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Conditional Statements

The if-else statement

Sometimes you need to specify different courses of action to be taken in a shell script, depending on the success or failure of a command. The if construction allows you to specify such conditions. The most common syntax of the if command is:

if TEST-COMMAND
then
    POSITIVE-CONSEQUENT-COMMANDS
else
    NEGATIVE-CONSEQUENT-COMMANDS
fi

If the TEST-COMMAND is successful (returns an exit status of 0), then the positive consequent commands are executed, otherwise, the negative consequent commands (if provided) are executed. The if statement is terminated with the fi command.

Testing files

Before you start, review the man page of the test command

The first example checks for the existence of a file:

echo "This script checks the existence of the messages file."
echo "Checking..."

if [ -f /var/log/messages ]
then
  echo "/var/log/messages exists."
fi

echo
echo "...done."

Testing exit status

Recall that the $? variable holds the exit status of the previously executed command. The following example utilize this variable to take a decision according to the success or failure of the previous command:

curl google.com &> /dev/null

if [ $? -eq 0 ]
then
  echo "Request succeeded"
else
  echo "Request failed, trying again..."
fi

Numeric comparisons

The below example demonstrates numeric comparison between a variable and 20. Don't worry is it doesn't work, you'll fix it soon 🙂

num=$(wc -l /etc/passwd)
echo $num

if [ "$num" -gt "20" ]; then
  echo "Too many users in the system."
fi

String comparisons

if [[ "$(whoami)" != 'root' ]]; then
  echo "You have no permission to run $0 as non-root user."
  exit 1;
fi

if-grep construct

echo "Bash is ok" > file

if grep -q Bash file
then
  echo "File contains at least one occurrence of Bash."
fi

Another example:

word=Linux
letter_sequence=inu

if echo "$word" | grep -q "$letter_sequence"
# The "-q" option to grep suppresses output.
then
  echo "$letter_sequence found in $word"
else
  echo "$letter_sequence not found in $word"
fi

[...] vs [[...]]

With version 2.02, Bash introduced the [[ ... ]] extended test command, which performs comparisons in a manner more familiar to programmers from other languages. The [[...]] construct is the more versatile Bash version of [...]. Using the [[...]] test construct, rather than [...] can prevent many logic errors in scripts.

Self-check questions

Enter the interactive self-check page

Exercises

✏️ Geo-location info

Write a bash script geo_by_ip.sh that given an ip address, prints geo-location details, as follows:

  1. The script first checks if jq cli is installed, if not, it prints a message to the user with a link to download the tool: https://stedolan.github.io/jq/download/
  2. The script checks that exactly one argument was sent to it, which represents the ip address to check. Otherwise, an informative message is being printed to stdout.
  3. The script checks that the given IP argument is not equal to 127.0.0.1
  4. The script performs an HTTP GET request to http://ip-api.com/json/<ip> while <ip> is the IP argument. The results should be stored in a variable.
  5. Using the jq tool and the variable containing the HTTP response, check that the request has been succeeded by checking that the status key has a value of success. The command jq -r '.<key>' can extract a key from the json (e.g. echo $RESPONSE | jq -r '.status')
  6. If the request succeed, print the following information to the user:
    • country
    • city
    • regionName

✏️ Extended availability test

Extending the above script to be more modular. Assume availability_test.sh is:

curl google.com &> /dev/null

if [ $? -eq 0 ]
then
    echo "Request succeeded"
else
    echo "Request failed, trying again..."
fi

Change the script such that can be working for any URL input from the user, in the following form:

myuser@hostname:~$ ./availability_test.sh google.com
...
myuser@hostname:~$ ./availability_test.sh cnn.com
...
myuser@hostname:~$ ./availability_test.sh abcdefg
...
myuser@hostname:~$ ./availability_test.sh
A valid URL is required