Linux shell array Deep learning understanding _linux shell

The bash shell supports only one-dimensional arrays, but there are no restrictions on the number of arguments.

Declare an array:
Declare-a Array
(In fact, you don't have to declare that you can assign a value directly to a variable in an array, and bash knows that it's an array)

Array Assignment:
(1) array= (var1 var2 var3 ... varn)
(2) array= ([0]=VAR1 [1]=var2] [2]=var3 ... [N]=varn)
(3) Array[0]=var1
Arrya[1]=var2
...
Array[n]=varn

To calculate the number of array elements:
${#array [@]} or ${#array [*]}

Bash's special parameters @ and * All represent the "extended positional argument, starting at 1", but the form is slightly different, but it seems to be common to use in arrays.

Referencing arrays:

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Echo ${array[n]}

To traverse an array:

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Filename= (' ls ')
For Var in ${filename[@]};d o
Echo $var
Done

Here are some small examples of shell arrays.

1, from "standard input" read n times string, each input string stored in array

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#!/bin/bash
I=0
N=5
While ["$i"-lt $n]; Todo
echo "Please input strings ... ' expr $i + 1 '"
Read array[$i]
b=${array[$i]}
echo "$b"
i= ' expr $i + 1 '
Done

2, put the letters in the string into the array, and output to the "standard output"

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#!/bin/bash
chars= ' ABCDEFGHIJKLMNOPQRSTUVWXYZ '
for ((i=0; i<26; i++)); Todo
array[$i]=${chars: $i: 1}
echo ${array[$i]}
Done

What's interesting here is ${chars: $i: 1}, which means to get 1 characters starting from the $i position of the chars string. If you change 1 to 3, you get 3 characters ~ The result is:
Abc
Bcd
...
Vxy
Xyz
YZ//There is not enough string to get the
z//Not enough string to get the

Here are some examples of applying arrays to shell environment variables.

3. Apply the array to the shell environment variable (1)

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Array Assignment:
[Root@pps ~]# season= ("srping" "Summer" "Autumn" "Winter")
When you find that the assignment is wrong, you can also immediately correct the new assignment, as Spring above is written as srping.
Re-assignment: (The original value is overridden)
[Root@pps ~]# season= ("Spring" "Summer" "Autumn" "Winter")

Check out the environment variables:
[Root@pps ~]# Set | grep SEASON
season= ([0]= "Spring" [1]= "Summer" [2]= "Autumn" [3]= "Winter")

Display the entire array:
[Root@pps ~]# echo ${season[*]} or echo ${season[@]}
Spring Summer Autumn Winter

To display an array element:
[Root@pps ~]# echo ${season[3]}
Winter

To assign a value to a single array element:
[Root@pps ~]# season[0]= "New_spring"

Look at the array again:
[Root@pps ~]# echo ${season[*]}
New_spring Summer Autumn Winter

Clears the specified single array element:
[Root@pps ~]# unset season[2]

Clear the entire array:
[Root@pps ~]# unset SEASON

4. Apply the array to the shell environment variable (2)
To convert a carriage return in a file to a space using the TR command:

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[Root@pps ~]# Cat/etc/shells | TR "\ n" "" ">/tmp/tmp.file

To assign the contents of a file to an array: (before the first return character is encountered)

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[Root@pps ~]# read-a SHELLS </tmp/tmp.file

To view array assignments:

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[Root@pps ~]# Set | grep "SHELLS"
shells= ([0]= "/bin/sh" [1]= "/bin/bash" [2]=]/sbin/nologin [3]= "/bin/tcsh" [4]= "/bin/csh" [5]= "/bin/ksh"]

You can later apply this array environment variable to other shell scripts or applications.

Summary: With these content, later in the Shell programming, the logarithmic group, should not be what the problem, you did it?!