Bash redirect stderr to file

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Bash redirect stderr to file

The first thing to notice is that there is a couple of ways depending on your purpose and shell, therefore, this requires slight understanding of multiple aspects. Furthermore, some commands such as the time and writing of the Strace Stderr output by default, and may or cannot provide a specific redirection method for that basic theory command

behind redirection is that a process generated by shell (assuming it is a External control and non-embedded shell) is created via Fork () and Execve () SYSCALLS, and before another DUP2 SYSCALL () occurs before it is necessary to redirect before EXECVE () happens. In this sense, redirects are inherited from the parent shell. The M &> Nem>

N.TXT inform the shell on how to run Open () and DUP2 () SYSCALL (see also as an entrance of redirect works, what is the difference between redirecting and tube, and what exactly does & average in outgoing redirection) More typical shell redirections, is via 2> to Bourne-like shells, such as dash (which is symbolically connected to / bin / sh) and

bash; First is the defect and shell posix-compliant and the other is what most users use for interactive session. They differ in syntax and features, but fortunately for us error flow redirection works the same (except the &> one non-standard). In the case of CSH and its derivatives, Stderr redirection does not work there. Let's go back to 2> Part. Two

fundamental things to note:> Redirection operator means, where a file is opened and 2 is for Stderr file descriptor; In fact this is exactly as standard POSIX shell language defines redirection in section 2.7: [N] Redir-op for simple> Redirection, the whole is implicit for stdout, ie echo hello world> / dev / null is the Same as echo hello world 1> / dev /

null. Note that the integer or redirection operator cannot be quoted, however Shell does not recognize them as such, and instead considers as a literal text string. As for spacing, it is important that whole is next to a redirection operator, but the file can be either next to the redirection operator or not, ie control 2> / dev / null and command 2> / dev /

null work well. The somewhat simplified syntax for a typical shell command would be command [arg1] [arg2] 2> / dev / null the trick is that redirection can appear anywhere. This is both> Command 2 [arg1] and command 2> [arg1] are valid. Note that for the bash shell, it does not exist and> mode to redirect both Stdout and Stderr Torrents, at the

same time, but once again - it is specific bash and if you are looking for the script portability, it may not work. See also Ubuntu wiki and what is the difference between and> and 2> & 1. Note: the> redirection operator truncates a file and overwrites, if the file exists. The 2 >> can be used for the Stderr addition to the file. If you can see,> you mean

for a single command. For scripts, we are able to redirect Stderr flow of all the script from the outside as in MyScript.sh 2> / dev / null or we can make use of exec built-in. The Exec Built-in has the power to reassure the flow for the entire shell session, so to say, is interactively or through scripts. Something similar to #! / Bin / sh Exec 2>

./my_log_file.txt stat / etc / non_existing_file In this example, the log file should show stat: it will not stat '/ etc / non_existing_file': no ?such file or directory. Another way is via functions. As Kopciuszek noted in his reply, we can write statement of function with the redirection already connected, which is a quilche_function () {command2 command1} 2>

my_log_file.txt commands by writing to stderr exclusively commands such as time and strace write their production to STDERR by default. In case of command time, Alternative possible is to redirect the entire command output, which is Eco Foo 2 time> & 1> Alternative file.txt, synchronous or sub-level list may be redirected if you want to separate

the output (as shown in Related post ): {Sleep Time 1 2> Sleep.stderr; } 2> Time.txt Other commands, such as strace or window to provide means to redirect STDERR. Strace has -O -O Option to specify the file name where you need to write the output. There's also an option to write a fleile for each sub-process that sees Strace. The command dialog

writes the user interface of the text of Stdout but output to STDERR, so to save his output variable (because ? var = $ (...) and conducted gets only STDERR) We must exchange the result of descriptors file = $ (Dialog --Inputbox Test 0 0 2> & 1 1> / dev / tty); But also, there --output-fd flag, we can also use. There is also the method of the named

pipes. Council to read the related post on the dialog command for a detailed description of what is happening. When we write something on our terminal program, we often see the production. For example, as we can see, Hello Echo is a command that means ? ? ?,?? Output Hello? ? ? ?. But where it's really gone this output? Output standards Each

