Sed is a stream editor in a UNIX-like operating system that is used for filtering and transforming text. Sed is derived originally from the basic line editor ‘ed’, an editor you will find on every Unix system but one that is rarely used because of its difficult user interface. How sed Works? As sed is […]
Shell Scripting
Backtick (`) symbol in Linux Shell Scripting
One of the most useful features of shell scripts is the lowly back quote character, usually called the backtick (`) in the Linux world. Be careful—this is not the normal single quotation mark character you are used to using for strings. Because it is not used very often outside of shell scripts, you may not […]
How to Find and Delete Empty Directories and Files in Linux
1. Find empty directories in the current directory using find -empty: $ find . -type d -empty 2. Use the following command to remove all empty directories under the current directory: $ find . -type d -empty -exec rmdir {} \; 3. Find empty files in the current directory using find -empty: $ find . […]
How to Check if the script is run by root at the start of the script
Check root at start of script Some scripts need to be run as root and you may want to check at the start of that script that it is running as root. This can be done by checking the environment variable $EUID. This variable will hold the value 0 if it’s being run as root. […]
How to Use user-defined Functions in awk
User-Defined Functions You can create user-defined functions in a awk script file using the func or function keywords. Placement in the file is not important; the function definition can occur anywhere in the awk script. The function can take arguments (local to the function only) or use any existing variable. The syntax of a function […]
Using Loops (while, for) in awk scripts
The awk programming language contains many of the programming concepts that are used in shell scripting. Conditionals, such as the if statement and loops, such as the following can also be used in awk programming. The while loop The do while loop The for loop The if Statement The if statement can have two branches: […]
Beginners Guide to Using “trap” to Catch Signals and Handle Errors in Shell Script
Shell Signal Values A signal is a message that some abnormal event has taken place or a message requesting another process do something. The signal is sent from one process to another process. Typically, a process sends a signal to one of its own subprocesses. You can obtain more information on signals from the signal […]
How to Make a Variable read-only (constant) in Bash and Korn Shell
Creating Bourne Shell Constants A variable can be made read-only with the following syntax: readonly var[=value] The square brackets around =value mean that the assignment of a value is not always necessary. For instance, if the variable had previously been created and assigned a value, and you now want to make it read-only (and not […]
Examples of “shift” Command in Shell Scripts
The shift Statement Sometimes a script does not require a specific number of arguments from users. Users are allowed to give as many arguments to the script as they want. In these situations, the arguments of the script are usually processed in a while loop with the condition being (($#)). This condition is true as […]
Korn Shell select Loop
The select statement in the Korn shell creates a menu. This construct is for the Korn shell only. The syntax for creating a menu is: select var in list do statement1 … statementN done The variables var and list follow the same syntactic rules used in the for loop (although the operation of the select […]