Read examples in linux

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Bash Read Command Examples

Receiving input from the read command

The read command is a built in function that allows scripts to catch information entered by users interactively. The basic syntax of the «read» command is as follows:

read [options] VAR1 VAR2 . VARN

The read command is used to get a line of input into a variable. Each argument must be a variable name without the leading «$«. The built in command reads a line of input and separates the line into individual words using the «IFS» inter field separator. By default the «IFS» is set to a space. Each word in the line is stored in a variable from left to right. The first word is stored in the first variable, the second word to the second variable and so on. If there are fewer variables than words, then all remaining words are then assigned to the last variable. If you have more variables than words defined, then any excess variables are set to null. If no variable names are supplied to the read line, then the read uses the default variable REPLY.

read example

Output from above read example

In the above «read» example, we passed the values «land«, «of» and «linux» into the variables «first«, «middle» and «last«. These are then displayed via the echo command.

Although our script allowed us to enter the information, it could not be classed as being very user friendly. To remedy this we can modify our script to use a simple echo statement:

read example

Output from above read example

We could also use the «-p» flag with the read command:

read -p example

read -p example

Output from above read -p example

In the above example we use the variables «first«, «middle» and «last» after our prompt. We could always split the prompt across several lines as follows:

read -p example

Output from above read -p example

read example — catch all

Sometimes there may be more words than defined variables. This is handled simply by all excess words being written to the last variable that we defined:

read -p example

Output from above read -p example

As we can see in the above example, the values «one» and «two» were assigned to variables «vara» and «varb«. The remaining input was assigned to a single variable «varc«. The variable «varc» contains the «three four five six».

Options available to the read command

Below is a table containing other parameters that may be used with the read built in in command:

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Команда read в Bash

Bash поставляется с рядом встроенных команд, которые можно использовать в командной строке или в сценариях оболочки.

В этой статье мы рассмотрим встроенную команду read .

Встроенное read Bash

read — это встроенная команда bash, которая считывает строку из стандартного ввода (или из файлового дескриптора) и разбивает строку на слова. Первое слово присваивается первому имени, второе — второму имени и так далее.

Общий синтаксис встроенной функции read имеет следующий вид:

Чтобы проиллюстрировать, как работает команда, откройте терминал, введите read var1 var2 и нажмите «Enter». Команда будет ждать, пока пользователь введет данные. Введите два слова и нажмите «Enter».

Слова присваиваются именам, которые передаются команде read качестве аргументов. Используйте echo или printf чтобы проверить это:

Вместо того, чтобы вводить текст на терминале, вы можете передать стандартный ввод для read с помощью других методов, таких как piping, here-string или heredoc :

Вот пример использования строки здесь и printf :

Если команде read не задан аргумент, вся строка присваивается переменной REPLY :

Если количество аргументов, предоставленных для read , больше, чем количество слов, прочитанных из ввода, оставшиеся слова присваиваются фамилии:

В противном случае, если количество аргументов меньше количества имен, оставшимся именам присваивается пустое значение:

По умолчанию read интерпретирует обратную косую черту как escape-символ, что иногда может вызывать неожиданное поведение. Чтобы отключить экранирование обратной косой черты, вызовите команду с параметром -r .

Ниже приведен пример, показывающий, как работает read при вызове с параметром -r и без него:

Как правило, вы всегда должны использовать read с параметром -r .

Изменение разделителя

По умолчанию при read строка разбивается на слова с использованием одного или нескольких пробелов, табуляции и новой строки в качестве разделителей. Чтобы использовать другой символ в качестве разделителя, присвойте его переменной IFS (внутренний разделитель полей).

Когда IFS установлен на символ, отличный от пробела или табуляции, слова разделяются ровно одним символом:

Строка разделена четырьмя словами. Второе слово — это пустое значение, представляющее отрезок между разделителями. Он создан, потому что мы использовали два символа-разделителя рядом друг с другом ( :: .

Для разделения строки можно использовать несколько разделителей. При указании нескольких разделителей присваивайте символы переменной IFS без пробела между ними.

