Write commands to file linux

How to write the output into the file in Linux

How do I save terminal output to a file?

A command can receive input from a file and send output to a file.

Writing the output into the file

The syntax is
command > filename
For example, send output of the ls command to file named foo.txt
$ ls > foo.txt
View foo.txt using the cat command:
$ cat foo.txt
Please note that when you type ‘ls > foo.txt’, shell redirects the output of the ls command to a file named foo.txt, replacing the existing contents of the file. In other words, the contents of the file will be overwritten.

Appending the output or data to the file

The syntax is
command >> filename
For example the following will append data:

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Verify it:
cat /tmp/data.txt

How to save the output of a command to a file in bash using tee command

The tee command read from standard input and write to standard output and files. The syntax is as follows for writing data into the file:
command | tee file.txt
Want to append data? Try
command | tee -a output.txt

Examples

Display output of the date command on screen and save to the file named /tmp/output.txt. If the output.txt already exists, it gets overwritten:
$ date | tee /tmp/output.txt
$ cat /tmp/output.txt
Same as above but append to the given files, do not overwrite file:
$ pwd | tee -a /tmp/test.txt
$ echo «Today is $(date)» | tee -a /tmp/test.txt
$ hostnamectl | tee -a /tmp/test.txt
$ cat /tmp/test.txt

The above commands will append the output to the end of the file, just like the shell >> operator as explained earlier.

I/O redirection summary for bash and POSIX shell

Shell operator	Description	Overwrite existing file?
command > output.txt	Save terminal output (standard output) to a file named output.txt	Yes
command >> output.txt	Append terminal output (standard output) to a file named output.txt	No
command	Takes standard input from output.txt file	N/A
command 0	Takes standard input from output.txt file	N/A
command 1> output.txt	Puts standard output to output.txt file	Yes
command 1>> output.txt	Appends standard output to output.txt	No
command 2> output.txt	Puts standard error to output.txt	Yes
command 2>> output.txt	Appends standard error to output.txt file	No
command &> output.txt	Puts both standard error and output to output.txt	Yes
command > output.txt 2>&1	<POSIX> Puts both standard error and output to file named output.txt	Yes
command &>> output.txt	Appends both standard error and output to file named output.txt	No
command >> output.txt 2>&1	<POSIX> Appends both standard error and output to file called output.txt	No
command | tee output.txt	Puts standard output to output.txt while displaying output on screen	Yes
command | tee -a output.txt	Appends standard output to output.txt while displaying output on screen	No
command |& tee output.txt	Puts both standard output and error to output.txt while displaying output on terminal	Yes
command 2>&1 | tee output.txt	<POSIX> Puts both standard output and error to file named output.txt while displaying output on terminal	Yes
command |& tee -a output.txt	Append both standard output and error to file called output.txt while displaying output on terminal	No
command 2>&1 | tee -a output.txt	<POSIX> Append both standard output and error to file named output.txt while displaying output on terminal	No

Conclusion

You learned how to write the output to the file in Linux or Unix-like system when using bash or POSIX shell. We have:

1. /dev/stdin (standard input) — File descriptor 0 is duplicated.
2. /dev/stdout (standard output) — File descriptor 1 is duplicated.
3. /dev/stderr (standard error) — File descriptor 2 is duplicated.

See I/O redirection documentation for more information. We can read bash man page as follows using the man command:
man bash

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How To Use Cat Command To Append Data To a File on Linux/Unix

I am a new Unix user. I have Debian Linux installed. I need to append text to a file called daily.log. How do I use the cat command to append data to a file?

You can use the cat command to append data or text to a file. The cat command can also append binary data. The main purpose of the cat command is to display data on screen (stdout) or concatenate files under Linux or Unix like operating systems. To append a single line you can use the echo command or printf command command. Let us see how to use the cat command to append data and update files without losing its content.

Tutorial details
Difficulty level	Easy
Root privileges	No
Requirements	cat on Linux or Unix
Est. reading time	2 mintues

How To Use Cat Command To Append Data To a File on Linux/Unix

Redirection symbol is as follows for appending data to a file:

Syntax

Examples

Create a text file called foo.txt, type:

To save the changes press CTRL-d i.e. press and hold CTRL and press d. Create another text file called bar.txt as follows:

Display both files on the screen, enter:

To append a contains of bar.txt to to foo.txt, enter:

To append a ‘Use unix or die’ text to foo.txt file, enter:

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Fig.01: Using the cat and echo command to append a text to a file.

