Linux Commands for Beginners-- Regular Expressions--the grep command

缺乏、安全感 2023-02-28 15:20 12阅读 0赞

**In this part,I wil show you the basis of Regular Expressions and the grep command  
  
  
1.Command:  
grep  
DESCRIPTION:  
print lines matching a pattern  
  
SYNOPSIS:  
 grep \[OPTIONS\] PATTERN \[FILE...\]  
OPTION:  
\-n, \--line-number print line number with output lines  
  
  
Regular expressions made up of anchors,character sets,modifiers  
Anchors:specify the position  
^ :at the beginnig of a line  
$ :at the end of a line  
Tips:If ^ is not placed at the beginning ,or $ at the end,the two won't act as anchors anymore  
  
character sets:what is seached  
  
  
Example:  
root@piniheaven:~/tutorial\#  ls  
A Girl.txt  
root@piniheaven:~/tutorial\# cat -n A\\ Girl.txt  
1 the tree has entered my hands  
2 the sap has ascended my arms  
3 the tree has grown in my breast-  
4 downward  
5 the branches grow out of me, like arms  
6  
7 tree you are  
8 moss you are  
9 you are violets with wind above them  
10 a child - so high - you are  
11 and all this is folly to the world  
  
print lines contain words "tree"  
root@piniheaven:~/tutorial\#  grep 'tree' A\\ Girl.txt  
the tree has entered my hands  
the tree has grown in my breast-  
tree you are  
  
print lines contain word "tree" and print which line they are in  
root@piniheaven:~/tutorial\# grep -n 'tree' A\\ Girl.txt  
1:the tree has entered my hands  
3:the tree has grown in my breast-  
7:tree you are  
  
  
print lines start with word "tree" and print which line they are in  
root@piniheaven:~/tutorial\# grep -n ^'tree' A\\ Girl.txt  
7:tree you are  
  
  
print lines contain word "are" and print which line they are in  
root@piniheaven:~/tutorial\# grep -n 'are' A\\ Girl.txt  
7:tree you are  
8:moss you are  
9:you are violets with wind above them  
10:a child - so high - you are  
  
  
  
print lines end with word "are" and print which line they are in  
root@piniheaven:~/tutorial\# grep -n 'are'$ A\\ Girl.txt  
7:tree you are  
8:moss you are  
10:a child - so high - you are  
  
  
  
Matching Character Sets  
\--"abc" finds lines with "abc" in them  
\--match any character with "."(dot)  
\--specify a range with \[\]  
\[123\] - lines that contain 1,2 or 3  
\[0-9\] - lines that contain at least a number  
\[A-Za-z0-9\] - lines that contain at leas a letters or a numbers  
  
Example:  
Find words start with letter 'm' and follow with a any character  
root@piniheaven:~/tutorial\# grep -n 'm.' A\\ Girl.txt  
1:the tree has entered my hands  
2:the sap has ascended my arms  
3:the tree has grown in my breast-  
5:the branches grow out of me, like arms  
8:moss you are  
  
Find words start with letter 'a' or 'b'  
root@piniheaven:~/tutorial\# grep -n ^'\[ad\]' A\\ Girl.txt  
4:downward  
10:a child - so high - you are  
11:and all this is folly to the world  
  
  
print lines start with a letter range from 'a' to 'm'  
root@piniheaven:~/tutorial\# grep -n ^'\[a-m\]' A\\ Girl.txt  
4:downward  
8:moss you are  
10:a child - so high - you are  
11:and all this is folly to the world  
  
Tips:  
If you want to search for "\[" or "\]",you should use backslash "\\"  
Example:  
\[\\\]\] - search lines with "\]" in them  
  
  
  
modifiers:specify how many times the previous charater set is repeated  
\* - matches zero or more copies of the chars  
\[\]\\\{1,4\\\} - lines that have between 1 to 4 mathes  
\\<word\\> - searches for a specific and separate word  
  
Example:  
root@piniheaven:~/tutorial\# ls  
A Girl.txt essay.txt  
root@piniheaven:~/tutorial\# cat -n essay.txt  
1 a  
2 aa  
3 aaa  
4 aaaa  
5 aaaaa  
6 aaaaaa  
7  
8 abc  
9 abbc  
10 abcc  
  
print lines contain word that mach \[b-c\]\[b-c\]  
root@piniheaven:~/tutorial\# grep -n '\[b-c\]\\\{2,3\\\}' essay.txt  
8:abc  
9:abbc  
10:abcc  
  
  
If you want to just search specify word,following command failed to work.  
root@piniheaven:~/tutorial\# grep -n 'aa' essay.txt  
2:aa  
3:aaa  
4:aaaa  
5:aaaaa  
6:aaaaaa  
  
therefore,you can type command like this  
root@piniheaven:~/tutorial\# grep -n '\\<aa\\>' essay.txt  
2:aa**