HAproxy logging

Conditional command execution

-it is possible to specify that a command in
a script will only run if particular condition Is met

- such conditions are always expressed in terms of
the exit status of another program, as follows:

Command1 && command2

Means that command2 will only run if command1
Completes with an exit status of 0



Command3 || command4

Means that command4 will only run if command3
completes with an exit status that is not 0

For example:
ls file1 && cp file1 ./tmp
cp abc xyz && echo The file was copied OK
diff fileA fileB || echo The fiels are different
ls file2 || exit


-The only problem with these constructs is that they are
very limited:

- You can only perform one command if the condition is met
(however, it is possible to group commands)
- You cannot specify a second command to be run
If the condition is not met


The if Statement

-A much more powerful (and readable) shell
programming construct is the if statement

-It's form is as follows:

If command1
Then
Command2
Command3
....
Fi

For example:

If diff file1 file2 > /dev/null
then
echo The files are the same
rm file2
fi



The else clause
-The if statement is a powerful language construct, but
we still have not seen a way to either:
--- execute commands on the condition that a given command
returns a non-zero exit status
---execute commands if a given condition is not met

-There is an optional component to the if statement, known as
the else clause, that will facilitate solutions to both of these problems,
as follows:

if command1
then
one set of commands
else
another set of commands
fo

For exaple:
if diff file1 file2 > /dev/null
then
echo the files are the same
rm file2
else echo The files are different!
diff file1 file2
fi


-We now have a way to execute commands if a given command
returns a non-zero exit status:

if ls file1 > /dev/null
then
: # ":" is the "do nothong" command
else
echo The file doesn't exist - exiting ...
exit
fi




The elif clause

-Often we need to write a conditional code construct in which there are
more than two mutually exclusive options
-The if statement also offers the elif clause (short else if), as follows:

if command1
then
command set 1
elif command2
then
command set 2
else command set 3
fi


For example:
if ls $file > /dev/null 2>&1
then
echo Sorry, the file already exists
elif who > $file
echo $file now contrains the user list
else Could not create $file
fi

- The elif clauses can be repeated indefinitely
(however, there can only be one else clause)