Append text to a file when using sudo command

We can use the echo command or printf command to append data to a file called sales.txt in the current directory:

Want to append to a file? Try:
cat filename | sudo tee -a foo_file.txt
In this example append data using the following syntax:
sudo sh -c ‘echo «192.168.1.253 wireless-router» >> /etc/hosts’
Verify it:
cat /etc/hosts

Summing up

We explained various Linux and Unix commands that one could use to append data to a file. Although I tested all examples on Bash running on macOS and Linux desktop, these examples should work with other shells, too, such as:

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Category	List of Unix and Linux commands
Documentation	help • mandb • man • pinfo
Disk space analyzers	df • duf • ncdu • pydf
File Management	cat • cp • less • mkdir • more • tree
Firewall	Alpine Awall • CentOS 8 • OpenSUSE • RHEL 8 • Ubuntu 16.04 • Ubuntu 18.04 • Ubuntu 20.04
Linux Desktop Apps	Skype • Spotify • VLC 3
Modern utilities	bat • exa
Network Utilities	NetHogs • dig • host • ip • nmap
OpenVPN	CentOS 7 • CentOS 8 • Debian 10 • Debian 8/9 • Ubuntu 18.04 • Ubuntu 20.04
Package Manager	apk • apt
Processes Management	bg • chroot • cron • disown • fg • glances • gtop • jobs • killall • kill • pidof • pstree • pwdx • time • vtop
Searching	ag • grep • whereis • which
Shell builtins	compgen • echo • printf
Text processing	cut • rev
User Information	groups • id • lastcomm • last • lid/libuser-lid • logname • members • users • whoami • who • w
WireGuard VPN	Alpine • CentOS 8 • Debian 10 • Firewall • Ubuntu 20.04

Comments on this entry are closed.

when i execute two commands like echo $val(nn) > t.txt and awk -f throughput.awk wpan.tr >t.txt
i want the file t.txt to have two columns side by side leaving a tab or space in between. but i m getting output like this

pls help in this regard…thanks

I’m not sure I understood your query. To merge corresponding or subsequent lines of files try paste command.

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Запись файлов в Bash

Одна из наиболее распространенных задач при написании сценариев Bash или работе в командной строке Linux — это чтение и запись файлов.

В этой статье объясняется, как записать текст в файл в Bash с помощью операторов перенаправления и команды tee .

Запись в файл с использованием операторов перенаправления

В Bash перенаправление вывода позволяет вам захватить вывод команды и записать его в файл.

Общий формат перенаправления и записи вывода в файл следующий:

• Оператор перенаправления > записывает вывод в указанный файл. Если файл существует, он обрезается до нулевой длины. В противном случае файл создается. Будьте особенно осторожны при использовании этого оператора, так как вы можете перезаписать важный файл.
• Оператор перенаправления >> добавляет вывод в указанный файл. Если файл не существует, он создается.

У вас должны быть права на запись в файл. В противном случае вы получите сообщение об ошибке в разрешении отказано.

Вот простой пример, показывающий, как перенаправить вывод команды echo в файл:

Чтобы предотвратить перезапись существующих файлов, включите опцию «noclobber» с помощью встроенной команды set :

>| оператор позволяет вам переопределить параметр Bash «noclobber»

Оператор >> добавляет вывод в конец файла, а не перезаписывает файл:

Используйте команду printf если вы хотите создать сложный вывод:

Если вы хотите записать несколько строк в файл, используйте перенаправление документа Here (Heredoc).

Например, вы можете передать содержимое команде cat и записать его в файл:

Чтобы добавить строки, замените > на >> перед именем файла:

Вы можете записать вывод любой команды в файл:

Вывод команды date будет записан в файл.

Запись в файл с помощью команды tee

Команда tee читает из стандартного ввода и записывает как в стандартный вывод, так и в один или несколько файлов одновременно.