operating system based on Unix has a concept of ? ? ? "a default location for the exit to go ... Since that sentence is a mouthful, everyone calls him" standard output ", or ? ? ? € ? Stdout? ? ? ?, pronounced standards. your shell (probably bash or zsh) you are constantly looking at that place of the default output. When you see the shell in there new

output, printing on the screen so that I, the human, can see it. Otherwise he would send echo hello ? ? ? "Hello? ? ? ? default to that place and would remain there for ever. standard input standard input (? ? ?,?? stdin? ? ? ?, pronounced standard) is the default location where the commands will hear the information. For example, if you type cat

without arguments, listen to the input on stdin, emitting what you type on Stdout, as long as ? not sending an EOF character (Ctrl + D): $ Cat Hello hello in there to say hello back again [Ctrl + D] As you can see, with standard input, you can directly send a string to a command. Pipes pipe Connect the standard output of a command input of another

standard. You do it by separating the two commands with the pipe symbol (|). Here's an example: $ echo "hello there" in there hello $ echo "hello in there" | sed 's / hello / hi / "echo hello" hello in there "print hello to stdout. But when we lead to SED "s / hello / hi /", SED takes that output as input and replaces ? ? ? ? "? ? ? Hello? with ? ? ?,?? HIA ?

? ?, then prints the result of Stdout. your shell sees only the final result after he was drafted by sED and prints that are on the screen. Hey, if sED sends its result to the standard, we cooing sed sed to another? Yes! $ echo "hello in there" | sed 's / hello / hi /' | sED "s / there / robot /" Hello robot over, Echo connected to sED, then connected to another

sED. the tubes are great to take the production of a command and turn it using commands like JQ. They a key part of the Unix philosophy of ? ? ?,?? small sharp tools ": ? since the commands can be chained together with tubes, each command should only do one thing and then deliver it to another command. Standard error standard error (? ? ?,??

Stderr? ? ? ?) is like the standard output and standard input, but it is the place where error messages go. To see a little 'STDERR output, try cats a file that does not exist: $ Cat cat does not exist: they do not exist: they do not exist: no such file or directory hey, that output looks just like Stdout! But wait a minute. I'll try to turn that output tubes: $

Cat does not exist | SED 's / no such / robot smash /' cat: do not exist: no such file or directory whoa - nothing has changed! Remember, the tubes take it stdoout the left of the pipe drive. The issuance of cat mistakes is going to stderr, not Stdout, so nothing has arrived through the tube towards the SED. It's nice that STDERRR not go through the pipe

by default: When giunchiamo tubing through something that does not take away the stdout of the terminal, we still want to see the errors immediately. Eg, A command that reads stdin and sends it to the printer: you will not want to walk the printer to see your mistakes. We need to redirect the Stderr cat to Stdout so that you cross the tube. and and

It means that we must know the redirection of production. Production redirection By default, Stdout and STDERR are printed at the terminal ? ? ?,? "that's because you can see them at all. But we can redirect that output to a file using the operator> Operator: $ Eco hello hello $ echo hello > new file $ cat new-file hello the second echo has not

printed anything for the terminal because we redirected its output aa file named new file. actually> new file does two things: create a file named new file if not You will exist; and replace the contents of the new file with the new content, so if the new file has already existed, and we echo hello> New-file, now would only hello in it. If you want to add to

the file, rather than Replace its contents, you can use the operator >>: $ cat new-file hello $ echo hello again >> new-file $ cat new-file hello hello again file descriptors a descriptor file, or fd, Is a positive integer that refers to a At sour source / output. For example, STDIN is 0, Stdout is 1, and STDERR is 2. Those may seem arbitrary numbers,

because they are: the POSIX standard defines them as such, and many operating systems (such as OS X and Linux) implement at least This part of the POSIX standard. To duplicate the output on a file descriptor, use the> & Operator plus the FD number. For example: # Redirect the Stdout to Stdout (FD 1) $ echo "Hi L?"> & 1 Hi L?? # Redirect

