This is a very helpful trick while writing Bash scripts. Performing mathematical comparisons: usually, conditions are enclosed in square brackets. Note that there is a space between and operands. It will show an error if no space is provided. If the script will be run using ash or dash for better performance , you cannot use the double square. For example, "A" is 0x41 and "a" is 0x Thus "A" is less than "a", and "AAa" is less than "Aaa".
The test command can be used for performing condition checks. This reduces the number of braces used and can make your code more readable. The same test conditions enclosed within can be used with the test command. Note that test is an external program which must be forked, while [ is an internal function in Bash and thus more efficient.
The test program is compatible with Bourne shell, ash, dash, and others. It's common to customize your shell by putting function definitions, aliases, and environment variable settings in one of these files. Linux and Unix have several files that might hold customization scripts. These configuration files are divided into three camps—those sourced on login, those evaluated when an interactive shell is invoked, and files evaluated whenever a shell is invoked to process a script file.
That's because the graphical window manager doesn't start a shell. When you open a terminal window, a shell is created, but it's not a login shell. Invoke a ssh login session, like this: ssh Linear arrays lists and associative arrays, are not supported in all shells.
Use these files to define non-exported items such as aliases desired by all users. They are useful for setting paths that must be inherited by other bash instances. Use these files to hold your personal values that need to be defined whenever a new shell is created.
Each command performs a simple function to make our work easier. These simple functions can be combined with other commands to solve complex problems. Combining simple commands is an art; you will get better at it as you practice and gain experience. For example, DBU can combine standard input data with data from a file. The DBU command can do this in a single invocation. The next recipes show basic and advanced usages of DBU.
The DBU command has many other options for viewing files. Getting rid of extra blank lines Some text files contain two or more blank lines together. Languages such as Python may treat tabs and spaces differently. Mixtures of tabs and spaces may look similar in an editor, but appear as different indentations to the interpreter.
It is difficult to identify the difference between tabs and spaces when viewing a file in a text editor. DBU can also identify tabs. This helps you to debug indentation errors. Do not attempt to use redirection to overwrite your input file. The shell creates the new output file before it opens the input file. The DBU command will not let you use the same file as input and redirected output. Trying to trick DBU with a pipe and redirecting the output will empty the input file.
Recording and playing back terminal sessions Recording a screen session as a video is useful, but a video is an overkill for debugging terminal sessions or providing a shell tutorial. The shell provides another option.
The TDSJQU command records your keystrokes and the timing of keystrokes as you type, and saves your input and the resulting output in a pair of files. You can create tutorials of command-line hacks and tricks by recording the terminal sessions.
You can also share the recorded files for others to playback and see how to perform a particular task with the command line. You can even invoke other interpreters and record the keystrokes sent to that interpreter.
You cannot record vi, emacs, or other applications that map characters to particular locations on the screen. This file will hold the keystrokes and the command results. We often record desktop videos to prepare tutorials. However, videos require a considerable amount of storage, while a terminal script file is just a text file, usually only in the order of kilobytes.
It is useful both at the command line and in shell scripts. This recipe deals with some common ways to utilize GJOE to locate files. Getting ready The GJOE command uses the following strategy: GJOE descends through a hierarchy of files, matches files that meet the specified criteria, and performs some actions.
This convention is followed throughout the Unix filesystem. The previous examples demonstrated using GJOE to list all the files and folders in a filesystem hierarchy. The GJOE command can select files based on glob or regular expression rules, depth in the filesystem tree, date, type of file, and more.
A typical example of text matching with regular expressions is to recognize all e-mail addresses. An e-mail address takes the OBNF! It can be generalized as! The characters inside the square brackets represent a set of characters. So, this regular expression translates to 'a sequence of letters or numbers, followed by an! PY next. PY new. PY Searching based on the directory depth The GJOE command walks through all the subdirectories until it reaches the bottom of each subdirectory tree.
By default, the GJOE command will not follow symbolic links. If a link references a link that points to the original, GJOE will be stuck in a loop. This will break the GJOE command from an otherwise infinite search. The folder hierarchy for a task is quite deep and includes symbolic links that loop back on themselves.
