LiveFire Labs' UNIX Tip, Trick, or Shell Script of the Week

Pattern Matching - Substrings - Part I

The Korn shell provides four pattern matching (substring) operators.  These operators give you the ability to extract a portion of a variable, or in other words, discard the part you do not want.

First, we'll discuss the two operators that remove characters from the beginning (or left side) of the variable:

${variable#pattern} - discards the smallest matching pattern; returns the rest

${variable##pattern} - discards the largest matching pattern; returns the rest

In the following example, since the pattern (*_) matches the beginning of the variable TEXT, the specified action will be performed.

$ TEXT="livefire_labs_provides_online_unix_training_with_hands_on_lab_exercises"
$ print ${TEXT#*_}
labs_provides_online_unix_training_with_hands_on_lab_exercises
$

From the output you can see that only "livefire_" was dropped.  This is because only one pound sign (#) was used in the statement.  Using two pound signs will delete the largest matching pattern from the variable:

$ print ${TEXT##*_}
exercises
$

Even though pattern matching has a wide variety of applications, it is frequently used to parse file or directory pathnames.  This example parses the full pathname of the present working directory, leaving only the name of the subdirectory the user is in:

$ print ${PWD}
/export/home/livefire
$ print ${PWD##*/}
livefire
$

A more common method for achieving this particular objective is to use the basename command:

$ print $(basename ${PWD})
livefire
$

Revew basename's man page ($ man basename) to learn more.

Read the NEXT article in this series - Pattern Matching - Substrings - Part II