Linux log output to file

how to log all the command output to one single file in bash scripting [duplicate]

In gnu/Linux i want to log all the command output to one particular file. Say in terminal,i am typing echo «Hi this is a dude» It should print in the file name specified earlier without using the redirection in every command.

5 Answers 5

$ script x1 Script started, file is x1 $ echo "Hi this is a dude" Hi this is a dude $ echo "done" done $ exit exit Script done, file is x1 

Then, the contents of file x1 are:

Script started on Thu Jun 13 14:51:29 2013 $ echo "Hi this is a dude" Hi this is a dude $ echo "done" done $ exit exit Script done on Thu Jun 13 14:51:52 2013 

You can easily edit out your own commands and start/end lines using basic shell scripting ( grep -v , especially if your Unix prompt has a distinctive substring pattern)

Commands launched from the shell inherit the file descriptor to use for standard output from the shell. In your typical interactive shell, standard output is the terminal. You can change that by using the exec command:

Following that command, the shell itself will write its standard output to a file called output.txt, and any command it spawns will do likewise, unless otherwise redirected. You can always «restore» output to the terminal using

Note that your shell prompt and text you type at the prompt continue to be displayed on the screen (since the shell writes both of those to standard error, not standard output).

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Redirect Output of a Process to a File and Standard Streams

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1. Overview

In this tutorial, we’ll explore few common strategies to redirect the output of a process to a file and standard streams such as stdout and stderr simultaneously.

2. The tee Command

The tee command is one of the most popular Linux commands that we can use for redirecting a process’s output.

2.1. Redirect stdout

Let’s take a simple use case of redirecting the output of the ls command to stdout and a temporary file /tmp/out.log:

$ ls -C | tee /tmp/out.log bin dev home lib32 libx32 mnt proc run srv tmp var boot etc lib lib64 media opt root sbin sys usr 

We can verify that the contents of the file are the same as the output generated from the executed command:

$ cat /tmp/out.log bin dev home lib32 libx32 mnt proc run srv tmp var boot etc lib lib64 media opt root sbin sys usr 

Another important thing to note is that the default behavior of the tee command is to overwrite the contents of the file. However, if required, we can choose the -a option to append the new content after the existing content of a file.

2.2. Redirect stdout and stderr to the Same File

We need to understand that internally, the tee command is acting as a T-splitter for the incoming stdin so that data can be redirected to the stdout and one or more files. Let’s use this understanding to redirect stderr of a process to stdout and a file:

$ (ls -C; cmd_with_err) 2>&1 | tee /tmp/out.log bin dev home lib32 libx32 mnt proc run srv tmp var boot etc lib lib64 media opt root sbin sys usr bash: cmd_with_err: command not found

We can notice that cmd_with_err is an unknown command, so it generates an error message. To make this available to the tee command, we redirect the stderr file descriptor (fd=2) to the stdout file descriptor (fd=1).

Alternatively, we can also use |& as a shorthand notation for 2>&1| to get the same result:

$ (ls -C; cmd_with_err) |& tee /tmp/out.log bin dev home lib32 libx32 mnt proc run srv tmp var boot etc lib lib64 media opt root sbin sys usr bash: cmd_with_err: command not found

Now, let’s verify the content of the /tmp/out.log file:

$ cat /tmp/out.log bin dev home lib32 libx32 mnt proc run srv tmp var boot etc lib lib64 media opt root sbin sys usr bash: cmd_with_err: command not found

2.3. Redirect stdout and stderr to Separate Files

In some scenarios, we might need to redirect the stdout and stderr of a process to separate files. We can do this by using process substitution while invoking the tee command. Before that, let’s take a look at a template code snippet that will enable the tee command to listen to a specific file descriptor and write back to the same file descriptor stream and a file:

We must note that fd is just a placeholder for a file descriptor, and the actual value will be 1 for stdout, 2 for stderr, and 0 for stdin.

Now, let’s use this understanding to redirect the stdout and stderr output of the process to /tmp/out.log and /tmp/err.log, respectively:

$ ((ls -C; cmd_with_err) 1> >(tee /tmp/out.log)) 2> >(tee /tmp/err.log 2>&2) bin dev home lib32 libx32 mnt proc run srv tmp var boot etc lib lib64 media opt root sbin sys usr bash: cmd_with_err: command not found

We can verify that /tmp/out.log contains the valid stdout message, whereas /tmp/err.log contains the error message from stderr:

$ cat /tmp/out.log bin dev home lib32 libx32 mnt proc run srv tmp var boot etc lib lib64 media opt root sbin sys usr $ cat /tmp/err.log bash: cmd_with_err: command not found

3. Redirection Delays

In a few scenarios, invocation of the tee command to redirect the process’s output to a file and stdout can introduce delay. In this section, we’ll explore such a scenario and learn to mitigate it.

3.1. Scenario

Let’s see a simple python script that prints the current time every one second:

$ cat time.py #!/usr/bin/python from datetime import datetime import time import sys from sys import stdout while True: sys.stdout.write(datetime.today().strftime("%H:%M:%S %p\n")) time.sleep(1)

If we execute this script, we’ll observe a delay of one second between any two consecutive timestamps written on the stdout:

$ ./time.py 6:49:48 PM 6:49:49 PM 6:49:50 PM 6:49:51 PM 6:49:52 PM 6:49:53 PM

3.2. Delayed Redirection

Now, let’s use the tee command to redirect the output of this process to stdout and the time.out file:

Unlike earlier, we’ll notice that there’s no output written on stdout for a long time, after which a huge chunk of output will be dumped on stdout in a single go.

The delay is introduced due to Linux’s stdio buffering policy in the glibc, a system library used by python internally. The buffering policy causes the writes to stdout to pass through a 4096–byte buffer, thereby reducing the number of I/O calls required to write on the stream.

For interactive applications, such delays in redirection are not acceptable. So, let’s find ways to mitigate the redirection delay issue.

3.3. Mitigation

As the root cause of the issue is associated with the delay in flushing of data to stdout, one way to solve this issue is by ensuring timely flushing of data to stream in the application code:

$ cat time.py #!/usr/bin/python from datetime import datetime import time import sys from sys import stdout while True: sys.stdout.write(datetime.today().strftime("%H:%M:%S %p\n")) sys.stdout.flush() time.sleep(1)

Let’s verify that the delay is indeed gone:

$ ./time.py | tee time.out 19:29:12 PM 19:29:13 PM

In this scenario, we had direct access to the application code, so we were able to modify it.

But, in many cases, the program could be an executable binary, and we might not have access to modify it. In such a scenario, we can use the unbuffer command in Linux to solve the delay caused by buffered writes to stdout.

Let’s remove the sys.stdout.flush() method call from our script and re-execute the redirection command using the unbuffer command:

$ unbuffer ./time.py | tee time.out 19:34:22 PM 19:34:23 PM

We can observe that there are no unexpected delays in the stdout writes now.

4. Conclusion

In this article, we explored several use cases of redirecting a program’s output to streams such as stdout and stderr simultaneously. Additionally, we learned few strategies to solve the issue of delayed redirection caused due to buffering.

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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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