Вот пример использования _ и — качестве разделителей:

Строка подсказки

При написании интерактивных сценариев bash вы можете использовать команду read для получения пользовательского ввода.

Чтобы указать строку приглашения, используйте параметр -p . Подсказка печатается перед выполнением read и не включает новую строку.

Вот простой пример:

Как правило, вы будете использовать для read команды внутри в while цикл , чтобы заставить пользователя дать один из ожидаемых ответов.

Приведенный ниже код предложит пользователю перезагрузить систему :

Если сценарий оболочки просит пользователей ввести конфиденциальную информацию, например пароль, используйте параметр -s который сообщает read не печатать ввод на терминале:

Назначьте слова в массив

Чтобы присвоить слова массиву вместо имен переменных, вызовите команду read с параметром -a :

Когда даны и массив, и имя переменной, все слова присваиваются массиву.

Выводы

Команда read используется для разделения строки ввода на слова.

Если у вас есть какие-либо вопросы или отзывы, не стесняйтесь оставлять комментарии.

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5 Practical Examples of the Read Command in Linux

What is the read command in Linux?

The read command in Linux is a way for the users to interact with input taken from the keyboard, which you might see referred to as stdin (standard input) or other similar descriptions.

In other words, if you want that your bash script takes input from the user, you’ll have to use the read command.

I am going to write some simple bash scripts to show you the practical usage of the read command.

Read command examples

The read command can be confusing to get started with, especially for those who are new to shell scripting. The scripts I am going to use here are very simple to understand and should be easy to follow, especially if you are practicing along with the tutorial.

Basic Programming Concepts

With almost every program or script, you want to take information from a user (input) and tell the computer what to do with that information (output).

When you use read, you are communicating to the bash terminal that you want to capture the input from the user. By default, the command will create a variable to save that input to.

Now let’s see some examples of read command to understand how you can use it in different situations.

1. Read command without options

When you type read without any additional options, you will need to hit enter to start the capture. The system will capture input until you hit enter again.

By default this information will be stored in a variable named $REPLY .

To make things easier to follow for the first example, I will use the ↵ symbol will show when the enter key is pressed.

More About Variables

As I mentioned earlier, the $REPLY variable is built into read , so you don’t have to declare it.

That might be fine if you have only one application in mind, but more than likely you’ll want to use your own variables. When you declare the variable with read, you don’t need to do anything other than type the name of the variable.

When you want to call the variable, you will use a $ in front of the name. Here’s an example where I create the variable Linux_Handbook and assign it the value of the input.

You can use echo command to verify that the read command did its magic:

Reminder: Variable names are case-senstive.

2. Prompt option -p

If you’re writing a script and you want to capture user input, there is a read option to create a prompt that can simplify your code. Coding is all about efficiency, right?

Instead of using additional lines and echo commands, you can simply use the -p option flag. The text you type in quotes will display as intended and the user will not need to hit enter to begin capturing input.

So instead of writing two lines of code like this:

You can use the -p option with read command like this:

The input will be saved to the variable $username.

3. “Secret”/Silent option -s

I wrote a simpe bash script to demonstrate the next flag. First take a look at the output.

Here is the content of secret.sh if you’d like to recreate it.

As you can see, the -s option masked the input when the password was entered. However, this is a superficial technique and doesn’t offer a real security.

4. Using a character limit with read option -n

You can add a constraint to the input and limit it to n number of characters in length.

Let’s use the same script from before but modify it so that inputs are limited to 5 characters.

Simply add -n N where N is the number of your choice.

I’ve done the same thing for our password.

As you can see the program stopped collecting input after 5 characters for the username.

However, I could still write LESS than 5 characters as long as I hit ↵ after the input.

If you want to restrict that, you can use -N (instead of -n) This modification makes it so that exactly 5 characters are required, neither less, nor more.

5. Storing information in an array -a

You can also use read command in Linux to create your own arrays. This means we can assign chunks of input to elements in an array. By default, the space key will separate elements.

