Bash-Beginners-Guide

Bash Guide for Beginners
Table 7-2. Combining expressions
Operation
Effect
[ ! EXPR ] True if EXPR is false.
[ ( EXPR ) ]
[ EXPR1 -a EXPR2
]
Returns the value of EXPR. This may be used to override the normal precedence of
operators.
True if both EXPR1 and EXPR2 are true.
[ EXPR1 -o EXPR2
True if either EXPR1 or EXPR2 is true.
]
The [ (or test) built-in evaluates conditional expressions using a set of rules based on the number of
arguments. More information about this subject can be found in the Bash documentation. Just like the if is
closed with fi, the opening square bracket should be closed after the conditions have been listed.
7.1.1.2. Commands following the then statement
The CONSEQUENT-COMMANDS list that follows the then statement can be any valid UNIX command,
any executable program, any executable shell script or any shell statement, with the exception of the closing
fi. It is important to remember that the then and fi are considered to be separated statements in the shell.
Therefore, when issued on the command line, they are separated by a semi-colon.
In a script, the different parts of the if statement are usually well-separated. Below, a couple of simple
examples.
7.1.1.3. Checking files
The first example checks for the existence of a file:
anny ~> cat msgcheck.sh
#!/bin/bash
echo "This scripts checks the existence of the messages file."
echo "Checking..."
if [ -f /var/log/messages ]
then
echo "/var/log/messages exists."
fi
echo
echo "...done."
anny ~> ./msgcheck.sh
This scripts checks the existence of the messages file.
Checking...
/var/log/messages exists.
...done.
7.1.1.4. Checking shell options
To add in your Bash configuration files:
# These lines will print a message if the noclobber option is set:
Chapter 7. Conditional statements 81