Shell Program Design (2)

5 while
Number of cycles
Foo = 1 // initial cyclic values
While ["$ foo"-le 2] // le: <= while (FOO <= 2)
Do
Echo "Are you there"
Foo = $ ($ Foo + 1) // This form is very important to foo ++
Done

6
Until Condition
Do
Statements
Done
The until loop is executed repeatedly until the condition is true, while the while loop is executed repeatedly when the condition is true.

Until who | grep "$1">/dev/null
Do
Sleep 600
Done
Echo-E //
Echo "***** $1 has just logged in ****"
Exit 0
Input:/bash/SH yzj
Output:
-E
* *** Yaozhangjun has just logged in ****

If you comment out>/dev/null, yes
Yzj tty7 2009-10-19 (: 0)
Yzj pts/0 2009-10-19 10:00 (: 0.0)
-E
* *** Yaozhangjun has just logged in ****
The function of the/dev/null recycle bin is displayed.

7 case
Case variable in
Pattern [| pattern]...) statements ;;
...
Esac

In fact, the case in shell is the same as the switch-case in C.
In this example, strings are compared in order, instead of finding the best match.
This causes
Case "$ timeofday" in
*) Echo "sorry, not be recognized ";;
Yes) echo "good morning ";;
No) echo "Good afternoon ";;
Y) echo "good morning ";;
N) echo "Good afternoon ";;
No matter what you enter, the reason why sorry will be executed

Case "$ timeofday" in
Yes | Y | Yes | yes) echo "good morning"; // here | it is not a pipeline but a logic or. Keep the distance.
N * | N *) echo "Good afternoon"; // double quotation marks
*) Echo "sorry ";;

Case "$ timeofday" in
[YY] | [YY] [EE] [ss]) // pay attention to the distance. If it is [YY] [EE] [ss], it will not work. Why not, because I tried syntax error: Word unexpected (expecting ")")
Echo "good morning" // for multiple statements in one case, you only need to add;
Echo "up bright and early this morning ";;

2.6.4 Functions
1. No return value is returned when the function is declared.
2. When using a function, you can directly use the function name or directly add real parameters without brackets.
3. The local variable is declared as a local variable.
4. Use the return command to let the function return a number group. A common way for the function to return a string value is to let the function Save the string in a variable.
5 echo> like touch Fred, Fred creates an empty file.
6 While: implement infinite loop instead of while true
7 break: jump out of the current loop (the loop that executes the break statement)
8. Continue: jump out of this loop (the rest of the current loop is still executed)
9. Command
# Classic_set.sh

Version = classic
PS1 = "classic>"

# Lastest_set.sh

Version = lastest
PS1 = "lastest version>"

Execute Command
$ ../Classic_set.sh
Classic>
Classic>. lastest_set.sh
Lastest version> echo $ version
Lastest
Through this experiment, I think it seems that the command can be executed instead of/bin/sh.
10. In bash, echo-n "hello" is used to delete line breaks.
11 Eval: allows evaluation of parameters
Foo = 10
X = foo
Y = '$ x
Echo $ Foo 10
Echo $ X foo
Echo $ y $ foo
While
Foo = 10
X = foo
Eval y = '$ X has the same functions as y =$ ($ X ).
Echo $ Foo 10
Echo $ X foo
Echo $ y 10
12 exit n
The exit command causes the script program to end with exit code n. 0 indicates success
13 Export
The Export command exports the variable used as its parameter to the sub-shell and stores it in the sub-shellzhong youxiao. export creates its own parameter as an environment variable, which can be seen by other scripts and programs called by the current program.

# Export2
Echo "$ foo"
Echo "$ bar"

# Export1
Foo = "the first meta-syntactic variable"
Export bar = "the second meta-syntactic varibale"
Export2

Run $/bin/sh export2 // The Wishing book uses export1. After the execution, the export2 not found is displayed.
// Empty rows
The second meta-syntactic varibale // indicates that the bar value is imported to exprot2, but Foo does not

E1 E2 E3
|
|
|
E2 E3
The bar value remains unchanged until E3, But it is strange that

E1 E2 E3
|
|
|
E2 E2

When, even if the bar values in E1 and E3 are different, E2 still outputs
Set-A or set-allexport will export all the variables declared after it

14 set: Set the parameter variables for shell. A field in the output result must be used.
Set $ (date)
Echo the month is $2 // equivalent to echo $ (date + % B)
Set is used to extract location parameters.

15 shift: Left all parameter variables to the left, $3-> $2. $4-> $3, and the original value of $1 will be discarded.

While ["$1 "! = ""]; Do
Echo "$1"
Shift
Done
Run:
$/Bin/sh shift1 A B C E
A
B
C
E

16 trap: the action taken after receiving the signal. When the script program is interrupted, the cleaning is completed.
Trap Command Signal
Command: The action to take when the specified quotation mark is received.
Signal: processing signal name
Role of command
-Reset the processing condition of a signal to the default value.
''Ignore a signal

The following example has already scared me.

Trap 'rm-F/tmp/my_tmp_file _ $ 'int

Echo creating file/tmp/my_tmp_file _ $
Date>/tmp/my_tmp_file _ $
Echo "Press interrupt to interrupt ..."
While [-F/tmp/my_tmp_file _ $]; do
ECHO file exists
Sleep 1
Done
Echo the file no longer exists
When:/bin/sh trap1 is executed
1 Creating file/tmp/my_tmp_file _ $ ($ indicates the number of generated files)
2. Write the current time
3 Press interrupt to interrupt...
4 file exists (occurs once every 1 minute)
5 Ctrl + c
6. Execute int and delete/tmp/my_tmp_file _ $.