01. Variables
1. Environmental variables
echo $PATH
2. Custom variable
hello="hello_world"
echo $hello
3. Store Linux command execution results as variables
(two methods are recommended. The second one is recommended. The first one is that the oblique point on the ~ key is difficult to identify)
files=ls -al
path=(pwd) note that there can be no spaces on both sides of the variable = sign. Variables need to be signed
02. Basic operation
1. Operator
+: add
-: minus
*: multiply
/: except
%: residual
==: judge whether it is equal
!=: Not equal to
: greater than =: Greater than or equal to <: less than <=: Less than or equal to
2. Integer operation (expr)
expr 10 + 3 echo $[10 + 3] There are two ways to store the calculation results as variables num=$(expr 10 + 3) num=`expr 10 + 3`
Attention
Operators must be preceded by spaces
*Multiply must be escaped with a backslash
3. Integer operation ($[])
num1=100 num2=200 sum_num=$[$num1+num2]
Attention
No spaces are allowed before and after operators
*Multiplication does not require an escape character
4. Floating point operation
num=$(echo "scale=2;10/3" | bc)
scale: number of decimal places reserved
Give 10 / 3 to bc calculator and keep two decimal places
03. Condition selection
If is followed by a command. In fact, it is the exit status code of the command. Normal exit, status code 0, and others are not 0. This means that if it is 0, execute then, otherwise do not execute
1,
if-then if command then "Successful execution" fi
2,
if-then-else
3,
if-then-elif-else if Judgment basic grammar,if start, fi ending
#/bin/bash NUM='4' if (( $NUM > 4 )) then echo "$NUM more then 4" elif (( $NUM == 4 )) then echo "$NUM Equal to 4" else echo "$NUM less then 4" fi
4. case statement
case $num in 1) echo "num=1" 2) echo "num=2" 3) echo "num=3" esac
04. Cycle
1,for - in
for i in list do commands done
2. C language style
for (( i = 0;i <= 10;i++)) do commands done
3. while loop
while test command do echo "Perform the operation here when the conditions are met" done
num=8 while (( $num < 10 )) do echo "Perform the operation here when the conditions are met" echo "$num Less than 10" done
4. until loop
until test command do echo "Execute when conditions are not met " done
explain:
1. If condition is a conditional expression and the value is false, continue to execute the statements in the loop body, otherwise jump out of the loop
2. The until loop is handled in the opposite way to the while loop
3. Control cycle
break jumps out of the outer loop, the same as python
continue terminates the current loop and enters the next loop, the same as python
while (( 1>0 )) do echo –n ""Enter a number" read num if (( $num>10 )) then break else continue fi done
05. Command line parameter processing
The bash shell can get parameters based on their location
Get the 1st to 9th command line parameters through $1 to $9
$0 is the shell name. If there are more than 9 parameters, you can get them through
06. Get user input (read)
Single input, specifying that the variable receives the input value (choice)
echo -n "yes or no(y/n)" read choice echo "you choice is : $choice"
For a single input, no variable is specified to receive the input value. read will put any data it receives into the special environment variable REPLY
echo -n "yes or no(y/n)" read echo "you choice is : $REPLY"
Multiple inputs
read -p "what is you name?" first second echo first:$first echo second:$second
The above example will first output what is you name? Then wait for user input in the line. read -p here realizes the effect of echo -n + read without line break in the above example. The input parameters are separated by spaces. If the input value exceeds the number of received variables, the shell will assign the remaining values to the last variable.
07. Timeout setting
if read -t 5 -p "enter you name:" name then echo "hello $name" else echo "time out" fi
Attention
1. Variable names are generally capitalized
2. Numerical comparison ((8 > 7))
3. String comparison [[STR1! = STR2]]
4. The test command can only judge the following three types of conditions
Numerical comparison string comparison File comparison
5. The flow control statement and end statement of 5 shell script are written in reverse of the start statement, such as if end statement fi and case end statement esac
Numerical comparison string comparison File comparison
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