SHELL arithmetic operations and Comparison

Arithmetic Operations and Comparison

1. Operator (let identifier)

+ Addition-subtraction * multiplication/division ** power operation 3 let "z = 5 ** 3" % modulo bash $ expr 5% 3

2. comparison operator (note that a space is required between [] and the variable "") integer comparison operator-eq is equal to, for example: if ["$ a"-eq "$ B"] // note Space

-Ne is not equal to, for example, if ["$ a"-ne "$ B"]-gt is greater than, for example: if ["$ a"-gt "$ B"]-ge is greater than or equal to, for example, if ["$ a"-ge "$ B"]-lt is less than, for example: if ["$ a"-lt "$ B"]-le is less than or equal to, for example: if ["$ a"-le "$ B"] <less than (double parentheses are required), such as: ("$ a" <"$ B ")) <= less than or equal to (double parentheses required), such as: ("$ a" <= "$ B")> greater than (double parentheses required ), for example: ("$ a"> "$ B")> = equal to or greater than (double parentheses are required), for example: ("$ a"> = "$ B ")) string comparison = equals, such as: if ["$ a" = "$ B"] = equals, such as: if ["$ a" = "$ B"], equivalent to = Note: The = function has different behaviors in [[] and [], as shown below: 1 [[$ a = z *] # If $ a starts with "z" (pattern matching) True 2 [[[$ a = "z *"] # If $ a is equal to z * (character matching ), the result is true 3 4 [$ a = z *] # File globbing and word splitting will occur 5 ["$ a" = "z *"] # If $ equal to z * (character matching ), the result is true. File globbing is a stenographer about a File, for example "*. c "is, like ~ Yes. But file globbing is not a strict regular expression. Although the structure is similar in most cases .! = Not equal to, such as: if ["$ "! = "$ B"] This operator uses pattern matching in the [[] structure. <less than, in the ASCII alphabetic order. for example, if [["$ a" <"$ B"] if ["$ a" \ <"$ B"] Note: in the [] structure, "<" needs to be escaped.> greater than, in the ASCII alphabetic order. for example, if [["$ a"> "$ B"] if ["$ a" \> "$ B"] Note: in the [] structure, ">" must be escaped. for more information, see Example 26-11. -The z string is "null ". the length is 0. -n string is not "null" Note: To use-n to test in the [] structure, you must use "" to cause the variable. use a string that is not! -Z or a string that is not referenced by "", put it in the [] structure (see Example 7-6). Although it can usually work, it is not safe. it is a good habit to use "" To test strings.

3. eg

#./Bin/bash
# Obtain the digit month corresponding to the English month
Getmonthofenglish ()
{
Case "$1" in
"Jan") msg_month = 1 ;;
"Feb") msg_month = 2 ;;
"Mar") msg_month = 3 ;;
"Apr") msg_month = 4 ;;
"May") msg_month = 5 ;;
"Jun") msg_month = 6 ;;
"Jul") msg_month = 7 ;;
"Aug") msg_month = 8 ;;
"Sept") msg_month = 9 ;;
"Oct") msg_month = 10 ;;
"Nov") msg_month = 11 ;;
"Dec") msg_month = 12 ;;
Esac
}
# Determine whether there are expired SMS messages and call C to delete the SMS function
Del_overtime_sms ()
{
Num = 'ipcs-q | grep "$1" | awk '{print $6 }''
If ("$ num"> "1"); then
Ipcs-q | grep "$1" | awk '{print $2, $6}'> overtimesms. ini
./Del_overtime_sms // result of Calling C function in shell
Else
Echo "No expired SMS messages in the message queue"
Fi
}
# Obtain the number of days for the current month
Cd/mas/shell/zhangweiatest/clear_overtime_sms
Mday = './getmonthofday'
Echo "the number of days corresponding to the previous month in the current month = $ mday"

# Obtain the year and month corresponding to the system time
Buff = 'date | awk' {print $2, $3 }''
Now_month =$ {buff: 0: 1}
Now_day =$ {buff: 3: 1}

# Obtain the survival time of the current queue
Ipcs-qt | awk '{print $1, $3, $4}'> msglivelytime. ini
Sed-n'4, $ P' msglivelytime. ini> msglivelytime1.ini // gets the characters starting from the fourth row to the end.
Mv msglivelytime1.ini msglivelytime. ini

While read msgid msg_month msg_day
Do
Echo
Echo "Message Queue ID = $ msgid created at: $ msg_month $ msg_day"
Echo "the system time is the $ now_day day of $ now_month"
Getmonthofenglish "$ msg_month"
Let "new_month = $ msg_month + 1"
If ["$ msg_month" = "Not"]; then // NULL
Echo "queue security"
Elif ["$ msg_month" = ""]; then // empty row
Echo "NULL"
Elif ["$ msg_month" = "$ now_month"]; then
Let "msg_day1 = $ msg_day + 2"
If ("$ now_day"> = "$ msg_day1"); then
If ("$ msg_day" <"$ mday"); then
Echo "insecure message queue"
Del_overtime_sms "$ msgid"
Fi
Fi
Elif ["$ now_month" = "$ new_month"]; then
Let"Now_day1 = $ now_day + $ mday"
Let "msg_day1 = $ msg_day + 2"
Echo "now_day1 = $ now_day1, msg_day1 = $ msg_day1"
If ("$ now_day1"> = "$ msg_day1"); then
# If ("$ msg_day1" <"$ mday"); then
Echo "insecure message queue"
Del_overtime_sms "$ msgid"
# Fi
Fi
Elif ("$ now_month"> "$ new_month"); then
Echo "" $ msg_month ">" $ now_month ""
Del_overtime_sms "$ msgid"
Echo "insecure message queue"
Else
Echo "msg queue safe"
Fi
Done <msglivelytime. ini