Appending text to a file in Bash on Linux

How to append to file on bash shell command line

The Bash shell is the most popular shell on Linux systems, and to use the shell efficiently, you need a little knowledge about Bash shell redirections. This is also an essential step in learning Bash scripting.

In this guide, we’ll show how to append text or command output to a file on the Bash shell command line. This will include several examples so you can pick the right method in any scenario.

In this tutorial you will learn:

  • How to append text or command output to a file
  • How to append and view command output at the same time
  • How to append multiple lines of text to a file
Appending text to a file in Bash on Linux

Appending text to a file in Bash on Linux

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Listing environment variables

How to set and list environment variables on Linux

Environment variables are part of the Linux system shell that contain changing values. They help facilitate scripts and system programs, so that code can accommodate a variety of scenarios. Unlike regular shell variables, environment variables can be accessed system-wide, by any user or process.

Let’s look at a very simple example to see how environment variables work and why they exist. There are many system programs and user made scripts that need to access a current user’s home directory. This can be done reliably through the HOME environment variable. Therefore, a script that contains the following line can be used by any user on the system and it will generate the same result.

$ echo $SHELL > $HOME/current-shell.log

$ cat $HOME/current-shell.log
/bin/bash

In this guide, we’ll show how to list all the environment variables on a Linux system, as well as set new ones. Setting new environment variables can either be done temporarily, or permanently if you need them to survive a reboot. We’ll show instructions for both methods below.

In this tutorial you will learn:

  • How to list environment variables on Linux
  • How to set a temporary environment variable on Linux
  • How to set a permanent environment variable on Linux

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How to Discover, From Inside a Bash Script, the Path the Script Is In

How to Discover, From Inside a Bash Script, the Path the Script Is In

When you develop complex Bash scripts and start putting various scripts into a folder, where one script interacts with another by, for example, starting it, it quickly becomes necessary to ensure we know the path the script was started from, so we can start the other scripts with a fully qualified pathname. This is important because the first script may have been started from outside the script’s directory. We could have also done so by using a relative path, so even – somehow – reading the command that started the current script will not work.

In this tutorial, you will learn:

  • What the pwd command is, and what it does
  • How to discover from inside a Bash script what path that same script is in

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Special Bash Variables with examples

Special Bash Variables with examples

Bash is a great coding language, which allows you to do complex things like Big Data Manipulation, or simply create sever or desktop management scripts.

The entry level skill required to use the Bash language is quite low, and one-liner scripts (an often used jargon, which indicates multiple commands executed at the command line, forming a mini-script), as well as regular scripts, can grow in complexity (and how well written they are) as the Bash developer learns more.

Learning to use special variables in Bash is one part of this learning curve. Whereas originally the special variables may look cryptic: $$, $?, $*, \$0, $1, etc., once you understand them and use them in your own scripts, things will soon become clearer and easier to remember.

In this tutorial you will learn:

  • How to use special variables in Bash
  • How to correctly quote variables, even special ones
  • Examples using special variables from the command line and scripts

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Introduction to named pipes on Bash shell

Introduction to named pipes on Bash shell

On Linux and Unix-based operating systems, pipes are very useful since they are a simple way to achieve IPC (inter-process communication). When we connect two processes in a pipeline, the output of the first one is used as the input of the second one. To build a so called “anonymous” pipe, all we have to do is to use the | operator. Anonymous, or unnamed pipes last just as long as the processes they connect. There is, however, another type of pipe we can use: a FIFO, or named pipe. In this article we will see how named pipes work and in what they are different from the standard pipes.

In this tutorial you will learn:

  • What is a named pipe
  • How to create a named pipe
  • How to recognize a named pipe
  • How named pipes work
  • How to delete a named pipe

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How to Use Bash Subshells Inside if Statements

How to Use Bash Subshells Inside if Statements

If you have ever used Bash subshells ($(...)), you know how flexible subshells can be. It only takes a few characters to start a subshell to process anything required, inline to another statement. The number of possible use cases is virtually unlimited.

We can also use Bash subshells inside if statements, inline with the statement. Doing so gives the user and developer much additional flexibility when it comes to writing Bash if statements.

