By: Dorris in Linux Tutorials on 2011-01-15
In Linux shell script, the
if statement is used for conditional branching based on the result of a test command. The basic syntax for the
if statement is as follows:
if [ condition ] then # commands to execute if condition is true else # commands to execute if condition is false fi
condition is the test command whose exit status determines whether the condition is true or false. The test command can be any command that returns an exit status of 0 for success and a non-zero value for failure.
Some common test commands that can be used in the
if statement include:
[ expression ]: tests a condition based on the values of its operands
test expression: same as above, can also be written as
[[ expression ]]
command: tests the exit status of a command
[[ -n string ]]: tests whether a string is non-empty
[[ -z string ]]: tests whether a string is empty
Here is an example of using the
if statement with the
[ expression ] test command:
#!/bin/bash if [ -f "/etc/passwd" ] then echo "File /etc/passwd exists" else echo "File /etc/passwd does not exist" fi
This script checks if the file
/etc/passwd exists and prints a message accordingly. If the file exists, the condition is true and the
echo command in the
then block is executed. If the file does not exist, the condition is false and the
echo command in the
else block is executed.
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