Match beginning and end of the filename using meta characters and regex

Last Updated on Friday, 01 April 2011 18:21

Question:

What is the name of the command which search for all the files starting with 'A' and end with 'K'?

Answer:

ls | grep ^A.*K$

Long answer:

Rather than searching for a single command we need a combination of commands to do this trick. Before we are able to do such a trick we need to get acquainted with couple bash features and terms:

Pipes

Pipe "|" allows us to redirect an output from one command to another command.

$ command-1 | command-1

Any output produced by command-1 is redirected for a further processing to a command-2. Here is a practical example:

$ ls
file1  file2  file3  file4

ls command returned names of all files and directories ( hidden files are not included ) currently residing in a current working directory. Redirecting an output from ls to wc command we can count number of files and directories located within a current working directory.

ls | wc -l
4

1. Matching a pattern

In addition to a command output redirection we can also search for a pattern within a filename ( or standard input ) using grep command. For example we want to count how many files in our current working directory contain a digit 4 within their filename:

$ ls | grep 4
file4

To pipe this output to yet another command such us wc we can also count number of files:

ls | grep 4 | wc -l
1

At this point we can clearly say that in our current working directory is only one file which contains a digit 4 in its file name.

2. Meta-characters

The real power comes when we start using meta-characters to search a pattern within a given string. The table below list all meta-characters and their meaning:

3. Regular expressions

Based on the knowledge above we can combine all what we have learned into a more complicated linux command using regular expression. Simply put, a regular expression allows us to search for a pattern within a string by use of meta-characters.

At this point we can go back to our original problem, which is to search for all files starting with 'A' and ending with 'K'.Here is a list of meta-characters we need to use in order to accomplish this task: "^", ".", "*" and "$".

Looking at the list of meta-characters and their meaning above we can construct a following command to display all files and directories starting with "a" and ending with "k" ( lowercase ! ) character.

$ cd /usr/bin/
$ ls | grep ^a.*k$
amarok
ark
authconfig-gtk
awk 

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