   %G   year of ISO week number (see %V); normally useful only with %V
   %h   same as %b
   %H   hour (00..23)
   %I   hour (01..12)
   %j   day of year (001..366)
   %k   hour, space padded ( 0..23); same as %_H
   %l   hour, space padded ( 1..12); same as %_I
   %m   month (01..12)
   %M   minute (00..59)
   %n   a newline
   %N   nanoseconds (000000000..999999999)
   %p   locale's equivalent of either AM or PM; blank if not known
   %P   like %p, but lower case
   %r   locale's 12-hour clock time (e.g., 11:11:04 PM)
   %R   24-hour hour and minute; same as %H:%M
   %s   seconds since 1970-01-01 00:00:00 UTC
   %S   second (00..60)
   %t   a tab
   %T   time; same as %H:%M:%S
   %u   day of week (1..7); 1 is Monday
   %U   week number of year, with Sunday as first day of week (00..53)
   %V   ISO week number, with Monday as first day of week (01..53)
   %w   day of week (0..6); 0 is Sunday
   %W   week number of year, with Monday as first day of week (00..53)
   %x   locale's date representation (e.g., 12/31/99)
   %X   locale's time representation (e.g., 23:13:48)
   %y   last two digits of year (00..99)
   %Y   year
   %z   +hhmm numeric time zone (e.g., -0400)
   %:z  +hh:mm numeric time zone (e.g., -04:00)
   %::z  +hh:mm:ss numeric time zone (e.g., -04:00:00)
   %:::z  numeric time zone with : to necessary precision (e.g., -04, +05:30)
   %Z   alphabetic time zone abbreviation (e.g., EDT)
   By default, date pads numeric fields with zeroes.
   The following optional flags may follow '%':
   -  (hyphen) do not pad the field
   _  (underscore) pad with spaces
   0  (zero) pad with zeros
   ^  use upper case if possible
   #  use opposite case if possible
   After any flags comes an optional field width, as a decimal number;
   then an optional modifier, which is either
   E to use the locale's alternate representations if available, or
   O to use the locale's alternate numeric symbols if available.
   Examples:
   Convert seconds since the epoch (1970-01-01 UTC) to a date
   $ date --date='@2147483647'
   Show the time on the west coast of the US (use tzselect(1) to find TZ)
   $ TZ='America/Los_Angeles' date
   Show the local time for 9AM next Friday on the west coast of the US
   $ date --date='TZ="America/Los_Angeles" 09:00 next Fri'