Shell script common function memo

bash common function memo

background

Occasionally I have to deal with shell scripts at work, but shell scripts are not frequently used tools, and the knowledge system of shell scripts itself is not systematic enough. Every time you use it, some basic usages need to be determined by google. Now, the common functions are specially organized into templates for future reference and use.

Basic Process Control

some instructions

The test command
The test command is used to evaluate an expression and return a boolean value. If the expression evaluation result is true, the "exit status" is 0, and if it is false, the exit status is a non-zero value. Use #? to view the "exit status"

test 1 -eq 1 
echo $?   # 显示为0  表示1 -eq 1执行结果为true

In addition, you can use [] instead of the test command [logical expression] Note: The logical expression and "[" "]" must have spaces such as

[ 1 -eq 1 ]

The test command usage restriction cannot have regular expression expansion

Integer String File Test

String judgment

Operator Operation Instructions

=	"string1" = "string2"	equal to operation
!=	"string1" != "string2"	not equal to operation
-z string	-z string	Check if the string length is equal to 0
-n string	-n string	Check if string length is not equal to 0

Integer comparison operations

Judgment statement

int1 –eq int2	Int1 is equal to int2
int1 - not int2	Int1 is not equal to int2
int1 –gt int2	Int1 is greater than int2
int1 –ge int2	Int1 is greater than or equal to int2
int1 –lt int2	Int1 is less than int2
int1 –the int2	Int1 is less than or equal to int2

file attribute judgment

Judgment statement

–b filename	Block special file
–c filename	Character special file
–d filename	Directory existence
–f filename	Regular file existence and not a directory
–g filename	Set-group-ID is set
–k filename	Sticky bit is set
–p filename	File is a named pipe
–r filename	File is readable
–s filename	File is nonzero size
–u filename	Set-user-ID bit is set
–w filename	File is writable
–x filename	File is executable

if elif else format

if format

if 逻辑表达式
then 
   执行语句
fi

There are two ways to write logical expressions with []
if [expression]

if [ 1 != 2]
  then
     echo "1 != 2"
  fi

Give an example of working day or weekend judgment

day_of_week=`date +"%u"`
echo "day_of_work=${day_of_week}"
if [ $day_of_week = 6 ] || [ $day_of_week = 7 ]; then
  echo "holiday"
else
   echo "workday"
fi

Note that the logical AND operation "||" is between two [ ] rather than expected

[ $day_of_week = 6 ||  $day_of_week = 7 ]

Use test if test expressions such as

if test 1 != 2
 then
   echo "1 != 2"
fi

if elif else format

if 条件表达式
 then
    执行语句
 elif 条件表达式
  then 
     执行语句
 else 
   执行语句
fi

code example

#!/bin/sh
current_hour=`date +%H`
echo "currnet time is $current_hour"  

if [ ${currnet_hour} -lt 12 ]; then
  echo "Good Morning"
elif [ ${currnet_hour} -lt 18 ]; then
  echo "Good Afternoon"
else 
 echo "Good evening"
fi

for loop

Format

for 变量名 in 列表
do
  命令
done

print string

for fruit in apple  orange banana
do
  echo $fruit
done

Display file contents by line

for line in `cat /etc/passwrd`
   echo $line
done

Difference between $* and $@

do while

while格式
while command 
do 
  command
done

example

num = 0
while [ $num -lt 10 ]
do 
   echo -n $num
  num = `expr $num + 1`
done 
echo "循环之后 num值是 $num "

util command

The util command format is the same as while but opposite to while, the loop will only be executed if the conditional expression is false, and the loop will be exited when the condition is non-zero.

util  条件表达式
do 
  command
done

shift command

Common operations

datetime operations

Get time date command ####Time format such as display the current date

date "+%Y%m%d"

%n : 下一行
%t : 跳格
%H : 小时(00..23)
%I : 小时(01..12)
%k : 小时(0..23)
%l : 小时(1..12)
%M : 分钟(00..59)
%p : 显示本地 AM 或 PM
%r : 直接显示时间 (12 小时制，格式为 hh:mm:ss [AP]M)
%s : 从 1970 年 1 月 1 日 00:00:00 UTC 到目前为止的秒数
%S : 秒(00..61)
%T : 直接显示时间 (24 小时制)
%X : 相当于 %H:%M:%S
%Z : 显示时区 %a : 星期几 (Sun..Sat)
%A : 星期几 (Sunday..Saturday)
%b : 月份 (Jan..Dec)
%B : 月份 (January..December)
%c : 直接显示日期与时间
%d : 日 (01..31)
%D : 直接显示日期 (mm/dd/yy)
%h : 同 %b
%j : 一年中的第几天 (001..366)
%m : 月份 (01..12)
%U : 一年中的第几周 (00..53) (以 Sunday 为一周的第一天的情形)
%w : 一周中的第几天 (0..6)
%W : Week of the year (00..53) (when Monday is the first day of the week)
%x : Display the date directly (mm/dd/yy)
%y : The last two digits of the year (00.99)
%Y : full year (0000..9999)

Time addition and subtraction

date +%Y%m%d //Display the current day, year, month, day date +%Y%m%d --date="+1 day" //Display the date of the next day
+%Y%m%d -- date="-1 day" //Display the date of the previous day
+%Y%m%d --date="-1 month" //Display the date of the previous month
+%Y%m%d --date ="+1 month" //Display the date of the next month
+%Y%m%d --date="-1 year" //Display the date of the previous year
+%Y%m%d --date ="+1 year" //Display the date of the next year

Or simpler date= date -d -${t}day '+%Y%m%d' // for t to be a few days ago

function

shell parameters

Variable meaning

$0	filename of the current script
$n	Arguments passed to scripts or functions. n is a number indicating the number of parameters. For example, the first argument is $1 and the second argument is $2.
$#	The number of arguments passed to the script or function.
$*	All arguments passed to the script or function.
$@	All arguments passed to the script or function. Slightly different from $* when enclosed in double quotes (" ")
$?	The exit status of the last command, or the return value of a function.
$$	Current shell process ID. For shell scripts, this is the process ID in which the scripts reside.

function definition

函数格式 function_name () { commands ; commands; } 将函数传入的第一个参数加一并返回

increment  () {
 sum = \`expr $1 + 1\`;
 return $sum;
}

执行函数 increment 2 查看函数执行结果 echo $?

函数参数和返回值 函数总结 shell脚本的函数不能定义形参，这是于通用编程语言的差别 函数内部的变量在函数外也可以返回 函数必须先定义再使用