【Bash百宝箱】shell内建命令之echo、printf

1、echo

在shell中，内建（builtin）命令echo，格式如下：

echo [-neE] [arg ...]

echo命令用于输出各参数arg，参数间以空格分隔，结尾是个换行符。选项“-n”禁止输出结尾的换行符。对于一些反斜线“\”转义的特殊字符，在echo命令中默认不进行转义，选项“-e”启用转义，“-E”禁止转义。

下面是反斜线“\”转义的特殊字符。

\a     警告（响铃）
\b     退格删除
\c     禁止继续输出
\e     转义字符
\E     转义字符
\f     换页
\n     新行
\r     换行
\t     水平制表符
\v     垂直制表符
\\     反斜线
\0nnn  八进制数nnn表示的八位字符
\xHH   十六进制数HH表示的八位字符
\uHHHH  一到四个十六进制数表示的Unicode字符
\UHHHHHHHH  一到八个十六进制数表示的Unicode字符

例子如下：

$ echo "a\tb"
a\tb
$ echo -e "a\tb"
a   b
$ echo -E "a\tb"
a\tb

2、printf

在shell中，内建（builtin）命令printf，格式如下：

printf [-v var] format [arguments]

printf命令用于把格式化的参数arguments打印到标准输出，格式由参数format控制。如果指定了选项“-v var”，结果会保存到变量var中，而非打印到标准输出。

格式format有三种形式。第一种是原始字符串，直接打印到标准输出。第二种是转义字符序列，先转义后打印到标准输出。第三种是格式控制字符串，与后面的参数arguments对应。

格式控制有以下几种形式：

\"     双引号
\\     反斜线
\a     警告（响铃）
\b     退格删除
\c     禁止继续输出
\e     转义字符
\f     分页
\n     新行
\r     换行
\t     水平制表符
\v     垂直制表符
\NNN   八进制数
\xHH   十六进制数
\uHHHH  十六进制Unicode
\UHHHHHHHH  十六进制Unicode
%%     百分号
%b     扩展printf中参数arguments的反斜线转义序列，但“\c”例外，“\'”、“\"”和“\?”中的反斜线也不会去掉，而且以“\0”开头的八进制数可能包含四个数字。
%q     把对应参数arguments以能够重新作为shell输入的格式打印出来。
%(datefmt)T    以格式datefmt输出日期，对应的参数argument为从1970年1月1日开始的秒数，有两个特殊的参数，-1表示当前时间，-2表示shell运行时间，默认为-1。

