Shell notes: for, while and until loops

One, for loop

Grammar one

for variable in value 1 value 2 value 3 ...
     do 
        program 
    done

Note: As long as there is "empty" between multiple values, not just spaces, line breaks, tabs, etc., for example, when reading a file, you can automatically traverse each line.

Example 1: Traversing certain items

#!/bin/bash

for time in morning noon afternoon evening
    do
        echo "This time is $time !"
    done

 

Example 2: Iterating over the values ​​in a variable

#! / bin / bash 

#Unzip all tar.gz compressed packages in the lamp directory 
cd / lamp
 ls *. tar .gz> ls .log
 for i in $ ( cat  ls .log)
     do 
        tar -zxf $ i $> / dev / null 
    done 
rm -rf / lamp / ls .log

 

Grammar 2

for ((initial value; loop control condition; variable change))
     do 
        program 
    done

Note: The execution flow is: first run the "initial value" statement, then execute the loop body once, then execute the "variable change" statement, and then judge whether it meets the "loop control condition", if it does, continue execution, otherwise exit the for loop.

Examples:

#!/bin/bash

s=0
for (( i=1;i<=100;i=i+1 ))
    do
        s=$(( $s+$i ))
    done
echo "The sum of 1+2+...+100 is: $s"

 

 

Two, while loop

grammar:

while [conditional judgment]
     do 
        program 
    done

As long as the judgment formula holds, the loop will continue to run.

The order of execution is to first determine whether the "conditional judgment formula" is true. If it is true, the loop body is executed. After the execution, the "conditional judgment formula" is judged again, and so on, until the "conditional judgment formula" is not established.

Examples:

#!/bin/bash

i=1
s=0
while [ $i -le 100 ]
    do
        s=$(( $s+$i ))
        i=$(( $i+1 ))
    done
echo "The sum is: $s"

 

 

Three, until cycle

grammar:

until [condition judgment type]
     do 
        program 
    done

 

Contrary to the while loop, when the conditional judgment is established, the loop is exited.

The order of execution is to first determine whether the "conditional judgment formula" is true. If not, execute the loop body. After the execution, judge the "conditional judgment formula" again, and so on, until the "conditional judgment formula" is established.

Examples:

#!/bin/bash

i=1
s=0
while [ $i -gt 100 ]
    do
        s=$(( $s+$i ))
        i=$(( $i+1 ))
    done
echo "The sum is: $s"