Under each process ID is a folder called DXE, which is a link to that task's current working directory. It can be used to find and print files that are located with a minimum level of depth from the base path. If they are specified as later arguments, it may affect the efficiency of GJOE as it has to do unnecessary checks. Searching based on file type Unix-like operating systems treat every object as a file. There are different kinds of file, such as regular files, directory, character devices, block devices, symlinks, hardlinks, sockets, FIFO, and so on.
But GJOE helps to do it. The creation time can be accessed with the stat command. Given that some applications modify a file by creating a new file and then deleting the original, the creation date may not be accurate. They can be specified with integer values in number of days. The GJOE command also supports options that measure in minutes.
As an example usage case, we can consider the case of the Apache web server. The PHP files in the web server require proper permissions to execute. You can delete files, or execute an arbitrary Linux command on the files. Consider the previous example. You must run the GJOE command as root if you want to change the ownership of files or directories.
Invoking a command for each file is a lot of overhead. If the command accepts multiple arguments as DIPXO does you can terminate the command with a plus instead of a semicolon. The plus causes GJOE to make a list of all the files that match the search parameter and execute the application once with all the files on a single command line. It accepts only a single command, but we can use a trick. For example, when searching for files in a development source tree under a version control system such as JU, the filesystem contains a directory in each of the subdirectories where version-control-related information is stored.
These directories may not contain useful files and should be excluded from the search. The technique of excluding files and directories is known as pruning. Previous examples have shown how to pass data from one application's standard output to another's standard input with a pipe.
We can invoke applications that accept command-line arguments in other ways. The YBSHT command can also convert any one-line or multiple-line text inputs into other formats, such as multiple lines specified number of columns or a single line, and vice versa. It uses standard input as the primary data source and executes another command using the values it reads from TUEJO as command-line arguments for the new command.
Splitting the input into elements based on whitespace becomes an issue when file and folder names have spaces or even newlines in them. We can define the delimiter used to separate arguments. The next examples show how to format sets of data on a command line. I need to apply the arguments in several styles. In the first method, I need one argument for each invocation, like this:. However, we may need to have a constant phrase at the end of the command and want YBSHT to substitute its argument in the middle, like this:.
All others should remain constant. However, take care to combine them carefully. It may cause removal of unexpected files. We can create a subshell to handle complex situations. We can pass the output to other commands without using pipes. However, this change resides inside the subshell only.
Immediately after the filename is a list of all the times NBJO appears in that file: find. It is used to craft elegant one-liner commands. It performs substitution of characters, deletes selected characters, and can squeeze repeated characters from the standard input.
Tr is short for translate, since it translates a set of characters to another set. In this recipe, we will see how to use US to perform basic translation between sets. Getting ready The US command accepts input through stdin standard input and cannot accept input through command-line arguments. We can specify custom sets as needed by appending characters or character classes. We can define sets easily. It can also be combined with any other characters or character classes. Using US with the concept of sets, we can map characters from one set to another set easily.
ROT13 is a well-known encryption algorithm. We saw some basic translations using the US command. Let's see what else can US help us achieve. For example, it can remove multiple occurrences of a character in a string.
Recursive right? With US, a one—liner does the trick. We can recalculate the key to confirm that a file has not changed. Files may be modified deliberately adding a new user changes the password file , accidentally a data read error from a CDROM drive , or maliciously a virus is inserted. Checksums let us verify that a file contains the data we expect it to. Checksums are used by backup applications to check whether a file has been modified and needs to be backed up.
Most software distributions also have a checksum file available. Even robust protocols such as TCP can allow a file to be modified in transit.
Hence, we need to know whether the received file is the original one or not by applying some kind of test. By comparing the checksum of the file we downloaded with the checksum calculated by the distributer, we can verify that the received file is correct.
If the checksum calculated from the original file at the source location matches the one calculated at the destination, the file has been received successfully.
If malware modifies a file, we can detect this from the changed checksum. In this recipe, we will see how to compute checksums to verify the integrity of data. The ms5sum and sha1sum programs generate checksum strings by applying the corresponding algorithm to the data. Let's see how to generate a checksum from a file and verify the integrity of that file.
