How to Iterate with Bash "for" Loop

System administrators often encounter repetitive tasks in their day-to-day activities. Repetitive tasks can take the form of executing an action multiple times on a target, such as checking a process every minute for 10 minutes to see if it has completed. Task repetition can also take the form of executing an action a single time across multiple targets, such as performing a database backup of each database on a system. The for loop is one of the multiple shell looping constructs offered by Bash and can be used for task iterations.

Using the for loop

Bash’s for-loop construct uses the following syntax. The loop processes the items provided in [LIST] in order one by one and exits after processing the last item on the list. Each item in the list is temporarily stored as the value of [VARIABLE], while the for loop executes the block of commands contained in its construct. The naming of the variable is arbitrary. Typically, the variable value is referenced by commands in the command block.

for <VARIABLE> in <LIST>;
     do
     <COMMAND>
     ...
     <COMMAND> referencing <VARIABLE>
done

The list of items provided to a for loop can be supplied in several ways. It can be a list of items entered directly by the user or be generated from different types of shell expansion, such as variable expansion, brace expansion, filename expansion, and command substitution. Some examples that demonstrate the different ways lists can be provided to for loops follow.

$ for HOST in host1 host2 host3
    do
    echo $HOST
done

Output:

host1
host2
host3

$ for HOST in host{1,2,3}; do echo $HOST; done
host1
host2
host3

$ for HOST in host{1..3}; do echo $HOST; done
host1
host2
host3

$ for FILE in file*; do ls $FILE; done
filea
fileb
filec

$ for FILE in file{a..c}; do ls $FILE; done
filea
fileb
filec

$ for PACKAGE in $(rpm -qa | grep kernel); do echo "$PACKAGE was installed on $(date -d @$(rpm -q --qf "%{INSTALLTIME}\n" $PACKAGE))"; done abrt-addon-kerneloops-2.1.11-12.el7.x86_64 was installed on Tue Apr 22 00:09:07 EDT 2014
kernel-3.10.0-121.el7.x86_64 was installed on Thu Apr 10 15:27:52 EDT 2014
kernel-tools-3.10.0-121.el7.x86_64 was installed on Thu Apr 10 15:28:01 EDT 2014
kernel-tools-libs-3.10.0-121.el7.x86_64 was installed on Thu Apr 10 15:26:22 EDT 2014

$ for EVEN in $(seq 2 2 8); do echo "$EVEN"; done; echo "Who do we appreciate?"
2
4
6
8
Who do we appreciate?