Write into 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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TechyTalk

Thoughts on Magento, PHP, Linux and open source

Linux system programming: Open file, read file and write file

This is my first article in what I’m hoping will be a series of articles on system programming for POSIX compliant operating systems with focus on Linux. Actually I’ve touched this topic a while ago when I wrote three articles about library programming on Linux (static libraries, dynamic libraries and dynamic libraries using POSIX API). In this series my goal is to go trough basics of Linux system programming from the easiest topics like open file, read file and file write to a bit more complicated things like Berkeley sockets network programming. So lets get started with environment setup and an example of program that copies source file into destination file using POSIX API system calls to demonstrate open(), read() and write() system calls on Linux operating system.

Configuring your environment

I’ll use my trustworthy Ubuntu Linux operating system but you can actually use any POSIX compliant operating system, the only difference will probably be that you will need to configure your environment differently. What we need to begin with Linux system programming is gcc compiler with related packages and POSIX related man pages. So here’s how to install this packages on Ubuntu based operating system:

Basically that’s all you need to create serious system tools for Linux operating system. Later we will probably need some more libraries but we will install them when necessary.

open(), read() and write() system calls

If you have named this code file sp_linux_copy.c and if you want to name executable file sp_linux_copy to compile this program you would probably use something like this:

Then if your source file is named source_file.txt and if you want to name the destination file destination_file.txt you would run this program like this:

Now lets go trough the code and explain tricky parts. First thing we must do is to include necessary header files. Man page of every system call tells you what header files you need to include to be able to use this system call. Second we will define constant we will use to define size of our buffer in bytes. Smaller buffer size will make our copy process longer but it will save memory. Next we open source and destination file descriptors, source with O_RDONLY to make it read only, destination with O_WRONLY | O_CREAT to make it writable and to create destination file with 0644 file system permission flags. In case of error we use perror() man 3 perror to print relatively user friendly error message.

Now we are ready to start copy process. We run read() and write() inside loop (because source file might be bigger than our buffer) to copy from one file into another. Important to notice is that write() is using number of bytes read from source file returned by read() so it would know how much to write into destination file. If number of bytes read (ret_in) and number of bytes written (ret_out) differ this indicates error so once again we use perror() to print out error description. At the end if all went well we do cleanup by closing both file descriptors and returning 0 (EXIT_SUCCESS) to indicate that program ended without errors.

That’s it for this introductory article on Linux system programming topic. In my next article I will show you few more examples on POSIX input/output and then move on to memory management related system calls.

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Thread: writing to a file from the command-line

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writing to a file from the command-line

I have created a file called myfile.txt. So could someone tell me how to write something into the file from the command-line please

Re: writing to a file from the command-line

Re: writing to a file from the command-line

$cat some text > myfile.txt

$ cat some text >> myfile.txt .. append to file

Re: writing to a file from the command-line

$cat some text > myfile.txt

$ cat some text >> myfile.txt .. append to file

echo «some text»>>myfile.txt
cat «somefile.txt>>myfile.txt»

If you use only a single more than symbol >, it will delete the file and write ONLY what you asked it to. a double >> adds it onto the end without deleting anything.

cat is for files (It reads the file and outputs it, i.e. cat file.txt gives back the contents of file.txt)
Echo is for plain text.

Re: writing to a file from the command-line

Re: writing to a file from the command-line

Shows just how much of a noob i am. So i shall start from the beginning.

I created a text file called myfile.txt by using: touch myfile.txt
I also created a directory by using: mkdir Mytext
I moved myfile.txt into the Mytext directory by using: mv myfile.txt

/Mytext
I typed ls to make sure that myfile was inside the Mytext directory

Now while i was still in the Mytext directory i typed:
cat «Up hill and over dale»>>myfile.txt (clicked on Enter)

And was rewarded with: No such file or diectory

Would someone please tell me where i am going wrong?

Re: writing to a file from the command-line

I used: echo «Up hill and over dale»>>myfile.txt (clicked on enter)
and the used: cat myfile.txt to view the contents

Thanks for the replies everyone

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How to Write to a File in Bash

There are multiple ways to read and write a file in bash. The simplest way is using operators “>” and “>>”.

• “>” operator will overwrite the existing data
• “>>” operator will append data

The general format of using redirection operators is:

Let’s understand the writing to a file procedure with an example:

How to write a file using redirection operators

As discussed above, the simple and straightforward approach of writing to a file is using redirection operators. For instance, if you want to change the text of an already existing file, then firstly create a text file by the name of “testfile.txt” and write anything in it:

Save the text file.

Type the below-mentioned command in the terminal:

Over wring may be risky; therefore, it is good practice to enable “noclobber”. Setting “noclobber” would block any overwriting to any exiting file.

But if you want to bypass “noclobber” then use the”>|” operator instead of “>”:

Or you can simply disable “noclobber”:

But this command will take away protection from all the files.

The above output is indicating that the existing text has been overwritten. Now, let’s use the “>>” operator:

“echo” is not always ideal to use since you cannot format text using it, therefore use “printf” in the place of “echo” to format the text as demonstrated in the following command:

Let’s understand the concept with a bash script example. Open Vim by typing “vim” in the terminal. If you do not have Vim editor on your device, then install it using:

Type the script:

The “cat” command is used to create and edit the files. Save the above script in Vim by switching mode after pressing the “Esc” key and then type “:w myscript.sh”. Open the terminal and run the code:

How to write a file using Heredoc

If you want to write multiple lines, then the easiest method is using “Heredoc”. Here document, also known as “Heredoc,” is a multi-purpose code block. The syntax of Heredoc is:

About the author

Sam U

I am a professional graphics designer with over 6 years of experience. Currently doing research in virtual reality, augmented reality and mixed reality.
I hardly watch movies but love to read tech related books and articles.

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write(2) — Linux man page

Synopsis

Description

The number of bytes written may be less than count if, for example, there is insufficient space on the underlying physical medium, or the RLIMIT_FSIZE resource limit is encountered (see setrlimit(2)), or the call was interrupted by a signal handler after having written less than count bytes. (See also pipe(7).)

For a seekable file (i.e., one to which lseek(2) may be applied, for example, a regular file) writing takes place at the current file offset, and the file offset is incremented by the number of bytes actually written. If the file was open(2)ed with O_APPEND, the file offset is first set to the end of the file before writing. The adjustment of the file offset and the write operation are performed as an atomic step.

POSIX requires that a read(2) which can be proved to occur after a write() has returned returns the new data. Note that not all file systems are POSIX conforming.

Return Value

If count is zero and fd refers to a regular file, then write() may return a failure status if one of the errors below is detected. If no errors are detected, 0 will be returned without causing any other effect. If count is zero and fd refers to a file other than a regular file, the results are not specified.

Errors

Other errors may occur, depending on the object connected to fd.

Conforming to

Under SVr4 a write may be interrupted and return EINTR at any point, not just before any data is written.

Notes

If a write() is interrupted by a signal handler before any bytes are written, then the call fails with the error EINTR; if it is interrupted after at least one byte has been written, the call succeeds, and returns the number of bytes written.

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