1. Shell Loop
1.1 The while loop
Syntax:

 while condition  
 do  
   Statement(s) to be executed if condition is true  
 done  

or

 while condition; do  
   Statement(s) to be executed if condition is true  
 done  

Ex:

 #!/bin/sh  
   
 a=0  
   
 while [ $a -lt 10 ]  
 do  
   echo $a  
   a=`expr $a + 1`  
 done  

 $./test.sh  
 0  
 1  
 2  
 3  
 4  
 5  
 6  
 7  
 8  
 9  

1.2 The for loop
Syntax:

 for var in word1 word2 ... wordN  
 do  
   Statement(s) to be executed for every word.  
 done  

Trong đó var là biến thay đổi, nó sẽ lần lượt được gán các giá trị từ word1 -> wordN.

Ex1:

 #!/bin/sh  
   
 for var in 0 1 2 3 4 5 6 7 8 9  
 do  
   echo $var  
 done  

 $./test.sh  
 0  
 1  
 2  
 3  
 4  
 5  
 6  
 7  
 8  
 9  

Ex2:

 #!/bin/sh  
   
 for FILE in $HOME/.bash*  
 do  
   echo $FILE  
 done  

 $./test.sh  
 /root/.bash_history  
 /root/.bash_logout  
 /root/.bash_profile  
 /root/.bashrc  

1.3 The until loop
Syntax:

 until condition  
 do  
   Statement(s) to be executed until condition is true  
 done  

 #!/bin/sh  
   
 a=0  
   
 until [ ! $a -lt 10 ]  
 do  
   echo $a  
   a=`expr $a + 1`  
 done  

 $./test.sh  
 0  
 1  
 2  
 3  
 4  
 5  
 6  
 7  
 8  
 9  

1.4 The select loop
Syntax:

 select var in word1 word2 ... wordN  
 do  
   Statement(s) to be executed for every word.  
 done  

Ex:

 #!/bin/ksh  
   
 select DRINK in tea cofee water juice appe all none  
 do  
   case $DRINK in  
    tea|cofee|water|all)   
      echo "Go to canteen"  
      ;;  
    juice|appe)  
      echo "Available at home"  
    ;;  
    none)   
      break   
    ;;  
    *) echo "ERROR: Invalid selection"   
    ;;  
   esac  
 done  

 $./test.sh  
 1) tea  
 2) cofee  
 3) water  
 4) juice  
 5) appe  
 6) all  
 7) none  
 #? juice  
 Available at home  
 #? none  
 $  

2. Nesting Loops (Vòng lặp lồng nhau)
Syntax:

 while condition1 ; # this is loop1, the outer loop  
 do  
   Statement(s) to be executed if condition1 is true  
   
   while condition2 ; # this is loop2, the inner loop  
   do  
    Statement(s) to be executed if condition2 is true  
   done  
   
   Statement(s) to be executed if condition1 is true  
 done  

Ex:

 #!/bin/sh  
   
 a=0  
 while [ "$a" -lt 10 ]  # this is loop1  
 do  
   b="$a"  
   while [ "$b" -ge 0 ] # this is loop2  
   do  
    echo -n "$b "  
    b=`expr $b - 1`  
   done  
   echo  
   a=`expr $a + 1`  
 done  

 $./test.sh  
 0  
 1 0  
 2 1 0  
 3 2 1 0  
 4 3 2 1 0  
 5 4 3 2 1 0  
 6 5 4 3 2 1 0  
 7 6 5 4 3 2 1 0  
 8 7 6 5 4 3 2 1 0  
 9 8 7 6 5 4 3 2 1 0