Red Hat System Administration II

In Bash, the for loop construct uses the following syntax:

for VARIABLE in LIST; do
COMMAND VARIABLE
done

The loop processes the strings that you provide in LIST and exits after processing the last string in the list. The for loop temporarily stores each list string as the value of VARIABLE, and then executes the block of commands that use the variable. The variable name is arbitrary. Typically, you reference the variable value with commands in the command block.

Provide the list of strings for the for loop from a list that the user enters directly, or that is generated from shell expansion, such as variable, brace, or file name expansion, or command substitution.

These examples demonstrate different ways to provide strings to for loops:

[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{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" $PACKAGE))"; done
kernel-tools-libs-5.14.0-70.2.1.el9_0.x86_64 was installed on Thu Mar 24 10:52:40 PM EDT 2022
kernel-tools-5.14.0-70.2.1.el9_0.x86_64 was installed on Thu Mar 24 10:52:40 PM EDT 2022
kernel-core-5.14.0-70.2.1.el9_0.x86_64 was installed on Thu Mar 24 10:52:46 PM EDT 2022
kernel-modules-5.14.0-70.2.1.el9_0.x86_64 was installed on Thu Mar 24 10:52:47 PM EDT 2022
kernel-5.14.0-70.2.1.el9_0.x86_64 was installed on Thu Mar 24 10:53:04 PM EDT 2022
[user@host ~]$ for EVEN in $(seq 2 2 10); do echo "$EVEN"; done
2
4
6
8
10

The simplest conditional structure is the if/then construct, with the following syntax:

if <CONDITION>; then
      <STATEMENT>
      ...
      <STATEMENT>
fi

With this construct, if the script meets the given condition, then it executes the code in the statement block. It does not act if the given condition is not met. Common test conditions in the if/then statements include the previously discussed numeric, string, and file tests. The fi statement at the end closes the if/then construct. The following code section demonstrates an if/then construct to start the psacct service if it is not active:

[user@host ~]$ systemctl is-active psacct > /dev/null 2>&1
[user@host ~]$ if  [[ $? -ne 0 ]]; then sudo systemctl start psacct; fi

You can further expand the if/then construct to take different sets of actions depending on whether a condition is met. Use the if/then/else construct to accomplish this behavior, as in this example:

if <CONDITION>; then
      <STATEMENT>
      ...
      <STATEMENT>
else
      <STATEMENT>
      ...
      <STATEMENT>
fi

The following code section demonstrates an if/then/else statement to start the psacct service if it is not active, and to stop it if it is active:

[user@host ~]$ systemctl is-active psacct > /dev/null 2>&1
[user@host ~]$ if  [[ $? -ne 0 ]]; then \
sudo systemctl start psacct; \
else \
sudo systemctl stop psacct; \
fi

Expand an if/then/else construct to test more than one condition and to execute a different set of actions when it meets a specific condition. The next example shows the construct for an added condition:

if <CONDITION>; then
      <STATEMENT>
      ...
      <STATEMENT>
elif <CONDITION>; then
      <STATEMENT>
      ...
      <STATEMENT>
else
      <STATEMENT>
      ...
      <STATEMENT>
fi

In this conditional structure, Bash tests the conditions as they are ordered in the script. When a condition is true, Bash executes the actions that are associated with the condition and then skips the remainder of the conditional structure. If none of the conditions are true, then Bash executes the actions in the else clause.

The following example demonstrates an if/then/elif/then/else statement to run the mysql client if the mariadb service is active, or to run the psql client if the postgresql service is active, or to run the sqlite3 client if both the mariadb and the postgresql service are inactive:

[user@host ~]$ systemctl is-active mariadb > /dev/null 2>&1
[user@host ~]$ MARIADB_ACTIVE=$?
[user@host ~]$ sudo systemctl is-active postgresql > /dev/null 2>&1
[user@host ~]$ POSTGRESQL_ACTIVE=$?
[user@host ~]$ if  [[ "$MARIADB_ACTIVE" -eq 0 ]]; then \
mysql; \
elif  [[ "$POSTGRESQL_ACTIVE" -eq 0 ]]; then \
psql; \
else \
sqlite3; \
fi