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tr
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===
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将字符进行替换压缩和删除
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## 补充说明
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**tr命令** 可以对来自标准输入的字符进行替换、压缩和删除。它可以将一组字符变成另一组字符，经常用来编写优美的单行命令，作用很强大。
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### 语法
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```
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tr(选项)(参数)
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```
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### 选项
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```
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-c或——complerment：取代所有不属于第一字符集的字符；
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-d或——delete：删除所有属于第一字符集的字符；
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-s或--squeeze-repeats：把连续重复的字符以单独一个字符表示；
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-t或--truncate-set1：先删除第一字符集较第二字符集多出的字符。
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```
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### 参数
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* 字符集1：指定要转换或删除的原字符集。当执行转换操作时，必须使用参数“字符集2”指定转换的目标字符集。但执行删除操作时，不需要参数“字符集2”；
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* 字符集2：指定要转换成的目标字符集。
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### 实例
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将输入字符由大写转换为小写：
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```
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echo "HELLO WORLD" | tr 'A-Z' 'a-z'
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hello world
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```
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'A-Z' 和 'a-z'都是集合，集合是可以自己制定的，例如：'ABD-}'、'bB.,'、'a-de-h'、'a-c0-9'都属于集合，集合里可以使用'\n'、'\t'，可以可以使用其他ASCII字符。
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使用tr删除字符：
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```
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echo "hello 123 world 456" | tr -d '0-9'
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hello world
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```
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将制表符转换为空格：
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```
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cat text | tr '\t' ' '
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```
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字符集补集，从输入文本中将不在补集中的所有字符删除：
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```
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echo aa.,a 1 b#$bb 2 c*/cc 3 ddd 4 | tr -d -c '0-9 \n'
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1 2 3 4
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```
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此例中，补集中包含了数字0~9、空格和换行符\n，所以没有被删除，其他字符全部被删除了。
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用tr压缩字符，可以压缩输入中重复的字符：
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```
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echo "thissss is a text linnnnnnne." | tr -s ' sn'
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this is a text line.
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```
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巧妙使用tr做数字相加操作：
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```
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echo 1 2 3 4 5 6 7 8 9 | xargs -n1 | echo $[ $(tr '\n' '+') 0 ]
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```
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删除Windows文件“造成”的'^M'字符：
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```
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cat file | tr -s "\r" "\n" > new_file
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或
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cat file | tr -d "\r" > new_file
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```
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**tr可以使用的字符类：**
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```
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[:alnum:]：字母和数字
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[:alpha:]：字母
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[:cntrl:]：控制（非打印）字符
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[:digit:]：数字
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[:graph:]：图形字符
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[:lower:]：小写字母
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[:print:]：可打印字符
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[:punct:]：标点符号
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[:space:]：空白字符
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[:upper:]：大写字母
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[:xdigit:]：十六进制字符
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```
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使用方式：
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```
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tr '[:lower:]' '[:upper:]'
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```
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<!-- Linux命令行搜索引擎：https://jaywcjlove.github.io/linux-command/ -->
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