Bash string processing (compared with Java)-3. assign values to the (string) Variable

Bash string processing (compared with Java)-3. assign values to the (string) Variable

In Java
Simple assignment
S = "Hello ";
 
Assignment of multiple variables at a time
S1 = s2 = "Same ";
 
Assign a value to an integer (convert an integer to a string)
Int I = 1234;
S = String. valueOf (I );
Or s = "" + I;
 
In Bash
Value assignment (=)
The value assignment operation is in the form of VAR = STRING
Remember: there cannot be spaces on both sides of the equal sign!
Advanced Bash-Scripting Guide: 4.2. Variable Assignment writes
=
The assignment operator (no space before and after)

Caution
Do not confuse this with = and-eq, which test, rather than assign!
Note that = can be either an assignment or a test operator, depending on context.
 
Assign a value to a String constant
Does not contain any special characters
Example: S1 = Literal
Example: S1 = 'literal'
Example: S1 = "Literal"
Example: S1 = $ 'literal'
 
Case with spaces
Example: S2 = Hello \ World
Example: S2 = 'Hello world'
Example: S2 = "Hello World"
Example: S2 = $ 'Hello world'
 
Case with single quotes
Example: S3 = Yes, \ I \'m \ Basher
Example: you cannot use single quotes.
Example: S3 = "Yes, I'm Basher"
Example: S3 = $ 'Yes, I \'m Basher'
 
Double quotation marks
Example: S4 = It \ is \ very \ "Stupid \".
Example: S4 = 'it is very "Stupid ".'
Example: S4 = "It is very \" Stupid \"."
Example: S4 = $ 'it is very "Stupid ".'
 
Multi-Line Text
Example: S5 = 'Hello
World'
Example: S5 = "Hello
World"
Example: S5 = $ 'Hello
World'
 
Value assigned to another variable
Common variables
Example: S2 = $ S1
Example: S2 =$ {S1}
 
Location parameters
Example: S3 = $1
Example: S3 =$ {1}
 
Assign a value to the command line to execute the output (replace the command)
Format: S2 = 'commandline'
Format: S2 = $ (commandline)
Format: S2 = "'commandline '"
Format: S2 = "$ (commandline )"
 
Create a script and output the Sha-Bang and Bash execution file paths, that is, the first line of the script.
[Root @ jfht ctmw] # echo '#! '$ (Which bash)> script. sh
[Root @ jfht ctmw] # cat script. sh
#! /Bin/bash
[Root @ jfht ctmw] #
 
Assign a value to a more complex string replacement result
Example: S1 = "Your account is $ {ACCOUNT }"
Example: S2 = "Current Working Directory is $ (pwd )"
 
Null String
Example: S =
 
Assign the same value to multiple variables at a time.
In Java, we can write a = B = "String ";
However, in Bash, we can only write A = $ B = "STRING"
 
Example: A = $ B = $ C
Assign variable C to B first, and then assign variable B to.
Note that it cannot be written as A = B = C, and the result of A = B = C is completely different.
 
Assign a value to the file content
Format: CONTENT = $ (cat filename)
Note: If the file specified by filename is a binary data file, the actual CONTENT may be inconsistent.
 
Text Files
[Root @ jfht ~] # Cat file.txt
1234
2345
4567
5678
[Root @ jfht ~] # Ls-l file.txt
-Rw-r -- 1 root 20 09-07 07:48 file.txt
[Root @ jfht ~] # S = $ (cat file.txt)
[Root @ jfht ~] # Echo $ S
1234 2345 4567 5678
[Root @ jfht ~] # Echo "$ S"
1234
2345
4567
5678
[Root @ jfht ~] # Echo $ {# S}
19
[Root @ jfht ~] #
The above $ {# S} is used to obtain the string length.
The length of a text file is inconsistent, but the string content is consistent.
 
Binary files
[Root @ smsgw root] # ls-l/usr/bin/ac
-Rwxr-xr-x 1 root 18452/usr/bin/ac
[Root @ smsgw root] # S = $ (cat/usr/bin/ac)
[Root @ smsgw root] # echo $ {# S}
13516
[Root @ smsgw root] # S = "$ (cat/usr/bin/ac )"
[Root @ smsgw root] # echo $ {# S}
13511
[Root @ smsgw root] #
The above $ {# S} is used to obtain the string length.
 
Q: Why is the content of the file loaded with cat different from that of the actual file?
 
Assign an integer
Since the variables in Shell are non-typed, all values can be directly assigned to the variables without any conversion means.
Format: STR = SOME_INTEGER
However, if a formula is assigned to a variable, it is not automatically calculated. You can use the following methods:
Format 1: STR = $ [EXPR]
Format 2: (STR = EXPR ))
Format 3: let STR = EXPR
Format 4: declare-I STR = EXPR
Format 5: STR = 'expr expr'
Format 6: STR = $ (expr EXPR)
 
[Root @ jfht ~] # STR = 1234
[Root @ jfht ~] # Echo $ STR
1234
[Root @ jfht ~] # STR = 1234 + 4321
[Root @ jfht ~] # Echo $ STR
1234 + 4321
Directly assigning the formula to the variable or the formula itself is not the calculation result.
[Root @ jfht ~] # STR = $[1234 + 4321]
[Root @ jfht ~] # Echo $ STR
5555
[Root @ jfht ~] # (STR = 1234 + 4321 ))
[Root @ jfht ~] # Echo $ STR
5555
[Root @ jfht ~] # Let STR = 1234 + 4321
[Root @ jfht ~] # Echo $ STR
5555
[Root @ jfht ~] # Declare-I STR = 1234 + 4321
[Root @ jfht ~] # Echo $ STR
5555
[Root @ jfht ~] # STR = 'expr 1234 + 4321'
[Root @ jfht ~] # Echo $ STR
5555
[Root @ jfht ~] # STR = $ (expr 1234 + 4321)
[Root @ jfht ~] # Echo $ STR
5555
[Root @ jfht ~] #

 

Author: "Bash @ Linux"