for loops

All our looping controls are simple and we will begin by looking at for loops. The word for is a keyword in bash and in terms of its working, it is similar to if. We can use the command type to verify this, as shown in the following example:

$ type for
for is a shell keyword

As a reserved shell keyword, we can use a for loop both in scripts and directly at the command line. In this way, we can utilize loops within and without the scripts, optimizing the use of the command line. A simple for loop is shown in the following example code:

# for u in bob joe ; do 
useradd $u 
echo '$u:Password1' | chpasswd  #pipe the created user to chpasswd 
passwd -e $u 
done 

The useradd command is used to create users and the chpasswd command is used to update passwords in batch mode.

Within a for loop, we read from the list on the right to populate the variable parameter on the left; in this case, we will read from the list containing bob and joe into the parameter variable u. Each item from the list...