Bash Shell String Operations Summary _linux Shell

1. Take the length

Copy Code code as follows:

Str= "ABCD"
Expr Length $str # 4
echo ${#str} # 4
Expr "$str": ". *" # 4

It seems that the second type is commonly used
2. Find the position of substring

Copy Code code as follows:

Str= "ABC"
Expr index $str "a" # 1
Expr index $str "B" # 2
Expr index $str "x" # 0
Expr index $str "" # 0

3. Select substring

Copy Code code as follows:

Str= "ABCdef"
Expr substr "$str" 1 3 # starting from the first position 3 characters, ABC
Expr substr "$str" 2 5 # Take 5 characters starting from the second position, Bcdef
Expr substr "$str" 4 5 # starting from fourth position 5 characters, Def
Echo ${str:2} # Extracts the string from the second position, Bcdef
Echo ${str:2:3} # Extracts 3 characters starting from the second position, BCD
echo ${STR: (-6): 5} # from the penultimate position to the left to extract the string, ABCDE
echo ${STR: (-4): 3} # Extract 6 characters to the left from the penultimate position, CDE

4. Intercept substring

Copy Code code as follows:

Str= "ABBC,DEF,GHI,ABCJKL"
echo ${str#a*c} # OUTPUT, def,ghi,abcjkl a well sign (#) to intercept the shortest match from the left (the ABBC string is removed here)
echo ${str# #a *c} # Output JKL, two well sign (# #) to intercept the longest match from the left (the ABBC,DEF,GHI,ABC string is removed here)
echo ${str# "A*c"} # Output ABBC,DEF,GHI,ABCJKL because there is no "a*c" substring in str
echo ${str## "A*c"} # Output ABBC,DEF,GHI,ABCJKL empathy
Echo ${str#*a*c*} # Empty
Echo ${str##*a*c*} # Empty
echo ${str#d*f) # output ABBC,DEF,GHI,ABCJKL,
echo ${str#*d*f} # OUTPUT, GHI,ABCJKL
Echo ${str%a*l} # Abbc,def,ghi a percent semicolon (%) that captures the shortest match from the right
Echo ${str%%b*l} # A two percent sign (%) indicates that the longest match is intercepted from the right
Echo ${str%a*c} # ABBC,DEF,GHI,ABCJKL
Memory can be so, the pound (#) is usually used to represent a number, it is placed in front of the percent (%) of the unload number behind; Or this memory, in the keyboard layout, the pound sign (#) is always on the left side of the percent (%) (that is, the front)

5. String replacement

Copy Code code as follows:

Str= "apple, tree and apple tree"
Echo ${str/apple/apple} # Replaces the first occurrence of Apple
Echo ${str//apple/apple} # replaces all Apple
echo ${str/#apple/apple} # If string str starts with Apple, replace it with Apple
Echo ${str/%apple/apple} # If string str ends with Apple, replace it with Apple

6. Compare

Copy Code code as follows:

[["A.txt" = *]] # logical true (pattern matching)
[["A.txt" =~. *\.txt]] # logical true (regex matching)
[["ABC" = "abc"]] # Logical True (string comparision)
[[< 2]] # logical True (string comparision), by ASCII value comparison

7. Connection

Copy Code code as follows:

s1= "Hello"
S2= "World"
Echo ${s1}${s2} # Of course it $s 1$s2, but it's best to add braces.

8. Replace all strings in a batch of files

Copy Code code as follows:

For I in File_list
Todo
VI $i <<-!
: g/xxxx/s//xxxx/g
: Wq
!
Done

9. Flip

Method One:
Use Rev command

Method Two:
Writing a script implementation

Copy Code code as follows:

#!/usr/bin/awk-f
################################################################
# Description:duplicate Rev in Awk
################################################################
{
Revline = ""
for (i=1;i<=length;i++)
{
Revline = substr (, i,1) revline
}
}
End{print Revline}

10. Matching

Copy Code code as follows:

Grep
Egrep
Fgrep

11. Get the number of repetitions of a character in a string

Copy Code code as follows:

echo $a |tr "x" "\ n" |wc-l

The resulting results need to be subtracted by 1.
Or

Copy Code code as follows:

echo $a |awk-f "x" ' {Print NF-1} '

12. How to insert a character in the middle of every two characters in a string
Using SED

Copy Code code as follows:

echo $test |sed ' s/. /&[insert char]/g '