1.通过whatis命令，知道了tr命令用来替换字符或者删除字符
2.通过tr --help命令可以知道tr命令的具体用法
[[email protected] ~]$ tr --help
Usage: tr [OPTION]... SET1 [SET2] #SET1 需要进行操作的字符集 SET2 指定要转换成的字符集
Translate, squeeze, and/or delete characters from standard input, #对来自标准输入的字符进行替换、压缩、删除操作，并写入到标准输出
writing to standard output.
-c, -C, --complement use the complement of SET1 #用来取代所有不属于第一字符集的字符
-d, --delete delete characters in SET1, do not translate #删除所有属于第一字符集的字符
-s, --squeeze-repeats replace each input sequence of a repeated character #把连续重复的字符以单独一个字符表示
that is listed in SET1 with a single occurrence
of that character
-t, --truncate-set1 first truncate SET1 to length of SET2 #将SET1用SET2转换，是tr默认选项，可以省略
--help display this help and exit
--version output version information and exit
SETs are specified as strings of characters. Most represent themselves. #也可以对特殊字符进行替换
Interpreted sequences are:
\NNN character with octal value NNN (1 to 3 octal digits)
\\ backslash
\a audible BEL
\b backspace
\f form feed
\n new line
\r return
\t horizontal tab
\v vertical tab
CHAR1-CHAR2 all characters from CHAR1 to CHAR2 in ascending order
[CHAR*] in SET2, copies of CHAR until length of SET1
[CHAR*REPEAT] REPEAT copies of CHAR, REPEAT octal if starting with 0
[:alnum:] all letters and digits
[:alpha:] all letters
[:blank:] all horizontal whitespace
[:cntrl:] all control characters
[:digit:] all digits
[:graph:] all printable characters, not including space
[:lower:] all lower case letters
[:print:] all printable characters, including space
[:punct:] all punctuation characters
[:space:] all horizontal or vertical whitespace
[:upper:] all upper case letters
[:xdigit:] all hexadecimal digits
[=CHAR=] all characters which are equivalent to CHAR
3.用法示例：
1.tr [-t] SET1 SET2
2..tr -c SET1 SET2
3.tr -d SET
4.tr -s SET1 SET2