【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\tb2、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