Shell programming process control

Branch statements

Single branch statement

if [ condition ];then
     Command line...
 fi

Example:

#!/bin/bash
 if [ -f "" ]; then
     echo "File exists"
 fi

Double branch statement

if [ condition ];then
     Command line...
 else
     Command line...
 fi

Example:

#!/bin/bash
 if [ -f "" ]; then
     echo "File exists"
 else
     echo "The file does not exist"
 fi

Multi-branch statement

if [ condition ];then
     Command line...
 elif [ condition ];then
     Command line...
 else
     Command line...
 fi

Example:

#!/bin/bash
 if [ -f "" ]; then
     echo "File exists"
 elif [ -d "/etc" ]; then
     echo "Directory exists"
 else
     echo "The file and the directory do not exist"
 fi

case statement

Similar to branch statements, case is generally used to implement scripts with multiple choices (menu selection)

grammar:

case $ variable in
     Mode 1)
         # Execute when matching pattern 1
         ;;
     Mode 2)
         # Execute when matching pattern 2
         ;;
     *)
         # Default
         ;;
 esac

Example:

#!/bin/bash
 read -p "Input yes/no: " answer
 case $answer in
     yes|Yes|YES)
         echo "You chose yes"
         ;;
     no|No|NO)
         echo "You chose no"
         ;;
     *)
         echo "Invalid input"
         ;;
 esac

Loop statement

for loop

grammar:

for variables in list; do
     # Circulation body
 done

Example:

#!/bin/bash
for num in {1..10}
do
  echo ${num}
done

While loop

grammar:

While [ condition ]
 do
     Order
 done

Violent loop syntax

While [ condition ]
 do
     Order
 done

Example: Use while loop to output 1..10

#!/bin/bash
num=1
while [ ${num} -le 10 ]
do
    echo ${num}
    let num++
done

Example: Calculate sums of 1-100 using while loop

#!/bin/bash
num=1
count=0
while [ ${num} -le 100 ]
do
    let count+=num
    let num++
done

echo ${count}

While reading the file

grammar:

while read variable
 do
     Commands/Content
 done <file path

Example:

#!/bin/bash
filename=/etc/passwd

while read line
do
    echo ${line}
done <${filename}

Loop control statement

break

break is used to terminate the current loop and will not continue to run the remaining loop.
Example:

#!/bin/bash
 for i in {1..10}; do
     if [ $i -eq 5 ]; then
         break
     fi
     echo "Current value: $i"
 done

continue

Continue is used to terminate this loop and enter the next loop
Example:

#!/bin/bash
 for i in {1..5}; do
     if [ $i -eq 3 ]; then
         Continue continue
     fi
     echo "Current value: $i"
 done