If you’re new to arrays, or seeing how them in bash for the first time, I’ll break down what’s happening.

• Enter desired elements, separated by spaces.
• If we put only the @ variable, it will iterate and print the entire loop.
• The @ symbol represents the element number and with the colons after, we can tell iterate from index 0 to index 3 (as written here).
• Prints element at index 0.
• Similar to above, but demonstrates that the elements are seperated by the space

Bonus Tip: Adding a timeout function

You can also add a timeout to our read. If no input is captured in the allotted time, the program will move on or end.

It may not be obvious looking at the output, but the terminal waited three seconds before timing out and ending the read program.

Conclusion

I hope this tutorial was helpful in getting you started with the read command in Linux. As always, we love to hear from our readers about content they’re interested in. Leave a comment below and share your thoughts with us!

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35 Linux Basic Commands Every User Should Know

When hearing about Linux, most people think of a complicated operating system that is only used by programmers. But it’s not as scary as it seems.

Linux is an entire family of open-source Unix operating systems, that are based on the Linux Kernel. This includes all of the most popular Linux based systems like Ubuntu, Fedora, Mint, Debian, and others. More accurately, they’re called distributions or distros.

Since Linux was first released in 1991, it has continued to gain popularity due to its open-source nature. People can freely modify and redistribute it under their own name.

When operating a Linux OS, you need to use a shell — an interface that gives you access to the operating system’s services. Most Linux distributions use a graphic user interface (GUI) as their shell, mainly to provide ease of use for their users.

That being said, it’s recommended to use a command-line interface (CLI) because it’s more powerful and effective. Tasks that require a multi-step process through GUI can be done in a matter of seconds by typing commands into the CLI.

So if you’re considering using Linux, learning basic command lines will go a long way. In this article, you’ll learn 35 basic Linux commands that will undoubtedly help you navigate through Linux as a newbie.

Linux Basic Commands

Before we go on to the list of commands, you need to open the command line first. If you are still unsure about the command-line interface, check out this CLI tutorial.

Although the steps may differ depending on the distribution that you’re using, you can usually find the command line in the Utilities section.

Here is a list of basic Linux commands:

1. pwd command

Use the pwd command to find out the path of the current working directory (folder) you’re in. The command will return an absolute (full) path, which is basically a path of all the directories that starts with a forward slash (/). An example of an absolute path is /home/username.

2. cd command

To navigate through the Linux files and directories, use the cd command. It requires either the full path or the name of the directory, depending on the current working directory that you’re in.

Let’s say you’re in /home/username/Documents and you want to go to Photos, a subdirectory of Documents. To do so, simply type the following command: cd Photos.

Another scenario is if you want to switch to a completely new directory, for example,/home/username/Movies. In this case, you have to type cd followed by the directory’s absolute path: cd /home/username/Movies.

There are some shortcuts to help you navigate quickly:

• cd .. (with two dots) to move one directory up
• cd to go straight to the home folder
• cd- (with a hyphen) to move to your previous directory

On a side note, Linux’s shell is case sensitive. So, you have to type the name’s directory exactly as it is.

3. ls command

The ls command is used to view the contents of a directory. By default, this command will display the contents of your current working directory.

If you want to see the content of other directories, type ls and then the directory’s path. For example, enter ls /home/username/Documents to view the content of Documents.

There are variations you can use with the ls command:

• ls -R will list all the files in the sub-directories as well
• ls -a will show the hidden files
• ls -al will list the files and directories with detailed information like the permissions, size, owner, etc.

4. cat command

cat (short for concatenate) is one of the most frequently used commands in Linux. It is used to list the contents of a file on the standard output (sdout). To run this command, type cat followed by the file’s name and its extension. For instance: cat file.txt.

Here are other ways to use the cat command:

• cat > filename creates a new file
• cat filename1 filename2>filename3 joins two files (1 and 2) and stores the output of them in a new file (3)
• to convert a file to upper or lower case use, cat filename | tr a-z A-Z >output.txt

5. cp command

Use the cp command to copy files from the current directory to a different directory. For instance, the command cp scenery.jpg /home/username/Pictures would create a copy of scenery.jpg (from your current directory) into the Pictures directory.

