Additional Commands and Functionality

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Wildcards

"Wildcards" in file and/or directory names allow display of selected file or directory names rather than listing all of them. * and ? are the most commonly used ones. "*" represents 0 or more characters so

[hes@vm18-165 ~]$ ls first*
will display the names of all files and directories starting with the letters "first" with 0 or more characters following in the name.

"?" represents exactly one character. So

[hes@vm18-165 ~]$ ls first-c?t
would display the file first-cat but not the file first-file. It would also display first-cot, if there was such a file.
Searching for Patterns
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The grep command format is

grep pattern file(s)

The pattern is a regular expression. The simplest one is just a character string, and grep looks for a match in the file(s). It doesn't have to be a complete word, just a string of characters. grep prints the lines of file(s) matching a pattern (a regex).

[hes@vm18-165 ~]$ grep line first-file
A regular expression example using a wildcard is sec.ond where the "." means that any one character is considered a match for that position. The "*" is commonly used to search many files, e.g. *html will include all the html files in the directory. (The notation differs between wildcards and regexes.)

[hes@vm16-203 ~]$ grep se.ond first-file
will look for 6 characters appearing anywhere in the file, where the first 2 are specified as are the last 3, but the 3rd can be any character. E.g. "se9ond" or "se+ond" appearing in the file would also match.

Regexes can be, and usually are, much more complicated than just using wildcards. Learning to formulate all the types of regular expressions is a more advanced topic, which can be very useful, and whole books have been written on this topic. E.g. Mastering Regular Expressions by Jeffrey Friedl. A much shorter treatment and more.

Piping Command Output to a Command Input
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The symbol for pipe is |
A very simple task illustrating its use is to search two files for a pattern. (It doesn't have to be in both files, and when it isn't, nothing is output.) We could search them individually and "print" the output of each command on the screen:

[hes@vm18-165 ~]$ grep se.ond first-cat [hes@vm18-165 ~]$ grep se.ond first-file
or put them together and do one search:
[hes@vm18-165 ~]$ cat first-cat first-file > tmp-file [hes@vm18-165 ~]$ grep se.ond tmp-file [hes@vm18-165 ~]$ rm tmp-file
where the last command removes the no longer needed temporary file, and both approaches work, but using the | command makes it more straight forward.

[hes@vm18-165 ~]$ cat first-cat first-file | grep se.ond
will more directly arrive at the needed result. This is a very simple example of using the pipe command to redirect the output of one command directly into the input of another command.
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Copyright 2019, 2020 by Henry E. Schaffer. Comments and suggestions are welcome, and should go to hes@ncsu.edu
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Last updated 1/9/2020