Stdout at Stderr (FD 2) $ echo "Hi L??"> & 2 Hi L¨¬ this is Very similar to the redirection of production aa file, as we did above, but you can think of stdouts and friends as special files that require us to use> & instead of>. Visually all the production above seems the same, but the changes become evident when we begin the production of pipes. See

what happens when it happens when we reinstate at Stdout against when we reinstate in Stderr: # Redirect to Stdout, then arrives through the $ echo tube "No modification"> & 1 | SED "S / NO / SOME /" SOME CHANGES # REINDING TO STDERR, then does not arrive through $ echo "No modification"> & 2 | SED "S / NO / SOME /" No

modification of ZSH users, take note! If you are using ZSH, you may have already noticed slightly different results. This is due to the Multi-scale option ZSH? ? ?,?, which is active by default. The Multios option means that echo something> & 1 | Other_command will be issued to FD 1 and lead output to another_command, rather than only lead. To

deactivate it, run MultiS Unetopt. Here's what the ZSH output with the Multios option looks like: # ZSH with Multios option on $ echo "Hi L?"> & 1 | sed "s / hello / hi /" hello hello last $ echo "hi l??"> & 2 | SED "S / Hello / Hi /" Hi Ciao L?? For this blog post, I suppose you are using bash or has ZSH with the Multi-ins for you. Advanced file

descriptors We know you have the STDRR output mixed with STDDROR output - perhaps the same command has been executed on many files and the command could emit on Stdout or STDERR each time. For convenience, the command outputs ? ? ?,? ? "stdout? ? ?,? at stdout, and ? ? ?,? ?" STDERR? ? ?,? to STDRR, in addition to the file name.

The visual output is similar to this: $ ./command file1 file2 file3 stdout file1 stderr file2 stddout file3 we want to transform all lines to have ? ? ?,? "dires says: ? ? ?,? before, but only dies the COND COMMAND WON 'T Work, because (again) Pipes Grab Stdout: $ ./command file1 file2 file3 | sed "s / ^ / robot says: /" stderr file2 robot says: stdout file1

robot says: stdout file3 this It is a case of common use for file descriptors: redirect STDERR to STDOUT to combine STDERR and STDOUT, so you can turn everything as stdout to another process. Try it: $ ./command file1 file2 file3 2> & 1 | sed " S / std / robot says: std / "robot says: stderr file2 robot says: File1 robot says: stdout file3 worked! We

have successfully redirected Stderr (FD 2) in Stdout (FD 1), combining them and inviting them the combined output through the Stdout. Common usage cases We could further redirect that output combined in a file, to review the combined output later in our free time: $ ./command ./command File2 file3> registry file 2> and 1 $ $ cat log-files stderr

file2 stdout file1 stdout file3 rents: we captured stderr and stdout in a file. An interesting point is that we have to do this: # Correct> Log-files 2> & 1 and not this: the correct version stdout points in the log file, then redirect Stderr to Stdout, then Stderr and Stdout point of the log file. The wrong version aims at Stderr on Stdout (which exits the

shell), then redirects the stdout on the file. So only the stdout points to the file, because Stderr is indicating the Stdoout ? ? ?,? ". Another common use for output redirection is redirecting Stderr only. To redirect a file descriptor, we use n>, where n Is a file descriptor. If there is no file descriptor, then stdout is used, as in echo hello> new file. We

can use this new syntax to silence stderr redirecting it to / dev / null, which swallows happily Whatever it receives and does nothing with it. It is the black input / output hole. Try it: # Redirects the stdout, because it's simple `> $. / command file1 file2 file3> file log-files stderr file2 # Redirect Stderrr, because it is "2>` $ ./command file1 file2 file3 2>

log1 -file stdout file1 stdout file3 excellent. Further reading Peteris Krumines wrote an excellent blog post on the redirection of the bash and goes into more detail on the descriptors of customized files. If you are a bit rusted on SED, he tests this delicate introduction. Jo?? ? ?I wrote a blog post on file descriptors and what they mapped in ? ? ?,? ?" I

in Ruby? ? ?,?. And if you want your own ./Command to test the examples in this post: #! / Bin / bash for f in $ @; do if [[$ f == "file2"]]; So echo "stderr file2"> & 2 else echo "stdout $ f" made

bash script redirect stderr to file. bash exec redirect stderr to file. bash redirect stderr to file append. bash redirect stderr to file and screen. bash redirect stderr to file and stdout. bash redirect stderr to file for whole script. bash redirect stderr to file and console. bash redirect stderr to files

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