String prefix removal (${var#pattern}) in Bash Scripting - Step-by-Step Execution

Concept Flow - String prefix removal (${var#pattern})

Start with variable var

↓

Apply pattern match from start

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Remove shortest matching prefix

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Return modified string

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End

This flow shows how bash removes the shortest matching prefix pattern from a variable's value.

Execution Sample

Bash Scripting

filename="report_2024.txt"
result=${filename#*_}
echo "$result"

Removes the shortest prefix ending with underscore from filename, outputting '2024.txt'.

Execution Table

Step	Variable 'filename'	Pattern	Action	Resulting Value	Output
1	report_2024.txt	"*_"	Find shortest prefix ending with '_'	report_
2	report_2024.txt	"*_"	Remove prefix 'report_'	2024.txt
3	report_2024.txt	N/A	Echo the result	2024.txt	2024.txt
4	N/A	N/A	End of script	N/A	N/A

💡 Pattern matched and shortest prefix removed; script ends after echo.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
filename	report_2024.txt	report_2024.txt	report_2024.txt	report_2024.txt
result	undefined	undefined	2024.txt	2024.txt

Key Moments - 2 Insights

Why does the pattern '*_' remove only up to the first underscore and not more?

Because ${var#pattern} removes the shortest match from the start. The pattern '*_' matches the shortest prefix ending with underscore, stopping at the first underscore as shown in execution_table step 1.

What happens if the pattern does not match anything in the variable?

If no match is found, the variable remains unchanged. This is implied because removal only happens when a prefix matches, as shown in execution_table step 2.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'result' after step 2?

A report_2024.txt

B 2024.txt

C report_

D txt

💡 Hint

Check the 'Resulting Value' column in row for step 2.

At which step does the script output '2024.txt'?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Output' column to find when '2024.txt' is printed.

If the pattern was '${filename##*_}', how would the result change after step 2?

A It would remove the longest prefix ending with underscore, resulting in 'txt'

B It would remove the shortest prefix ending with underscore, resulting in '2024.txt'

C It would remove nothing, result stays 'report_2024.txt'

D It would remove the entire string, result is empty

💡 Hint

Double # removes longest match; compare with single # behavior in variable_tracker.

Concept Snapshot

Syntax: ${var#pattern}
Removes shortest matching prefix pattern from var's value.
Useful to strip parts before a delimiter.
If no match, var stays unchanged.
Double # (##) removes longest matching prefix.
Example: filename="a_b_c"; echo ${filename#*_} outputs 'b_c'.

Full Transcript

This lesson shows how bash removes the shortest matching prefix from a variable using ${var#pattern}. We start with a variable 'filename' holding 'report_2024.txt'. The pattern '*_' matches the shortest prefix ending with underscore, which is 'report_'. Removing it leaves '2024.txt'. Then echo prints this result. If the pattern does not match, the variable stays the same. Using double # removes the longest matching prefix instead. This helps to extract parts of strings easily in scripts.