If you are not familiar yet (or would like to learn more about) Bash if statements, please see our Bash If Statements: If Elif Else Then Fi article.

In this tutorial you will learn:

  • How to incorporate Bash subshells inside if statements
  • Advanced methods to incorporate Bash subshells inline with other commands
  • Examples demonstrating the use of Bash subshells in if statements

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Process List Management and Automatic Process Termination

Process List Management and Automatic Process Termination

As sever optimal use/maximization continues to grow, it becomes more and more important to manage processes well. One aspect of this is automatic process termination. When a process has gone rogue, and is consuming too much resources, it can be terminated automatically.

This is especially suited to servers which have a lot of temporary or disposable processes. It is also well suited for testing servers which are running many test trials and where such test trials prove to be unstable or cause the software under testing to behave erratically (for example by using too much memory)

In this tutorial you will learn:

  • How to manage processes in an automated fashion
  • Which resources you may want to monitor, and why
  • Example code showing how automatic process termination can work for memory hogging issues

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Timing bash script execution

Time Your Bash Scripts and Procedures From Inside the Code

In general, one may use the time Bash utility (see man time for more information) to run a program, and obtain runtime duration and system resource usage summaries. But how can one time particular sections of code, directly from within the Bash source code?

Using some easy variable assignments and calculations, it is possible to achieve accurate timing metrics for Bash script executions.

In this tutorial you will learn:

  • How to time Bash scripts using variable assignments and calculations
  • How to use overlapping timers to time specific sections of your scripts
  • Examples which exemplify how specific sections of code can be timed

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How to Correctly Grep for Text in Bash Scripts

How to Correctly Grep for Text in Bash Scripts

grep is a versatile Linux utility, which can take a few years to master well. Even seasoned Linux engineers may make the mistake of assuming a given input text file will have a certain format. grep can also be used, directly in combination with if based searches to scan for the presence of a string within a given text file. Discover how to correctly grep for text independent of character sets, how to use the -q option to text for string presence, and more!

In this tutorial you will learn:

  • How to do correct character set-independent text searches with grep
  • How to use advanced grep statements from within scripts or terminal oneliner commands
  • How to test for string presence using the -q option to grep
  • Examples highlighting grep usage for these use cases

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Bash Advanced Variable Idioms for Case Sensitivity Management

Bash Advanced Variable Idioms for Case Sensitivity Management

Whenever we work with textual strings, sooner or later the issue of case comes up. Does a word need to be fully uppercase, fully lowercase, with a capitalized letter at the start of the word or sentence, and so on.

An idiom is a natural language expression of a simple programming task. For example, in the sleep 10 command (which will pause the terminal one is working in for ten seconds), the word sleep is a natural language expression of what is a time based coding construct, developed in the Bash GNU coreutils software package.

There are a number of special variable-bound idioms (i.e. suffixes which can be added to a variable name, indicating what we would like to do with a given variable), which can be used in Bash to more easily do these types of conversions on the fly instead of having to use for example the Sed Stream Editor with a Regular Expression to do the same.

If you are interested in using regular expressions, have a look at our Bash Regexps For Beginners With Examples Advanced Bash Regex With Examples articles!

This makes working with variables that need case modification, or if statement testing a whole lot easier and provides great flexibility. Idioms can be added directly inside the if statements and do not need to employ a subshell with sed.

While the syntax looks slightly complex to start with, once you learn a little mental support trick to remember the right keys, you will be well on your way to use these idioms in your next script or Bash one-liner script at the command line!

In this tutorial you will learn:

  • How to use the ^, ^^, , and ,, Bash variable suffix idioms
  • How to use a regular expression [] range idiom in combination with these
  • How to use the ^ and , idioms directly from within if statements
  • Detailed examples exemplifying the use of ^, ^^, , and ,,

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String concatenation in Bash

String concatenation in Bash

This tutorial will explain the Bash string concatenation by using examples. When it comes to bash scripting or programming in general, the concatenation refers to joining two or more string together to produce single unified output. Using Bash shell and bash scripting the string concatenation can be achieved in number for ways.

In this tutorial you will learn:

  • How to concatenate strings using echo command
  • How to concatenate strings in loop
  • How to concatenate string with command output

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