Bash redirection issues. How to get rid of them


This guide was created to assist you in receiving a Bash error redirect error message.

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  1. Redirect stdout to a file and stderr to another file: command> out 2> error.
  2. Redirect standard output to file (> out), then redirect standard output to standard output (2> & 1): Command> out 2> & 1.

error output redirection bash


How do I redirect stdout?

File descriptors are used to identify stdout (1) and stderr (2); Command> Exit is just a shortcut to command 1> Exit; You can use & [FILE_DESCRIPTOR] to refer to the value of the file descriptor. If you use 2> & 1, stderr is redirected to the value set in stdout (and 1> & 2 does the opposite).


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Standard Version

Any Unix-based Operating System has the concept of "standard place of production." Since this offer is a sip, Everyone calls it the "standard version," or "standard output", very standard output. Your shell (probably bash or zsh) constantly monitors this default location. If your shell sees new editions, he prints it on the screen so you man can see it. Otherwise, echo hello will send "hello" to this default location and stay there forever.

Standard Record

standard input ("standard", pronounced standard) is the default location where teams expect information. For example, if you enter cat with no arguments, he listens to enter standard input, Take away what you print on stdout, until you send him the EOF character ( CTRL + d ):


Pipes connect the standard output of one control to the standard input of another. To do this, separate the two commands with the channel symbola ( | ). Here is an example:

echo "hello there" outputs hello there to standard output. But if we pass it to sed "s / hello / hi /" , sed takes this output as input and replaces “Hello” with “Hello”. then output the result to standard output. Your shell sees the end result only after processing sed. and displays the result on the screen.

Above, we connected Echo to sed, then connected to another sed. Pipes are ideal for placing an order. and convert it using other commands like jq. They are an integral part of the Unix “small sharp tools” philosophy: because orders can be chained, Each order has only one then transfer it to another order.

Standard Error

The standard error ("stderr") is similar to standard output and standard input. But error messages are displayed here. To see the problem with stderr, Try to intercept a file that does not exist:

Wow - nothing has changed! Remember that pipes use the standard control outlet to the left of the pipe. Error output from cat went to stderr, not to stdout. so nothing came through the pipe. It is good that stderr does not pass by default: If we direct the output through something that does not generate standard output to the terminal, We always want to see errors right away. For example, imagine a command that reads stdin and sends it to the printer: You do not need to go to the printer to see errors.

We need to redirect stderr from cat to stdout so it goes through the pipe. This means that we must learn to redirect costs.

Redirect Output

By default, stdout and stderr are printed on your terminal - That is why you can see them at all. However, we can redirect this output to a file using the > :


Second Echo did not print anything on the terminal because we redirected the output to a file named new-file . Actually, > new file does two things:

So, if a new file already exists, and we have a echo hello> new file , now it will only contain hello . If you want to attach to a file, instead of replacing it with contents е You can use the >> operator:

File Descriptors

The file descriptor, or FD, is a positive integer that refers to Input / output source. For example, stdin is 0, stdout is 1, and stderr is 2. They seem to be arbitrary numbers: POSIX The standard defines them as such and many operating systems (for example, OS X. and Linux) implement at least this part of the POSIX standard.

This is very similar to redirecting output to a file. as we did above But you can treat stdout and his friends as special files To do this, we must use > & instead of > .

Visually, all of the problems listed are the same. However, the changes will become clear when we start the pipeline. Let's see what happens when we Switch to standard output and pass to standard output:

Attention ZSH Users!

This is due to the ZSH MULTIOS option. This is enabled by default. The MULTIOS option means that repeat something> & 1 | other_order dispatched in FD 1, and the output is passed to other_command , instead of just directing it. To disable this, run and disable MULTIOS .

Extended File Descriptors

Suppose you mix stderr output with stdout output - Perhaps you run the same command for many files. and the command can be sent to stdout or stderr every time. For simplicity, the stdout command is sent to standard output. and "stderr" in stderr plus the file name. The visual output looks like this:

We want to convert each line so that "Robot says:" is in front. But just passing the command to sed does not work. because (again) channels only capture standard output:

This is a common use case for file descriptors: redirect stderr to stdout to combine stderr and stdout, Thus, you can transfer everything as a standard to another process.

Common Use Cases

The correct version points stdout to the log file and passes stderr stdout, that is, both stderr and stdout point to a log file. Invalid impression version stderr goes to standard output (which came out in the shell), Then redirect standard output to a file. Thus, only standard output points to a file, because stderr points to the "old" standard output.

Another common use for cost redirection is to redirect only stderr. To redirect the file descriptor, we use N> , where N is the file descriptor. If the file descriptor is missing, standard output is used, as in echo Hello> New File .

We can use this new syntax disable stderr by redirecting it to / dev / null , who happily swallows what gets and do nothing with it. This is a black hole in / out. Let's try this:

Chapter 20. I / O Redirection

There are always three standard files [1] Open, stdin (keyboard), stdout (screen) and stderr (error messages are sent to Screen). These and other open files can be redirected. Redirecting simply means capturing the output of a file . A program, script, or even a block of code in a script (see Example 3-1 and Example 3-2) and send as It is printed in another file, command, program or script.

A file descriptor is assigned to each open file. [2] File Descriptors for stdin , stdout and stderr 0, 1 or 2. To open additional files descriptors 3 through 9 remain. Sometimes it is useful to assign one of these additional file descriptors for stdin , stdout or stderr as a temporary double link. [3] This simplifies normal recovery after complex redirection. and stirring (see example 20-1).

Multiple instances of input and output redirection and / or pipes can be combined in one order Line.

Child processes inherit open file descriptors. it why pipes work. Close it to prevent fd inheritance.



What happens if I first redirect stdout to a file and then redirect stderr to the same file?

If you redirect standard output and standard error to the same file, you may get unexpected results. This is because STDOUT is a buffered stream, while STDERR is always buffered.

What does 2 &1 mean in shell script?

2 refers to the second file descriptor of the process, i.e. H. Stderr. > means sabotage. & 1 means that the redirection destination must be in the same place as the first file descriptor, i.e., H. Standard Edition. So> / dev / null 2> & 1 first redirects standard output to / dev / null, and then redirects stderr there.


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