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What are Variables? A variable is a temporary storage area in memory that is either set by the user, shell, system, or any program that loads another program. There are two categories of variables: The environment variables are valid for the duration of the session. The shell variables apply only to the current instance of the shell and are used to set short-term working conditions. Note: When a shell variable follows the dollar $ sign character, the shell interprets that the value … [Read more...] about Understanding Variables in Bash Shell Under Linux

Shell metacharacters Shell metacharacters are specific characters, generally symbols, that have special meaning within the shell. The metacharacters supported in bash are listed as follows: | : Sends the output of the command to the left as the input to the command on the right of the symbol & : Runs the process in the background, allowing you to continue working on the command line ; : Allows you to list multiple commands on a single line, separated by this character () : Groups commands … [Read more...] about How to use command redirection under Linux

Shell Expansions While working in a shell, sets or ranges of information are often repeated. Shell expansion helps generate a large number of shell tokens using compact syntaxes. The expansion is performed on the command line after the command is split into tokens. Of the many expansions available, the path name, file name, and brace expansions are explained ahead. Path Name Expansion The pathname expansion simplifies location changes within the directory hierarchy. The pathname expansion … [Read more...] about How to use shell expansions for generating shell tokens under Linux

Functions, a powerful feature of shell programming, is a group of commands organized by common functionality. These easy-to-manage units, when called return a single value, and do not output anything. Using a function involves two steps: 1. Defining the function 2. Invoking the function Shell function Vs shell alias Shell functions and aliases are different on two counts. - aliases do not take arguments as functions do. - if a command name is defined as a function and an alias, the … [Read more...] about How to use command line shell functions in Linux

What is an alias An alias is a shorthand shell notation that allows you to customize and abbreviate commands. Aliases are available in all shells. A common syntax to define an alias on command line is as follows: $ alias name=command_string If the first word on the command line is an alias, the shell replaces that word with the text of the alias. The shell maintains a list of aliases that it searches when a command is entered. The following rules apply while creating an alias: There can … [Read more...] about How to use shell aliases in Linux

Alias for commands lets you define your own short easy to remember command shortcuts. Below are some examples of defining command aliases permanently into the different shells like bash, ksh and sh. You can also define aliases on command line, but they will not persist after you change the shell or logout of the shell. Bash Shell To create a command alias for the bash shell: 1. Login as user. 2. Add the following lines to .bashrc in home directory of the user : $ vi ~/.bashrc alias … [Read more...] about Examples of creating command alias in different shells

Introduction to the sed Editor The term sed stands for stream editor. Sed can take its input from standard in, apply the requested edits on the stream, and automatically put the results to standard out. The sed syntax allows for an input file to be specified on the command line. You do not need to interact with the sed editor while it is running; therefore, it has also been termed a batch editor. This is in contrast to such editors as vi and ed, which are interactive. Because sed does not … [Read more...] about Introduction to sed (Stream Editor) : Useful sed Command Examples

There are many times in a shell script when you want to repeatedly execute a statement or group of statements until some specified condition occurs. In addition, you might want to execute a group of statements for each element in a certain list. The shell provides three looping constructs for these situations: the for, while, and until constructs. The for loop allows you to specify a list of values. A group of statements is executed for each value in the list. For loop syntax The for loop … [Read more...] about Bash for loop Examples