Bash Script to Extract Substring from a String

In Bash, extract a substring using ${string:position:length}, where position is the start index (0-based) and length is how many characters to take, for example: substring=${string:2:5}.

Examples

Inputstring='HelloWorld', position=0, length=5

OutputHello

Inputstring='abcdefg', position=2, length=3

Outputcde

Inputstring='short', position=3, length=10

Outputrt

How to Think About It

To extract a substring in Bash, think of the string as a sequence of characters starting at index 0. You choose where to start and how many characters to take. Bash lets you do this easily with ${string:position:length} syntax, which slices the string from the start position for the given length.

Algorithm

1

Get the input string.

2

Decide the start position (0-based index) for the substring.

3

Decide the length of the substring to extract.

4

Use Bash parameter expansion <code>${string:position:length}</code> to get the substring.

5

Print or return the extracted substring.

Code

bash

#!/bin/bash
string="HelloWorld"
position=2
length=5
substring=${string:position:length}
echo "$substring"

Output

lloWo

Dry Run

Let's trace extracting substring from 'HelloWorld' starting at position 2 with length 5.

1

Set variables

string='HelloWorld', position=2, length=5

2

Extract substring

substring=${string:2:5} → 'lloWo'

3

Print result

echo 'lloWo'

Step	Operation	Value
1	string	HelloWorld
2	substring extraction	lloWo
3	output	lloWo

Why This Works

Step 1: Parameter Expansion

Bash uses ${string:position:length} to slice strings without external commands.

Step 2: Zero-based Indexing

The position starts at 0, so 2 means the third character.

Step 3: Length Limits

If length exceeds string end, Bash returns characters up to the string's end.

Alternative Approaches

Using cut command

bash

string="HelloWorld"
echo "$string" | cut -c3-7

Uses external command; less efficient but simple for fixed positions.

Using expr substring

bash

string="HelloWorld"
expr substr "$string" 3 5

Legacy method; requires external command and 1-based indexing.

Complexity: O(n) time, O(n) space

Time Complexity

Extracting a substring takes time proportional to the length of the substring, O(n), where n is the substring length.

Space Complexity

The substring requires additional space proportional to its length, O(n), as it creates a new string.

Which Approach is Fastest?

Bash parameter expansion is fastest as it is built-in and avoids external commands, unlike cut or expr.

Approach	Time	Space	Best For
Parameter Expansion	O(n)	O(n)	Fast, built-in, simple substring extraction
cut command	O(n)	O(n)	Simple fixed-position extraction, external tool
expr substring	O(n)	O(n)	Legacy scripts, external tool, 1-based indexing