Easy Way To Fix Redirecting Output Errors In Linux
November 30, 2021
Over the past few days, a number of readers have informed us that they have encountered Linux output error redirection.
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Redirect stdout one file and stderr another file: Command> Online 2> Error.Redirect stdout to file (> out), then redirect stderr to stdout (2> & 1): direction> out 2> & 1.
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The first thing to know about queries is that there are different options depending on your goal and reach. Therefore, a little understanding of several aspects is required. In addition, some types of commands, such as time and strace , write source code to stderr by default and can provide some sort of redirection system to those who appear to be on the command.
The basic theory of redirection is that a specific process created by the shell (assuming it is an external command, not just a built-in shell), the fork () and method. are execve () system calls , and before it happens another system call will call dup2 () to redirect before execve () happens. In the latter sense, redirects are inherited from the parent shell. m &> n and just m> n.txt tell the shell that they are open () and dup2 () <. you should run / code> syscall (see also How well input redirection works, What is the difference between redirection and a pipeline, and What exactly works and what redirecting output means)
Shell Redirects
Most often this is 2> from bourne-like wrappers, such as dash (which should be denoted as / bin / sh ) and therefore bash ; one is a standard POSIX-compliant shell, and the other is the actions that most users use for interactive control. They differ in syntax and usage, but luckily error-free redirection works the same (except for &> , which is not standard). Because of csh and its derivatives, which the stderr redirection does not fully control, they take effect.
Let's return to 2> in part. There are two important things to keep in mind: > OpA process redirection script where we open your file and the whole 2 is in the stderr file descriptor; At this point, the POSIX shell language standard defines redirection in clause 2.7:
[n] word redir-op For simple redirection, > is an integer 1 for stdout , that is, echo Hello World> / dev / null < / code> can be exactly the same as echo Hello World 1> / dev / null . Note that a real integer redirection or real integer operator may not always be quoted, otherwise the shell will not recognize any of them and will instead treat them as literal text strings. To have spaces, it is important that the integer is likely to go directly to the next redirection operator, so the file can either close the redirection operator or not, i.e. command and 2> / dev / null command 2> / dev / null should work fine.
Slightly simpler syntax for typical upper / lower case commands:
command [arg1] [arg2] 2> / dev / null CunningThe trick is that a lot of redirects can happen anywhere. That is, both command line 2> [arg1] and command 2> [arg1] are checked. Note that for bash shell, there is a &> method inside that will help you redirect stdout and stderr streams at the same time, but rather c 'is bash specific and if you find yourself striving for portability of software packages, this may not work. See Ubuntu and Wiki. What is the specific difference between &> and 2> & 1.
Note. The redirection operator > strips off the information and overwrites it if the exact file exists. 2 >> can be used a little more to add stderr to the file. You
If you've noticed this, > is suggested for one command. For software packages, we can redirect the stderr stream from the entire script, as in myscript.2> sh / dev / null , where many people can use built-in Govt. The built-in exec has the ability to redirect the flow for the most important whole session, so to speak shells, whether interactive or script. ... Something. Like.
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#! / bin / shexec .2> ../my_journal_file.txtstat / etc / nonexistent_file In which example the log file stat: cannot provide stat '/ etc / non_existing_file': do not declare such or directory .
Another idea is complete functionality. As you know from Kopchyushek's answer, we can write a function declaration with redirection already added, i.e.
some_function () Command1 Command2 2> my_file_journal.txt Commands That Only Write To Stderr
Command variants such as time and strace write this output to stderr by default. In the case of the time command, the only viable alternative is to completely redirect the command performance, including
time echo foo 2> & 1> file.txt 
Alternatively, synchronous BOMs or subshells can be sent directly if you want to split their output (as shown in a sequential article):
sleep time specific only 2> sleep.stderr; 2> hour.txt You will most likely send the output to / dev / null using the command syntax> / dev / null. However, this probably won't work if the command is associated with a standard error (FD number 2). So, at best, you will need to modify> / dev / null as follows to redirect the result and errors to / dev / null.
Other requirements such as strace or dialog sources provide redirection from stderr. strace functions -o allows you to specify the name of the file into which the output will be compiled. It is also possible to write a text file for the strace subprocess that it sees. When a dialog is received, the UI text is written to stdout, but printed to stderr to store its source in a variable, because ( var = $ (...) additionally only passes pipeline get stderr) you need to swap descriptors
result = $ (dialog ended --inputbox test 0 0 2> & 1 1> / dev / tty); Command name> command name output.txt> stdout.txt.Command name 2> error.txt command name 2> stderr.txt.Command1> Output.txt 2> Fehler.txt Command2 -f -z -y> Output.txt 2> Fehler.txt.Command1> all.txt 2> & 1 Command1 -arg> all.txt 2> & 1.
but there is also a --output-fd flag that we can use as well. There is also a named pipe method. I recommend reading the post pertaining to the dialog command topic to see what's going on.
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