In shell programming, there are different ways to provide an input (for example, via a keyboard and terminal) and display an output (for example, terminal and file) and error (for example, terminal), if any, during the execution of a command or program.

The following examples show the input, output, and error while running the commands:

When a program executes, by default, three files get opened with it which are stdin, stdout, and stderr. The following table provides a short description of each of these...

We have an option to redirect standard input, output, and errors, for example, to a file, another command, intended stream, and so on. Redirection is useful in different ways. For example, I have a bash script whose output and errors are displayed on a standard output—that is, terminal. We can avoid mixing an error and output by redirecting one of them or both to a file. Different operators are used for redirection. The following table shows some of operators used for redirection, along with its description:

The outputs of the programs are generally saved in files for further use. Sometimes, temporary files are created in order to use an output of a program as an input to another program. We can avoid creating temporary files and feed the output of a program as an input to another program using bash pipe and pipelines.

$ cat /proc/cpuinfo | less

$ ps -aux | wc -l    # Showing...

Regular expression (also known as regex or regexp) provides a way of specifying a pattern to be matched in a given big chunk of text data. It supports a set of characters to specify the pattern. It is widely used for a text search and string manipulation. A lot of shell commands provide an option to specify regex such as grep, sed, find, and so on.

The regular expression concept is also used in other programming languages such as C++, Python, Java, Perl, and so on. Libraries are available in different languages to support regular expression's features.

One of the powerful and widely used command in shell is grep. It searches in an input file and matches lines in which the given pattern is found. By default, all the matched patterns are printed on stdout that is usually terminal. We can also redirect the matched output to other streams such as file. Instead of giving an input from a file, grep can also take the input from the redirected output of the command executed on the left-hand side of '|'.

The sed command is a non-interactive stream editor that allows you to modify the content of the standard input or file. It performs an operation on each line in a pipeline. The syntax will be:

sed [OPTIONS]... {script} [input-file …]

By default, the output is displayed on stdout, but can be redirected to a file if specified.

The input-file are the files on which sed needs to be run. If no files are specified, it reads from stdin.

The script can be a command or a file with multiple commands to pass to sed, and OPTIONS to sed are described in the following table:

In some cases, it's necessary to print an output on stdout and save an output in a file. In general, command output can either be printed or can be saved in a file. To solve it, the tee command is used. This command reads from the standard input and writes to both standard output and files. The syntax of tee will be as follows:

tee [OPTION] [FILE …]

The tee command copies the output to each FILE and also to stdout. The OPTIONS can be as follows:

Writing an output to stdout and file: In general, to write an output to stdout and file, we will call the same command twice, with and without redirection. For example, the following command shows how to print an output on stdout and save it to a file:

$  ls /usr/bin/*.pl  # Prints output on stdout
/usr/bin/rsyslog-recover-qi.pl  /usr/bin/syncqt.pl
$  ls /usr/bin/*...