From time to time I need to troubleshoot the execution of a command that produces verbose output. Redirecting output to a file is fundamental to analyzing the execution. However, sometimes I also need to get immediate feedback that the command is running. In that case, it is usually enough to send all output to the console and either all or part of it to a file. But what if you need to look at more than one aspect of the output and thus you need that output in more than one file?
Output to a single file
Sending output to the console and to a file is precisely the purpose
tee reads from standard input and outputs what reads to
the console and to a given file.
Consider the following program that outputs to standard output and
I send standard output and error to the console and only standard
output to a file by means of
tee in the following way.
I send both outputs to the console and to the same file in the following way.
I can even send both outputs to the console and only standard error to a file like so.
Sometimes troubleshooting is easier when you look at standard output and error separately. How do you do that?
Output to two or more files
I can send standard output and standard error to the console and to a
file by running two instances of
I do so by means of process
To illustrate process substitution, consider the following code that
sends standard error and standard output to separate files.
I send standard error and standard output the console and to separate files in like so.
What the previous line of Bash does is execute commands
OUT and connect the standard error of
CMD to the input of
and the standard output of
CMD to the input of
Sometimes I still need to put error output in context. In that case, I send standard error and standard output to the console, to separate files, and to the same file like this.
When I only need to look at errors and put them in context, I do the following. I send standard error and standard output to the console and to the same file. While doing that, I send standard error to a separate file. The following Bash line illustrates the technique.
Do you have any interesting techniques for redirecting output? Let me know in the comments.
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I love to explain and answer questions on programming problems, the kind you find in coding interviews. I publish a new programming problem and its solution every Sunday. Did I mention that I love to answer questions?