How to Use For Loop in Bash Script

Using for loop to iterate commands

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 once across multiple targets, such as backing up 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.

Processing Items from the Command Line

Bash’s for loop construct uses the following syntax.

for VARIABLE in LIST
do
    COMMAND VARIABLE
done

The loop processes the strings provided in LIST in order one by one and exits after processing the last string in the list. Each string 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.

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

[user@host ~]$ for HOST in host1 host2 host3; do echo $HOST; done
host1
host2
host3

[user@host ~]$ for HOST in host{1,2,3}; do echo $HOST; done
host1
host2
host3

[user@host ~]$ for HOST in host{1..3}; do echo $HOST; done
host1
host2
host3

[user@host ~]$ for FILE in file*; do ls $FILE; done
filea
fileb
filec

[user@host ~]$ for FILE in file{a..c}; do ls $FILE; done
filea
fileb
filec

[user@host ~]$ 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

[user@host ~]$ for EVEN in $(seq 2 2 10); do echo "$EVEN"; done
2
4
6
8
10