Conditional statements

Arithmetic comparison

Conditions are usually placed within enclosing square brackets. Be sure to note that there is a space between [ or ] and the operand . If you forget this space, the script will report an error. For example:
[ $var-eq 0 ] When var is equal to 0, return true; [ $var-ne 0 ] When var is non-zero, return true

Important operators:

grammatical role

-gt	more than the.
-lt	less than.
-give	greater than or equal to.
-the	less than or equal to
-eq	equal
-born	not equal to

Tests can be performed in combination with multiple conditions as follows:

[ $var1 -ne 0 -a $var2 -gt 2 ] # Use logical AND -a
[ $var -ne 0 -o $var2 -gt 2 ] # logical or -o

string comparison

When using string comparisons, it's best to use double square brackets , because sometimes a single square bracket can produce errors, so it's best to avoid them.
Two strings can be checked to see if they are the same.

grammatical role

[[ $str1 = $str2 ]]	Returns true when $str1 is equal to $str2. That is, $str1 and $str2 contain exactly the same text.
[[ $str1 == $str2 ]]	This is another way to check if strings are equal. It is also possible to check if two strings are different.
[[ $str1 != $str2 ]]	Returns true if $str1 and $str2 are not the same. We can also check the alphabetical order of the string as shown below.
[[ $str1 ＞ $str2 ]]	Returns true if $str1 is alphabetically greater than $str2.
[[ $str1 ＜ $str2 ]]	Returns true if $str1 is less alphabetically than $str2.
[[-z $ str1]]	Returns true if $str1 contains the empty string.
[[ -n $str1 ]]	Returns true if $str1 contains a non-empty string.

Multiple conditions can easily be combined using the logical operators && and || ; if [[ -n $str1 ]] && [[ -z $str2 ]], the semantics of not if [[ ! -n $str1 ] ] ; pay attention to spaces

File system

grammatical role

[ -f $file_var ]	Returns true if the given variable contains a normal file path or file name .
[ -d $var ]	Returns true if the given variable contains a directory.
[ -e $var ]	Returns true if the file contained in the given variable exists.
[ -w $var ]	Returns true if the file contained in the given variable is writable.
[ -r $var ]	Returns true if the file contained in the given variable is readable.
[ -c $var ]	Returns true if the given variable contains the path to a character device file.
[ -b $var ]	Returns true if the given variable contains the path to a block device file.
[-x $ var]	Returns true if the file contained in the given variable is executable.
[ -L $var ]	if the given variable contains a symbolic link

control statement

if statement

We can perform tests with if, if else, and logical operators, and compare data items with some comparison operators.

if [ condition]
then 
	commands
else
	commands
fi

It is worth noting that if then and if judgments are on the same line, you need to add ";"

for loop

Form 1

Values are separated by spaces

for variable in XXX 
	do 
		program 
	done 
# example 1 
for item in 1 2 3 
    do 
        echo $item 
    done	 
# example 2	 
for item in $(ls ~) 
    do 
        echo $item 
    done 
# example 3 
for variable in `seq 1 100` 
	do 
		program 
	done

Form 2

with seq

for ((initial value; control condition; variable change)) 
	do 
		program 
	done

while loop

while [conditional judgment] 
	do 
		program 
	done

with special symbols

symbolic role

‘’	apostrophe. All special symbols such as "$" and "`" (backticks) in single quotes have no special meaning.
“”	Double quotes. Special symbols in double quotes have no special meaning, but "$", "`" and "\" are exceptions, which have special meanings for "calling the value of a variable", "quoting a command" and "escape character".
``	backticks. The content enclosed in backticks is a system command, which is executed first in Bash. Same as $(), but $() is recommended because backticks are easy to read wrong.
$()	Like backticks, it is used to quote system commands.
#	In shell scripts, lines starting with # represent comments.
$	It is used to call the value of the variable. If you need to call the value of the variable name, you need to use $name to get the value of the variable.
\	转义符，跟在\之后的特殊符号将失去特殊含义，变为普通字符。如\$将输出“$”符号，而不当做是变量引用。

Process control based on linux bash

Conditional statements

Arithmetic comparison

string comparison

File system

control statement

if statement

for loop

Form 1

Form 2

while loop

with special symbols

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