It generates a character hex code from the input. Checksums are useful to verify the integrity of files downloaded from the Internet.
ISO images are susceptible to erroneous bits. A few wrong bits and the ISO may be unreadable, or, worse, it might install applications that fail in strange ways. Checksums are also useful when used with a number of files. Let's see how to apply checksums to a collection of files and verify the accuracy.
Calculating the checksum for a directory requires recursively calculating the checksums for all the files in the directory. These programs may not be installed on your system.
Only the hash for a password is stored. When a user needs to be authenticated, the password is read and converted to the hash and that hash is compared to the stored hash.
If they are the same, the password is authenticated and access is provided. Storing plain—text password strings is risky and poses a security risk. This is because the rise in computing power in recent times that makes it easier to crack them. Shadow-like hash salted hash The next recipe shows how to generate a shadow-like salted hash for passwords. In some situations, we need to write scripts to edit passwords or add users.
In that case, we must generate a shadow password string and write a similar line to the preceding one to the shadow file. Shadow passwords are usually salted passwords. Salt consists of random bits that are used as one of the inputs to a key derivation function that generates the salted hash for the password. Unlike the checksum algorithms we just discussed, encryption programs can reconstruct the original data with no loss.
HQH signatures are also widely used in e-mail communications to "sign" e- mail messages, proving the authenticity of the sender. We are not covering HQH in much detail in this book. Base64 is a group of similar encoding schemes that represent binary data in an ASCII string format by translating it into a radix representation.
These programs are used to transmit binary data via e-mail. It can be coupled with other commands to produce the required output. The following recipes illustrate some sort and VOJR use cases. To sort by months in the order Jan, Feb, March, It will help you understand what that strange little server is doing in the corner of your office, what the mysterious virtual machine languishing in Azure is crunching through, what that circuit-board-like thing is doing under your office TV, and why the LEDs on it are blinking rapidly.
By the end of the book, you will have gained practical knowledge of Linux, which will serve as a bedrock for learning Linux administration and aid you in your Linux journey. The book then proceeds to explain text processing, web interaction and concludes with backups, monitoring and other sysadmin tasks.
This book is written in cookbook style and it offers learning through recipes with examples and illustrations. Each recipe contains step-by-step instructions about everything necessary to execute a particular task.
You can start writing scripts and one-liners by simply looking at the similar recipe and its descriptions without any working knowledge of shell scripting or Linux. What this book provides instead is task-oriented coverage designed around the needs of the Oracle Database Administrator. Find the right chapter. Look up the task to perform. See the solution. Implement the solution straight away in your own environment.
Get the job done. New in this edition is coverage of Oracle's own Solaris operating system. Oracle Corporation has been working diligently to bring commonality between Solaris and and Linux, and this book takes advantage of those efforts to provide task-oriented solutions that work on common distributions of Linux such as Red Hat Enterprise Linux and Oracle Enterprise Linux while also accommodating the growing number of Oracle Solaris customers.
Examples in the book match the tasks DBAs perform daily, even hourly. Solutions come first in the book, but alway are followed by close explanations of the details. It's the book to buy if you're after clear and reliable examples to help in getting the job done, and getting home to your family. Takes you directly from problem to solution Covers the "right" mix of operating-system tasks for database administrators Respects your time by being succinct and to—the—point Includes coverage of Solaris in addition to common Linux distributions What You Will Learn Execute Linux and Solaris commands applicable to Oracle Database.
Automate critical DBA tasks via operating-system shell scripts. Monitor, tune, and optimize Linux and Solaris servers for Oracle. Setup a VirtualBox environment for the Oracle database. Perform system administration tasks relevant to Oracle Database. Remotely and securely! Pro Bash Programming teaches you how to effectively utilize the Bash shell in your programming. Overview Master the art of crafting one-liner command sequence to perform text processing, digging data from files, backups to sysadmin tools, and a lot more And if powerful text processing isn't enough, see how to make your scripts interact with the web-services like Twitter, Gmail Explores the possibilities with the shell in a simple and elegant way - you will see how to effectively solve problems in your day to day life In Detail The shell remains one of the most powerful tools on a computer system - yet a large number of users are unaware of how much one can accomplish with it.