6. mv command

The primary use of the mv command is to move files, although it can also be used to rename files.

The arguments in mv are similar to the cp command. You need to type mv, the file’s name, and the destination’s directory. For example: mv file.txt /home/username/Documents.

To rename files, the Linux command is mv oldname.ext newname.ext

7. mkdir command

Use mkdir command to make a new directory — if you type mkdir Music it will create a directory called Music.

There are extra mkdir commands as well:

• To generate a new directory inside another directory, use this Linux basic command mkdir Music/Newfile
• use the p (parents) option to create a directory in between two existing directories. For example, mkdir -p Music/2020/Newfile will create the new “2020” file.

8. rmdir command

If you need to delete a directory, use the rmdir command. However, rmdir only allows you to delete empty directories.

9. rm command

The rm command is used to delete directories and the contents within them. If you only want to delete the directory — as an alternative to rmdir — use rm -r.

Note: Be very careful with this command and double-check which directory you are in. This will delete everything and there is no undo.

10. touch command

The touch command allows you to create a blank new file through the Linux command line. As an example, enter touch /home/username/Documents/Web.html to create an HTML file entitled Web under the Documents directory.

11. locate command

You can use this command to locate a file, just like the search command in Windows. What’s more, using the -i argument along with this command will make it case-insensitive, so you can search for a file even if you don’t remember its exact name.

To search for a file that contains two or more words, use an asterisk (*). For example, locate -i school*note command will search for any file that contains the word “school” and “note”, whether it is uppercase or lowercase.

12. find command

Similar to the locate command, using find also searches for files and directories. The difference is, you use the find command to locate files within a given directory.

As an example, find /home/ -name notes.txt command will search for a file called notes.txt within the home directory and its subdirectories.

Other variations when using the find are:

• To find files in the current directory use, find . -name notes.txt
• To look for directories use, / -type d -name notes. txt

13. grep command

Another basic Linux command that is undoubtedly helpful for everyday use is grep. It lets you search through all the text in a given file.

To illustrate, grep blue notepad.txt will search for the word blue in the notepad file. Lines that contain the searched word will be displayed fully.

14. sudo command

Short for “SuperUser Do”, this command enables you to perform tasks that require administrative or root permissions. However, it is not advisable to use this command for daily use because it might be easy for an error to occur if you did something wrong.

15. df command

Use df command to get a report on the system’s disk space usage, shown in percentage and KBs. If you want to see the report in megabytes, type df -m.

16. du command

If you want to check how much space a file or a directory takes, the du (Disk Usage) command is the answer. However, the disk usage summary will show disk block numbers instead of the usual size format. If you want to see it in bytes, kilobytes, and megabytes, add the -h argument to the command line.

17. head command

The head command is used to view the first lines of any text file. By default, it will show the first ten lines, but you can change this number to your liking. For example, if you only want to show the first five lines, type head -n 5 filename.ext.

18. tail command

This one has a similar function to the head command, but instead of showing the first lines, the tail command will display the last ten lines of a text file. For example, tail -n filename.ext.

19. diff command

Short for difference, the diff command compares the contents of two files line by line. After analyzing the files, it will output the lines that do not match. Programmers often use this command when they need to make program alterations instead of rewriting the entire source code.

The simplest form of this command is diff file1.ext file2.ext

20. tar command

The tar command is the most used command to archive multiple files into a tarball — a common Linux file format that is similar to zip format, with compression being optional.

This command is quite complex with a long list of functions such as adding new files into an existing archive, listing the content of an archive, extracting the content from an archive, and many more. Check out some practical examples to know more about other functions.

21. chmod command

chmod is another Linux command, used to change the read, write, and execute permissions of files and directories. As this command is rather complicated, you can read the full tutorial in order to execute it properly.