По умолчанию команда tee перезаписывает указанный файл, как и оператор > . Чтобы добавить вывод в файл, вызовите команду с параметром -a ( —append ):

Если вы не хотите, чтобы tee выводил данные на стандартный вывод, вы можете перенаправить его на /dev/null :

Чтобы записать текст в несколько файлов, укажите файлы в качестве аргументов команды tee :

Еще одно преимущество команды tee заключается в том, что вы можете использовать ее вместе с sudo и записывать в файлы, принадлежащие другим пользователям. Чтобы добавить текст в файл, в который у вас нет прав на запись, добавьте sudo перед tee :

Выходные данные команды echo передаются как входные в tee , который повышает разрешения sudo и записывает текст в файл.

Выводы

В Linux для записи текста в файл используйте операторы перенаправления > и >> или команду tee .

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

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Writing to Files Using cat Command on Linux

Introduction

The cat command is a Unix tool used for manipulating and displaying file contents. The command gets its name from the word «concatenate» because it has, among other things, the ability to concatenate files.

In this article, we’ll go through a few easy ways to use this command to write text to a file with examples. Using cat is very straightforward, so no prior programming or Unix experience is needed to follow along.

cat Command Basics

Starting, we’ll just sum up the basics of the cat command to help you out if you’ve never used it before or need a brief overview.

Syntax

The syntax looks like this:

To quickly look up the syntax or command options, run cat with the help option:

Or, you can use the manual pages:

These commands should display the following list of options:

Displaying File Contents on the Standard Output

To print the contents of a file to the standard output just name the file you want to display:

If the file is in a different directory, you’ll have to navigate to it:

We’ll be expecting to see the contents of this file, printed to the standard output, in this case — the terminal:

This is the most common use of the cat command since it makes it easy to peek into the contents of a file without opening a text editor.

Writing Text to a File Using cat

To redirect the output of the cat command from the standard output to a file, we can use the output redirection operator > :

This will overwrite the contents of filename2 with the contents of filename1 , so make sure that filename2 doesn’t contain anything you would mind losing. Now, filename2 contains:

The output redirection operator will redirect the output of any command we call. For example, let’s try it out with the pwd command, which prints the name of the current working directory:

If we take a look at the testfile now:

It contains the current working directory’s path:

If the file you are redirecting to doesn’t exist, a file by that name will be created:

Concatenating Files with cat

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Concatenating multiple files using cat is simple — just list the files in the desired order:

This takes the filename1 and filename2 files, concatenates them, and outputs the into a new outputfile :

Standard Input Between Files

When the name of the input file isn’t listed, cat starts reading from the standard input until it reaches EOF (end-of-file). The end-of-file signal is sent by the ctrl+d command line shortcut:

This would output:

We can even add text from the standard input in between files we wish to concatenate by using — to indicate where we expect standard input. If we have files such as filename1 , filename2 , and filename3 , and we want some text from the standard input in between filename1 and filename2 , we would write:

Checking output , we’ll see something along the lines of:

Appending Files with cat

In the previous examples, using the redirection operator discarded the previous contents of the output file. What if we want to append the new content to the old content? To append files we use the >> operator:

And that should result in:

Concatenating Contents of all Files Directory with cat

To concatenate all contents of all files in a directory, we use the wildcard * :

To concatenate all contents of all files in the current working directory, we’d use:

* can also be used to concatenate all files with the same extension:

Enumerating Line Numbers

Enumeration of all lines of output is done with the -n option:

Which would write something along the lines of:

Write $ at the End of Every Line

The -E option marks the end of every line in the file with the $ character:

Sorting Lines of Concatenated Files by Piping

This one is a bit of a cheat. The cat command can’t sort, but we can use piping to achieve that. The pipe command ( | ) is used to turn the output of one command into the input of another. To sort the lines of a file, we’ll use both cat and another command, called sort :

This results in:

Conclusion

Cat is a simple, yet powerful Unix tool that comes pre-installed on most systems. It can be used on its own or in combination with other commands using pipes. Originally made by Ken Thompson and Dennis Ritchie in 1971, cat ‘s easy to use and intuitive functionalities stand the test of time.

In this article, we’ve explored some of the possibilities of using the cat command to write text to files, check contents, concatenate and append files as well as enumerate lines and sort them.

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