DOWNLOAD NOW » Author : Clif Flynt Publisher: Packt Publishing Ltd ISBN: Category: Computers Page: View: Do amazing things with the shell About This Book Become an expert in creating powerful shell scripts and explore the full possibilities of the shell Automate any administrative task you could imagine, with shell scripts Packed with easy-to-follow recipes on new features on Linux, particularly, Debian-based, to help you accomplish even the most complex tasks with ease Who This Book Is For If you are a beginner or an intermediate Linux user who wants to master the skill of quickly writing scripts and automate tasks without reading the entire man pages, then this book is for you.
You can start writing scripts and one-liners by simply looking at the relevant recipe and its descriptions without any working knowledge of shell scripting or Linux. What You Will Learn Interact with websites via scripts Write shell scripts to mine and process data from the Web Automate system backups and other repetitive tasks with crontab Create, compress, and encrypt archives of your critical data.
Configure and monitor Ethernet and wireless networks Monitor and log network and system activity Tune your system for optimal performance Improve your system's security Identify resource hogs and network bottlenecks Extract audio from video files Create web photo albums Use git or fossil to manage revision control and interact with FOSS projects Create and maintain Linux containers and Virtual Machines Run a private Cloud server In Detail The shell is the most powerful tool your computer provides.
Despite having it at their fingertips, many users are unaware of how much the shell can accomplish.
Using the shell, you can generate databases and web pages from sets of files, automate monotonous admin tasks such as system backups, monitor your system's health and activity, identify network bottlenecks and system resource hogs, and more. This book will show you how to do all this and much more.
This book, now in its third edition, describes the exciting new features in the newest Linux distributions to help you accomplish more than you imagine. It shows how to use simple commands to automate complex tasks, automate web interactions, download videos, set up containers and cloud servers, and even get free SSL certificates. Starting with the basics of the shell, you will learn simple commands and how to apply them to real-world issues.
From there, you'll learn text processing, web interactions, network and system monitoring, and system tuning. Software engineers will learn how to examine system applications, how to use modern software management tools such as git and fossil for their own work, and how to submit patches to open-source projects. Finally, you'll learn how to set up Linux Containers and Virtual machines and even run your own Cloud server with a free SSL Certificate from letsencrypt.
Style and approach This book will take you through useful real-world recipes designed to make your daily life easier when working with the shell. DOWNLOAD NOW » Author : Ganesh Sanjiv Naik Publisher: Packt Publishing Ltd ISBN: Category: Computers Page: View: Unleash the power of shell scripts to solve real-world problems by breaking through the practice of writing tedious code About This Book Learn how to efficiently and effectively build shell scripts and develop advanced applications with this handy book Develop high quality and efficient solutions by writing professional and real-world scripts, and debug scripts by checking and shell tracing A step-by-step tutorial to automate routine tasks by developing scripts from a basic level to very advanced functionality Who This Book Is For This book is ideal for those who are proficient at working with Linux and who want to learn about shell scripting to improve their efficiency and practical skills.
By the end of this book, you will be able to confidently use your own shell scripts in the real world. What You Will Learn Familiarize yourself with the various text filtering tools available in Linux Combine the fundamental text and file processing commands to process data and automate repetitive tasks Understand expressions and variables and how to use them practically Automate decision-making and save a lot of time and effort of revisiting code Get to grips with advanced functionality such as using traps and signals and using dialogs to develop screens Start up a system and customize a Linux system Take an in-depth look at regular expressions and pattern matching to understand the capabilities of scripting In Detail Linux is the one of the most powerful and universally adopted OSes.
Shell is a program that gives the user direct interaction with the operating system. Scripts are collections of commands that are stored in a file. The shell can read this file and act on the commands as if they were typed on the keyboard. Shell scripting is used to automate day-to-day administration, and for testing or product development tasks.
We start with an introduction to the Shell environment and explain basic commands used in Shell. Next we move on to check, kill, and control the execution of processes in Linux OS.
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