Stream to file linux

Bash output stream write to a file

And this just keeps running and as soon as it has any cause it outputs something. So there is no end to execution. No the echo is working perfectly, however when i am trying to use the ‘>’ operator this doesn’t seem to write to file. so for instance

#somedevice -getevent > my_record_file 

this doesn’t work properly, my_record_file only gets data written to it in intervals, however i want to be written immediately. Any ideas?

The lag may be due to output buffering. Can you change the program to do either do a flush() after each print or set auto-flush on its standard output?

Okay, i figured out a solution for my issue, it was a buffering issue somedevice -getevent | grep » » —line-buffered > myrecordfile solved the issue.. perhaps its not the best idea, but for now this hack did the job.. thanks for all you help!

4 Answers 4

The output is being buffered because the C standard library changes the output buffering mode depending on whether or not stdout is a terminal device. If it’s a terminal device (according to isatty(3) ), then stdout is line-buffered: it gets flushed every time a newline character gets written. If it’s not a terminal device, then it’s fully buffered: it only gets flushed whenever a certain amount of data (usually something on the order of 4 KB to 64 KB) gets written.

So, when you redirect the command’s output to a file using the shell’s > redirection operator, it’s no longer outputting to a terminal and it buffers its output. A program can change its buffering mode with setvbuf(3) and friends, but the program has to cooperate to do this. Many programs have command line options to make them line-buffered, e.g. grep(1) ‘s —line-buffered option. See if your command has a similar option.

If you don’t have such an option, you can try using a tool such as unbuffer(1) to unbuffer the output stream, but it doesn’t always work and isn’t a standard utility, so it’s not always available.

The command somedevice probably uses the «Standard Input/Output Library», and in that library, the buffering is on by default. It is switched off when the output does to a terminal/console.

Can you modify the somedevice program? If not, you can still hack around it. See http://www.pixelbeat.org/programming/stdio_buffering/ for details.

somedevice -getevent | tee -a my_record_file 

The ‘-a’ option is to append instead of just replacing the content.

This is probably because your » somedevice -getevent » command’s stdout is being block-buffered. According to this, stdout is by default line-buffered (i.e. what you want) if stdout is a terminal, and block-buffered otherwise.

I’d have a look at the manual for your somedevice command to see if you can force the output to be unbuffered or line-buffered. If not, stdbuf -oL somedevice -getevent > my_record_file should do what you want.

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Redirect all output to file in Bash [duplicate]

I know that in Linux, to redirect output from the screen to a file, I can either use the > or tee . However, I’m not sure why part of the output is still output to the screen and not written to the file. Is there a way to redirect all output to file?

9 Answers 9

That part is written to stderr, use 2> to redirect it. For example:

foo > stdout.txt 2> stderr.txt 

or if you want in same file:

Note: this works in (ba)sh, check your shell for proper syntax

well, i found the reference and have deleted my post for having incorrect information. from the bash manual: ‘»ls 2>&1 > dirlist» directs only the standard output to dirlist, because the standard error was duplicated from the standard output before the standard output was redirected to dirlist» 🙂

also from the bash man «There are two formats for redirecting standard output and standard error: &>word and >&word Of the two forms, the first is preferred. This is semantically equivalent to >word 2>&1»

Two important addenda: If you want to pipe both stdout and stderr, you have to write the redirections in the opposite order from what works for files, cmd1 2>&1 | cmd2 ; putting the 2>&1 after the | will redirect stderr for cmd2 instead. If both stdout and stderr are redirected, a program can still access the terminal (if any) by opening /dev/tty ; this is normally done only for password prompts (e.g. by ssh ). If you need to redirect that too, the shell cannot help you, but expect can.

All POSIX operating systems have 3 streams: stdin, stdout, and stderr. stdin is the input, which can accept the stdout or stderr. stdout is the primary output, which is redirected with > , >> , or | . stderr is the error output, which is handled separately so that any exceptions do not get passed to a command or written to a file that it might break; normally, this is sent to a log of some kind, or dumped directly, even when the stdout is redirected. To redirect both to the same place, use:

EDIT: thanks to Zack for pointing out that the above solution is not portable—use instead:

If you want to silence the error, do:

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How do I Redirect Output to a File in Linux

Redirecting is a method of changing the standard input and output devices when executing a command. Linux commands work by taking input and producing an output.

We can use redirection to send and use the data present in the system so far. By using Redirect, we will not send copies of our data to the system. The following article explains how to use redirect output to a file in Linux.

How do I Redirect Output to a File in Linux?

Direct Redirection is an interesting Linux feature, which allows users to send requests directly to the kernel (stdin) while receiving output from the kernel (stdout) only through direct redirection. Linux allows users to save files using the stdout (output) command, also known as the stream command. Transmitting data between a server and a computer is done with stream commands.

