That's a great follow-up question! Just as the % operator handles the suffix (the end of a string), Bash uses the # operator to handle the prefix (the beginning of a string).
Here is how prefix expansion works:
1. The # Operator (Shortest Match)
The ${variable#pattern} syntax removes the shortest match of the pattern from the beginning of the string.
Example:
Imagine you have a variable representing a path:
file_path="Logs/system.log"
echo ${file_path#*/}
# Output: system.log#looks at the start.*/matches everything up to and including the first slash.
2. The ## Operator (Longest Match)
The ${variable##pattern} syntax removes the longest match of the pattern from the beginning. This is very common for extracting filenames from full paths.
Example:
full_path="/home/labex/project/script.sh"
echo ${full_path##*/}
# Output: script.sh##*/matches everything from the start up to the last slash, leaving only the filename.
Summary Comparison
| Direction | Operator | Description |
|---|---|---|
| Prefix (Start) | # |
Remove the shortest match from the beginning. |
| Prefix (Start) | ## |
Remove the longest match from the beginning. |
| Suffix (End) | % |
Remove the shortest match from the end. |
| Suffix (End) | %% |
Remove the longest match from the end. |
A Pro-Tip for Memory:
#is on the left side of the$key on a US keyboard (above '3'), and it deals with the beginning of the string.%is on the right side of the$key (above '5'), and it deals with the end of the string.
This makes parameter expansion a very powerful tool for string manipulation without needing to call external tools like sed or awk!
