Shell variable operation ${} detailed usage

${} Basic function

In general $var And ${var} There is no difference , But with ${ } Will be more accurate to define the scope of variable name

[[email protected] ~]# A=Linux
[[email protected] ~]# echo $AB    # Said variable AB

[[email protected] ~]# echo ${A}B    # Said variable A Connected behind B
LinuxB

besides ${} There are many other functions as follows :

Declare test variables first :

file=/dir1/dir2/dir3/my.file.txt

Take substring and replace

Assign values to variables based on status

tips:

The above understanding lies in , You must distinguish unset And null And non-null These three assignment States . generally speaking , : And null of , If you don't bring : Words , null Unaffected , If : Zelian null Also affected .

• my.file.txt Before “-” It can be understood as “ No definition , Replace with word”;“+” It can be understood as “ Defined , Replace with word”.

• my.file.txt Before “?” It can be understood as “ Is the parameter defined , No definition , hold word When the error message is printed .”

${} String interception

Examples are as follows ：
command | explain | result
${file#*.} Take off the first one . And the string to the left of it file.txt

[[email protected] ~]# echo ${file#*.} 
file.txt

${file##*.} Take off the last one . And the string to the left of it txt

[[email protected] ~]# echo ${file##*.}
txt

${file%.*} Take off the last one . And the string to the right of it /dir1/dir2/dir3/my.file

[[email protected] ~]# echo ${file%.*}
/dir1/dir2/dir3/my.file

${file%%.*} Take off the first one . And the string to the right of it /dir1/dir2/dir3/my

[[email protected] ~]# echo ${file%%.*}
/dir1/dir2/dir3/my

The memory method is as follows ：

• # Take out the left side ( On the keyboard # stay $ On the left )
• % Take out the right side ( On the keyboard % stay $ To the right )
• A single sign is the minimum match ; The two symbols are the maximum match
• * Is used to match unwanted characters , That is, the part you want to remove
• Also specify the character separator , And * coordination , Decide which part to take

The above is an online explanation , Follow the example above , As if ${} It can only be delimited with a single character , You can actually use string , And it is not only able to match one side of the deleted delimited string , In fact, both sides can be matched and deleted at the same time （ But because there is a line beginning or end delimitation ,# The pattern specifies that one side of the beginning of the line must be deleted when matching the beginning of the line ,% Pattern homology , See the explanation below ）

Add ： Test found , Actually, here # and % It refers to the beginning or end of the matching string , Specify to match ^ perhaps $. Successive # and $, Similar to regular greedy matching and shortest matching ,* Equivalent to .* , Similar to shell Match pattern in command . ${} In fact, it matches strings in this non-standard regular way , Then intercept and delete .

Examples are as follows ：

${file##/*.fi} The longest match , But it doesn't match the later "." , Because what matches here is "/*.fi",.fi And .txt Mismatch , And the front /*
Equivalent to regular ^/*, You can match the front part of the string .

[[email protected] ~]# echo ${file##/*.fi}
le.txt

If replaced {file##d*.}, Because it matches the pattern at the beginning of the string , Ahead d* Equivalent to matching regular ^d*, If matching fails, the original string will be returned

[[email protected] ~]# echo ${file##d*.}
/dir1/dir2/dir3/my.file.txt

${file#*.t} The shortest match , The match here is ".*t", Can match .txt The front part , give the result as follows

[[email protected] ~]# echo ${file#*.*t}
xt

${file%dir*xt} Match the end of the line

[[email protected] ~]# echo ${file%dir*xt}
/dir1/dir2/

there * It's not necessary , No match can succeed

[[email protected] ~]# echo ${file%le.txt}
/dir1/dir2/dir3/my.fi

therefore ${} in # and $ It is to use a regular way to specify the match at the beginning or end of the line to match the deletion character

Array

A="a b c def"   #  Defining variables 
A=(a b c def)   #  Define an array 

• In fact, shell Medium variable A And array elements A[0] It is equivalent. （ A string is an array with only one element ）

After declaring variables separately , You can also take values in array

[[email protected] ~]# B="12 3 4"
[[email protected] ~]# echo ${B[*]}
12 3 4
[[email protected] ~]# echo ${#B[*]}
1
[[email protected] ~]# echo ${B[0]}
12 3 4