Linux bash string processing

Returns the length of a string.

Copy codeThe Code is as follows:
% X = "abcd"
# Method 1
% Expr length $ x
4
# Method 2
% Echo $ {# x}
4
# Method 3
% Expr "$ x ":".*"
4
# Expr help
# STRING: REGEXP anchored pattern match of REGEXP in STRING

Search for substrings

Copy codeThe Code is as follows:
% Expr index $ x "B"
2
% Expr index $ x ""
1
% Expr index $ x "B"
2
% Expr index $ x "c"
3
% Expr index $ x "d"
4

Obtain the substring

Copy codeThe Code is as follows:
# Method 1
# Expr <string> startpos length
% Expr substr "$ x" 1 3
Abc
% Expr substr "$ x" 1 5
Abcd
% Expr substr "$ x" 2 5
Bcd
# Method 2
# $ {X: pos: lenght}
% Echo $ {x: 1}
Bcd
% Echo $ {x: 2}
Cd
% Echo $ {x: 0}
Abcd
% Echo $ {x: 0: 2}
AB
% Pos = 1
% Len = 2
% Echo $ {x: $ pos: $ len}
Bc

Match Regular Expression
Copy codeThe Code is as follows:
# Print matching length
% Expr match $ x "."
1
% Expr match $ x "abc"
3
% Expr match $ x "bc"
0

The beginning and end of a string

Copy codeThe Code is as follows:
% X = aabbaarealwwvvww
% Echo "$ {x % w * w }"
Aabbaarealwwvv
% Echo "$ {x % w * w }"
Aabbaareal
% Echo "$ {x # a * }"
Lwwvvww
% Echo "$ {x # a * }"
Bbaarealwwvvww

Where, # indicates the header, because # on the keyboard is on the left of $.
Here, % indicates % because % on the keyboard is on the right of $.
A single parameter indicates the minimum matching, and a double parameter indicates the maximum matching.
That is to say, when there are multiple matching schemes, select the maximum length or the minimum length of the matching.

String replacement

Copy codeThe Code is as follows:
% X = abcdabcd
% Echo $ {x/a/B} # Replace only one
Bbcdabcd
% Echo $ {x // a/B} # Replace all
Bbcdbbcd

Regexp cannot be used, *? .