bash split str by : using param expansion. take last part and rest

In Bash, splitting a string by a delimiter and extracting specific parts can be elegantly accomplished using parameter expansion. This technique is particularly useful when working with strings that contain delimiters, such as colons (:). Below, we'll explore how to split a string by a colon, extract the last part, and get the rest of the string using parameter expansion.

Understanding Parameter Expansion in Bash

Parameter expansion is a powerful feature of Bash that allows you to manipulate variables and strings without the need for external commands. The syntax typically used is ${variable}, and we can extend this with various operators to achieve splitting and extraction.

Example Scenario

Let's assume we have the following string:

my_string="part1:part2:part3:last_part"

Step 1: Extracting the Last Part

To extract the last part of the string after the final colon, you can use the following parameter expansion technique:

last_part=${my_string##*:}

Explanation:

• ## is used to perform a greedy match, which removes everything before the last : including the colon itself. After executing this line, last_part will contain last_part.

Step 2: Extracting the Rest of the Parts

To extract everything except the last part, you can use the following syntax:

rest_of_string=${my_string%:*}

Explanation:

• % is used to remove the shortest match from the end of the variable, which in this case is everything after and including the last :. The variable rest_of_string will then contain part1:part2:part3.

Complete Script Example

Here’s how the full Bash script would look:

#!/bin/bash

# Original string to be processed
my_string="part1:part2:part3:last_part"

# Extracting the last part
last_part=${my_string##*:}
echo "Last part: $last_part"

# Extracting the rest of the string
rest_of_string=${my_string%:*}
echo "Rest of the string: $rest_of_string"

Output

When you run the script, you should see the following output:

Last part: last_part
Rest of the string: part1:part2:part3

Conclusion

Using Bash parameter expansion is an efficient way to manage string manipulations without relying on external commands. By applying the greedy and non-greedy substring removal techniques (## and %), you can easily extract both the last element after a specified delimiter and the remainder of the string. This method is both fast and efficient, making it ideal for scripts where performance is a consideration.

For further reading and various techniques on string manipulation in Bash, you might find useful discussions on sources like Stack Overflow and Baeldung helpful.