Shell fundamentals-variables, judgments, loops
1. The basic format and variables of shell
# add an interpreter to the shell script #! / bin/bash
# print out hello world with script
#! / bin/bash
Echo "hello world"
Exit
Parameter: a parameter is an entity that stores real values, which is generally referenced in the script
# using variables to echo hello world, the following example echoes two hello world.
#! / bin/bash
A = "hello world"
Echo $a
Echo "hello world"
Exit
If something else is added after the parameter, you need to use {}
#! / bin/bash
A = "hello world"
Echo ${a}, i am coming
Exit
Echo result:
# calculate string length, hello world with spaces, a total of 11
2. Script exit
# the return value of a script that executes successfully is 0. If it is not successful, it is not 0.
[root@localhost script] # clear
[root@localhost script] # sh hello.sh
Hello world,i am coming
eleven
[root@localhost script] # echo $?
0
# determine whether there is a file in a directory. If so, echo file exsit. If not, return 1.
[root@localhost script] # cat test.sh
#! / bin/bash
Cd / home
If [- f file]; then
Echo "file exsit"
Else
Exit 2
Fi
[root@localhost script] # sh test.sh
[root@localhost script] # echo $?
two
[root@localhost script] #
3. Test statement, if statement
Whether the test-d xx directory exists
Whether the test-f xx file exists
Whether test-b xx is a block file
Is test-x xx an executable file
A-eq B judges whether they are equal or not.
A-ne B judgment is not equal
A-le B is less than or equal to
A-lt B less than
A-gt B is greater than
# if conditional judgment statement
If []; then
.
Else
.
Fi
# if statement nesting
If []; then
.
Elif []; then
.
Elif []; then
.
Else
.
Fi
# test gives an example to determine whether the test file exists, and if so, back up it. If the message is not echoed
# example: enter a value to determine which interval it is in. Less than 80, greater than 100, between 80 and 100, using if nesting!
[root@localhost script] # cat if.sh
#! / bin/bash
Read-p "plz input a $num:" num
If [$num-le 80]; then
Echo "num is less 80"
Elif [$num-ge 80] & & [$num-le 100]; then
Echo "num between 80 and 100"
Else
Echo "num is greater than 100"
Fi
4. Circular statements for, while
For var in $var1 $var2 $var3 var4... $var
Do
.
.
Done
# example: print 1 to 10 numbers
[root@localhost home] # cat for1.sh
#! / bin/bash
For var in {1..10}
Do
Echo "$var"
Sleep 2
Done
[root@localhost home] # sh for1.sh
one
two
three
four
five
six
seven
eight
nine
ten
[root@localhost home] #
# print box content for nesting
[root@localhost home] # sh for2.sh
*********
*********
*********
*********
*********
*********
*********
*********
*********
[root@localhost home] # cat for2.sh
#! / bin/bash
For ((iTun1)
