环境
1 [root@osker ~]# cat /etc/redhat-release 2 CentOS Linux release 7.6.1810 (Core) 3 [root@osker ~]# uname -r 4 3.10.0-957.el7.x86_64
1 [root@osker ~]# cat 3.txt
2 Mike Harrington:[510] 548-1278:250:100:175
3 Christian Dobbins:[408] 538-2358:155:90:201
4 tom
5 Chet Main:[510] 548-5258:50:95:135
6 Tom Savage:[408] 926-3456:250:168:200
文档中有tom和Tom两个关键词
vim
:set ic (ignorecase 的缩写)忽略大小写
:set noic (noignorecase 的缩写)不忽略大小写
grep
查询man帮助可以找到，使用-i参数可以忽略大小。
-i, --ignore-case：Ignore case distinctions in both the PATTERN and the input files. (-i is specified by POSIX.)
1 [root@osker ~]# grep -i 'tom' 3.txt 2 tom 3 Tom Savage:[408] 926-3456:250:168:200
sed
1 [root@osker ~]# sed -n '/tom/Ip' 3.txt 2 tom 3 Tom Savage:[408] 926-3456:250:168:200 4 [root@osker ~]# sed -n '/tom/ip' 3.txt 5 p
可以看出在/后加入 I 可以忽略大小写过滤，使用i会出错。
1 [root@osker ~]# sed -n 's#tom#qiu#gp' 3.txt 2 qiu 3 [root@osker ~]# sed -n 's#tom#qiu#gpi' 3.txt 4 qiu 5 qiu Savage:[408] 926-3456:250:168:200 6 [root@osker ~]# sed -n 's#tom#qiu#gpI' 3.txt 7 qiu 8 qiu Savage:[408] 926-3456:250:168:200
在使用替换功能时候可以时用i或者I，对需要替换的词忽略大小写。
在GNU找了好久，找到这么一段话：
i I ：The I modifier to regular-expression matching is a GNU extension which makes sed match regexp in a case-insensitive manner.
引用链接：
https://www.gnu.org/software/sed/manual/html_node/The-_0022s_0022-Command.html#The-_0022s_0022-Command
awk
1 [root@osker ~]# awk '/tom/' IGNORECASE=1 3.txt 2 tom 3 Tom Savage:[408] 926-3456:250:168:200
查看man帮助可以查询到，awk内置了一个IGNORECASE变量，专门用于处理大小写的忽略。当IGNORECASE的值为真时，则进行忽略大写的匹配。
IGNORECASE：
Controls the case-sensitivity of all regular expression and string operations. If IGNORECASE has a non-zero value, then string comparisons and pattern matching in rules, field splitting with FS and FPAT, record separating with RS, regular expression matching with ~ and !~, and the gensub(), gsub(), index(), match(), patsplit(), split(), and sub() built-in functions all ignore case when doing regular expression operations. NOTE: Array subscripting is not affected. However, the asort() and asorti() functions are affected.
Thus, if IGNORECASE is not equal to zero, /aB/ matches all of the strings "ab", "aB", "Ab", and "AB". As with all AWK variables, the initial value of IGNORECASE is zero, so all regular expression and string operations are normally case-sensitive.