22. chown command

In Linux, all files are owned by a specific user. The chown command enables you to change or transfer the ownership of a file to the specified username. For instance, chown linuxuser2 file.ext will make linuxuser2 as the owner of the file.ext.

23. jobs command

jobs command will display all current jobs along with their statuses. A job is basically a process that is started by the shell.

24. kill command

If you have an unresponsive program, you can terminate it manually by using the kill command. It will send a certain signal to the misbehaving app and instructs the app to terminate itself.

There is a total of sixty-four signals that you can use, but people usually only use two signals:

• SIGTERM (15) — requests a program to stop running and gives it some time to save all of its progress. If you don’t specify the signal when entering the kill command, this signal will be used.
• SIGKILL (9) — forces programs to stop immediately. Unsaved progress will be lost.

Besides knowing the signals, you also need to know the process identification number (PID) of the program you want to kill. If you don’t know the PID, simply run the command ps ux.

After knowing what signal you want to use and the PID of the program, enter the following syntax:

kill [signal option] PID.

25. ping command

Use the ping command to check your connectivity status to a server. For example, by simply entering ping google.com, the command will check whether you’re able to connect to Google and also measure the response time.

26. wget command

The Linux command line is super useful — you can even download files from the internet with the help of the wget command. To do so, simply type wget followed by the download link.

27. uname command

The uname command, short for Unix Name, will print detailed information about your Linux system like the machine name, operating system, kernel, and so on.

28. top command

As a terminal equivalent to Task Manager in Windows, the top command will display a list of running processes and how much CPU each process uses. It’s very useful to monitor system resource usage, especially knowing which process needs to be terminated because it consumes too many resources.

29. history command

When you’ve been using Linux for a certain period of time, you’ll quickly notice that you can run hundreds of commands every day. As such, running history command is particularly useful if you want to review the commands you’ve entered before.

30. man command

Confused about the function of certain Linux commands? Don’t worry, you can easily learn how to use them right from Linux’s shell by using the man command. For instance, entering man tail will show the manual instruction of the tail command.

31. echo command

This command is used to move some data into a file. For example, if you want to add the text, “Hello, my name is John” into a file called name.txt, you would type echo Hello, my name is John >> name.txt

32. zip, unzip command

Use the zip command to compress your files into a zip archive, and use the unzip command to extract the zipped files from a zip archive.

33. hostname command

If you want to know the name of your host/network simply type hostname. Adding a -i to the end will display the IP address of your network.

34. useradd, userdel command

Since Linux is a multi-user system, this means more than one person can interact with the same system at the same time. useradd is used to create a new user, while passwd is adding a password to that user’s account. To add a new person named John type, useradd John and then to add his password type, passwd 123456789.

To remove a user is very similar to adding a new user. To delete the users account type, userdel UserName

Bonus Tips and Tricks

Use the clear command to clean out the terminal if it is getting cluttered with too many past commands.

Try the TAB button to autofill what you are typing. For example, if you need to type Documents, begin to type a command (let’s go with cd Docu, then hit the TAB key) and the terminal will fill in the rest, showing you cd Documents.

Ctrl+C and Ctrl+Z are used to stop any command that is currently working. Ctrl+C will stop and terminate the command, while Ctrl+Z will simply pause the command.

If you accidental freeze your terminal by using Ctrl+S, simply undo this with the unfreeze Ctrl+Q.

Ctrl+A moves you to the beginning of the line while Ctrl+E moves you to the end.

You can run multiple commands in one single command by using the “;” to separate them. For example Command1; Command2; Command3. Or use && if you only want the next command to run when the first one is successful.

To Sum Up

Basic Linux commands help users execute tasks easily and effectively. It might take a while to remember some of the basic commands, but nothing is impossible with lots of practice.

In the end, knowing and mastering these basic Linux commands will be undoubtedly beneficial for you. Good luck!

Artūras is an experienced technical content writer. Bringing in a lot of knowledge about WordPress and web hosting to the team, he strives to write pristine content about any IT related subject. He also loves dogs.

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