上面提到的日期格式如下（摘录自“man 3 strftime”）：

%a     The abbreviated name of the day of the week according to the current locale.
%A     The full name of the day of the week according to the current locale.
%b     The abbreviated month name according to the current locale.
%B     The full month name according to the current locale.
%c     The preferred date and time representation for the current locale.
%C     The century number (year/100) as a 2-digit integer. (SU)
%d     The day of the month as a decimal number (range 01 to 31).
%D     Equivalent to %m/%d/%y.  (Yecch—for Americans only.  Americans should note that in other countries %d/%m/%y is rather common.  This means that in international context this  format  is
ambiguous and should not be used.) (SU)
%e     Like %d, the day of the month as a decimal number, but a leading zero is replaced by a space. (SU)
%E     Modifier: use alternative format, see below. (SU)
%F     Equivalent to %Y-%m-%d (the ISO 8601 date format). (C99)
%G     The  ISO 8601  week-based year (see NOTES) with century as a decimal number.  The 4-digit year corresponding to the ISO week number (see %V).  This has the same format and value as %Y,
except that if the ISO week number belongs to the previous or next year, that year is used instead. (TZ)
%g     Like %G, but without century, that is, with a 2-digit year (00-99). (TZ)
%h     Equivalent to %b.  (SU)
%H     The hour as a decimal number using a 24-hour clock (range 00 to 23).
%I     The hour as a decimal number using a 12-hour clock (range 01 to 12).
%j     The day of the year as a decimal number (range 001 to 366).
%k     The hour (24-hour clock) as a decimal number (range 0 to 23); single digits are preceded by a blank.  (See also %H.)  (TZ)
%l     The hour (12-hour clock) as a decimal number (range 1 to 12); single digits are preceded by a blank.  (See also %I.)  (TZ)
%m     The month as a decimal number (range 01 to 12).
%M     The minute as a decimal number (range 00 to 59).
%n     A newline character. (SU)
%O     Modifier: use alternative format, see below. (SU)
%p     Either "AM" or "PM" according to the given time value, or the corresponding strings for the current locale.  Noon is treated as "PM" and midnight as "AM".
%P     Like %p but in lowercase: "am" or "pm" or a corresponding string for the current locale. (GNU)
%r     The time in a.m. or p.m. notation.  In the POSIX locale this is equivalent to %I:%M:%S %p.  (SU)
%R     The time in 24-hour notation (%H:%M).  (SU) For a version including the seconds, see %T below.
%s     The number of seconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC). (TZ)
%S     The second as a decimal number (range 00 to 60).  (The range is up to 60 to allow for occasional leap seconds.)
%t     A tab character. (SU)
%T     The time in 24-hour notation (%H:%M:%S).  (SU)
%u     The day of the week as a decimal, range 1 to 7, Monday being 1.  See also %w.  (SU)
%U     The week number of the current year as a decimal number, range 00 to 53, starting with the first Sunday as the first day of week 01.  See also %V and %W.
%V     The ISO 8601 week number (see NOTES) of the current year as a decimal number, range 01 to 53, where week 1 is the first week that has at least 4 days in the new year.  See also %U  and
%W.  (SU)
%w     The day of the week as a decimal, range 0 to 6, Sunday being 0.  See also %u.
%W     The week number of the current year as a decimal number, range 00 to 53, starting with the first Monday as the first day of week 01.
%x     The preferred date representation for the current locale without the time.
%X     The preferred time representation for the current locale without the date.
%y     The year as a decimal number without a century (range 00 to 99).
%Y     The year as a decimal number including the century.
%z     The +hhmm or -hhmm numeric timezone (that is, the hour and minute offset from UTC). (SU)
%Z     The timezone name or abbreviation.
%+     The date and time in date(1) format. (TZ) (Not supported in glibc2.)
%%     A literal '%' character.

对于标准C中printf函数的转换字符，百分号后跟“diouxXfeEgGcs”中的任意一个字符，在printf命令中也可以使用，而且标准C中printf函数的输出样式控制标记、输出字符串宽度、数字精度等也适用于printf命令。下面以例子说明。

打印十六进制整数：

$ printf "%x\n" 17
11
$ printf "%X\n" 17
11
$ printf "%#x\n" 17
0x11
$ printf "%#X\n" 17
0X11

设置输出字符串宽度和对齐方式：

$ printf "%d\n" 123
123
$ printf "%6d\n" 123
123
$ printf "%-6d\n" 123
123

设置输出字符串宽度和填充方式：

$ printf "%d\n" 123
123
$ printf "%6d\n" 123
123
$ printf "%06d\n" 123
000123

在正数前面添加空格：

$ printf "%d\n" 123
123
$ printf "% d\n" 123
123
$ printf "%d\n" -123
-123
$ printf "% d\n" -123
-123

在正数前面添加加号：

$ printf "%d\n" 123
123
$ printf "%+d\n" 123
+123
$ printf "%d\n" -123
-123
$ printf "%+d\n" -123
-123

设置数字输出格式为千分位：

$ printf "%d\n" 123456789
123456789
$ printf "%'d\n" 123456789
123,456,789

设置浮点数精度：

$ printf "%f\n" 123
123.000000
$ printf "%.f\n" 123
123
$ printf "%.1f\n" 123
123.0
$ printf "%.3f\n" 123
123.000