Parameter Expansion for Length/Substring

Getting String Length with ${#s}

${#s} expands to the number of characters currently stored in the variable s, which is the standard length-checking idiom in Bash. It counts characters, not bytes, in a locale-aware way for most standard configurations.

Example: Getting String Length with ${#s}

#!/bin/bash
word="Bash"
echo "The word '$word' has ${#word} characters"

Extracting a Substring with ${s:pos:len}

${s:pos:len} extracts len characters from s starting at zero-based index pos. This is a purely Bash built-in operation, requiring no external tools like cut or awk.

Example: Extracting a Substring with ${s:pos:len}

#!/bin/bash
sentence="The quick brown fox"
word=${sentence:4:5}
echo "Extracted word: $word"

Omitting the Length

Leaving out the length argument, ${s:pos}, extracts everything from index pos all the way to the end of the string. This is convenient when you don't know or don't care exactly how long the remaining text is.

Example: Omitting the Length

#!/bin/bash
path="/usr/local/bin/mytool"
remainder=${path:5}
echo "Everything after index 5: $remainder"

Negative Starting Position

A negative pos, written with a leading space like ${s: -4}, counts backward from the end of the string instead of from the beginning. This is a quick way to grab the last few characters of a string, such as a file extension.

Warning: ${filename:-4} (no space) would instead trigger the default-value expansion, not a substring extraction.

Example: Negative Starting Position

#!/bin/bash
filename="report_final.txt"
last_four=${filename: -4}
echo "Last four characters: $last_four"