Basic Shell Programming __ programming

if else and greater than, less than, equal to logical expressionIf ...; Then
....
Elif ...; Then
....
Else
....
fi [F "somefile"]: judge whether it is a file
[-X "/bin/ls"]: Determine if/bin/ls exists and has executable permissions
[-N ' $var]: Determine if the $var variable has a value
["$a" = "$b"]: Determine if $a and $b are equal
-r file user readable as True
-W file user can write as true
-X file user can execute as true
-F file is true for regular files
-D file is true for directory
-C file is true for character special files
-B file is a block special file is True
-S file file non-0 o'clock is true
-T file True when the device specified by the file descriptor (default is 1) is a terminal
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Integer comparisons
-eq equals, such as: if ["$a"-eq "$b"]
-ne is not equal to, such as: if ["$a"-ne "$b"]
-GT is greater than, such as: if ["$a"-gt "$b"]
-ge is greater than or equal to, such as: if ["$a"-ge "$b"]
-lt is less than, such as: if ["$a"-lt "$b"]
-le is less than or equal, such as: if ["$a"-le "$b"]

< less than (requires double brackets), such as: (("$a" < "$b")
<= is less than or equal (requires double parenthesis), such as: (("$a" <= "$b")
> Greater than (requires double brackets), such as: (("$a" > "$b")
>= is greater than or equal (requires double parenthesis), such as: (("$a" >= "$b")

String comparisons

= equals, such as: if ["$a" = "$b"]
= = equals, such as: if ["$a" = = "$b"], and = equivalent

Note: the behavior in [[]] and [] is different in some cases:

[[$a = = z*]] # If $a starts with "Z" (pattern match) then it will be true
[[$a = = "z*"]] # if $a equals z* (character match), then the result is true

[$a = = z*] # File globbing and word splitting will occur
["$a" = "z*"] # if $a equals z* (character match), then the result is true

For i in {1..10}
Todo
Echo $i
Done

#详细列出 (not many characters and few items)
For File in 1 2 3 4 5
Todo
Echo $File
Done

#对存在的文件进行循环
For shname in ' LS *.sh '
Todo
Name= ' echo ' $shname | Awk-f. ' {print '} '
Echo $name
Done

#查找循环 (LS data is too large to use this method)
For Shname in ' find. -type f-name "*.sh"
Todo
Name= ' echo ' $shname | awk-f/' {print $} '
Echo $name
Done

# (Grammar loop--a bit like C syntax, but remember double bracket
For ((i=1;i<100;i++))
Todo
if ((i%3==0))
Then
Echo $i
Continue
Fi
Done

#seq形式 starting from 1
For i in ' SEQ 100 '
Todo
if ((i%3==0))
Then
Echo $i
Continue
Fi
Done

#while循环注意为方括号 [], and note that spaces
Min=1
max=100
While [$min-le $max]
Todo
Echo $min
min= ' expr $min + 1 '
Done

#双括号形式, the internal structure is somewhat like the syntax of C, note assignment: i=$ (($i + 1))
I=1
while (($i <100))
Todo
if (($i%4==0))
Then
Echo $i
Fi
i=$ (($i + 1))
Done