With Redirect, you can make the commands visible in the terminal and save them to review later. Below are a few ways to explain Linux redirection:

Send a Single File Output

We can redirect in bash by using a script or command in the form of > or >>. This command provides the path of the file.

• The output redirect for the > command changes the file itself.
• The following command >> adds the given output to the output redirect file.

Stdout displays vital reductions as a standard command. ls is the next command, which displays everything as listed on the screen.

This command doesn’t need to specify any path; it automatically displays and saves it.

If you look at the file, the output of the ls command will appear as if the cat command prints the contents to the terminal.

The following command is to be used to print the output redirection.

Operator > is given when only one output is issued, but >> is used to save the output in the following command:

Bash creates the file if no file exists; otherwise leaves it as it is and appears at the end if it is appended.

Redirect from Terminal to a Single Output

Given method is used when > or >> is not to be used. In this, instead of > Tee command is used which performs both printing and saving functions. So here is the following command:

Reducing the terminal with the appropriate command overwrites the output by appending > as in the tee command.

Using Top command

The top command is used to view essential information like real-time, memory used, processes being used in the Linux system. B plague is used in the given function. Which gets all the information about the given command, including the full command permissions, and provides the output by specifying its number. The puzzle command is given to redirect the output specified as well as its less command.

Now N attention command will be given, which will be used to determine their number in which one or number is written after n.

Finally, the cat command is used to give details of what is going on.

Conclusion

This article also explains how to redirect the output into a file and add and re-creating the file through the output. We hope that this article will prove to be of your use as well as have learned something from it.

About the author

Prateek Jangid

A passionate Linux user for personal and professional reasons, always exploring what is new in the world of Linux and sharing with my readers.

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How to Save Command Output to a File in Linux

Linux command line interface (CLI) is a powerful tool that allows users to interact with the operating system using text-based commands. One common task in the CLI is executing commands and capturing their output to a file for later use. In this article, we’ll explore various ways to save command output to a file in Linux.

Understanding Output Streams

Before we dive into the different ways to save command output, it’s important to understand how output streams work in Linux. There are three main types of output streams:

• Standard Output (stdout): Standard Output is the default output stream where a command sends its output. By default, stdout sends output to the terminal window.
• Standard Error (stderr): Standard Error outputs error messages or diagnostics. Like stdout, stderr also sends its output to the terminal window.
• Standard Input (stdin): Standard Input accepts input from the user. Unlike stdout and stderr, stdin is not visible in the terminal window.

Saving Command Output to a File

Now that we understand the different output streams let’s explore how to save command output to a file in Linux.

Saving Standard Output to a File

To save the standard output of a command to a file, we can use the “>” (redirect) operator followed by the filename. For example, the following command will save the output of the “ls” command to a file named “filelist.txt”:

If the file “filelist.txt” already exists, its contents will be overwritten. To append the output to the end of an existing file, we can use the “>>” (append) operator instead of the “>” operator:

Saving Standard Error to a File

To save the standard error of a command to a file, we can use the “2>” (redirect standard error) operator followed by the filename. For example, the following command will save any error messages produced by the “ls” command to a file named “errorlog.txt”:

ls /invalid/path 2> errorlog.txt

The file “errorlog.txt” will be empty if there are no errors.

Saving Both Standard Output and Standard Error to a File

To save both the standard output and standard error of a command to a file, we can use the “&>” (redirect both stdout and stderr) operator followed by the filename. For example, the following command will save both the output and any error messages produced by the “ls” command to a file named “output.txt”:

ls /invalid/path &> output.txt

Using Pipes to Save Output to a File

In addition to the above methods, we can also use pipes to save the output of a command to a file. A pipe is a way to redirect the output of one command as input to another command. To save the output of a command to a file using a pipe, we can use the “|” (pipe) operator followed by the “tee” command and the filename. For example, the following command will save the output of the “ls” command to a file named “filelist.txt” using the “tee” command:

The “tee” command displays the output on the terminal window and saves it to the file.

Using xargs to Save Output to a File

Another useful tool to save command output to a file is xargs. xargs is a command that reads items from standard input and executes a command for each item. To save the command output to a file using xargs, we can use the “>” (redirect) operator followed by the filename, as shown below:

echo "file1.txt file2.txt file3.txt" | xargs cat > combined.txt 

The above command will concatenate the contents of the files file1.txt, file2.txt, and file3.txt and save the output to a file named combined.txt.

Saving Command Output to Multiple Files

Sometimes, we may want to save the output of a command to multiple files. We can achieve this by using brace expansion in the command. For example, the following command will save the output of the “ls” command to files named “file1.txt”, “file2.txt”, and “file3.txt”:

Conclusion

Once you understand the different output streams and redirection operators, saving command output to a file in Linux is a simple task. Using these methods, you can easily save the output of a command to a file for later use. Always refer to the official documentation and community resources for more information and help with Linux commands.

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