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I've written a 'generic' bash script (= generic_script.sh) which serves as a wrapper to start an actual bash script (= actual_script.sh), depending on the parameters given. The stdout of both the generic_script as well as the actual_script should be written to a specific file/folder. The same goes for the stderr of the generic_script as well as the actual_script.

The write path of those two files (stdout and stderr) is dependent on the arguments, which get parsed in the script. Here's a simplified call:

# wrapper-script     job-id job-script                args
./generic_wrapper.sh JOB001 /path/to/actual_script.sh arg1 arg2

generic_wrapper.sh:

#!/bin/bash
{
    # Guarding clauses
    ...

    # Parsing arguments
    stdout_file="/some/path/$1/timestamp.stdout"         # Creating job-id specific folders
    stderr_file="/some/path/$1/timestamp.stderr"

    # Sourcing actual_script.sh
    source "${path}/$2" 1>"${stdout_file} 2>"${stderr_file}"

    # Statistics
    ...

} > "${stdout_file}" 2>"{$stderr_file}" # throws error
echo "${stdout_file}" # returns correct file path

actual_script.sh

#!/bin/bash
echo "Just an example."

However, executing this code returns /wrappers/generic_at_wrapper.sh: line 108: : No such file or directory error. This strongly implies, that at the time of redirection, the variable stdout_file has not been filled. I reckon that this is tied to the order, in which the variables are resolved or the bash is interpreted.

If I echo the value of said variable stdout_file(as well as stderr_file) on the next line, I get the correct value whatsoever, meaning that this is tied to the {} > construct. The redirection method is from this SO-Question.

How can I redirect the stdout and stderr to a file path stored in a variable? The file path variable itself gets calculated in the {} construct and doesn't seem available right after closing the brackets.

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  • Are you missing a closing quote here } > "${stdout_file}? Commented May 31, 2021 at 22:23
  • @Cole Tierney: you're absolutely right. But this was solely an issue in my pseudo code above. Fixed it. Commented May 31, 2021 at 22:40
  • @Lino: You are using stdout_file outside the {...} sequence, but define it inside. When bash sets up the outside redirection, the variables are still undefined. Your redirection is expanded to > "", and this is what the error message says. Commented Jun 1, 2021 at 8:03

2 Answers 2

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If you want to redirect for the entirety of your code, instead of using blocks, use the exec command. That is:

stdout_file="/some/path/$1/timestamp.stdout"         # Creating job-id specific folders
stderr_file="/some/path/$1/timestamp.stderr"
exec >"$stdout_file" 2>"$stderr_file"

# ...all code below this point has stdout going to stdout_file, and stderr to stderr_file
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3 Comments

I'd prefer this method of redirection, as it immediately shows, that a redirection takes place without having to read the entire script/block. I've removed the block {}, created the stdout and stderr variables before doing the exec and the timestamp specific files get created. Surprisingly, there is no output (to either stdout or stderr) before I have sourced the actual_script.sh. So doing an echo "test" immediately after the exec will not be visible in the output. According to the example given, the first line written is Just an example. Any thoughts on this?
I'd need to see a reproducer. With some commands you might see libc's buffering behaviors causing writes to be deferred (until there's enough content to be able to do a larger, lower-overhead write) when stdout is going somewhere other than a tty, but echo is not such a command.
The cause was actually much simpler. I forgot to adjust the redirection on the source, which is in fact not needed at all using exec to redirect. See explanation in this question: stackoverflow.com/questions/67839242/…
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The {} > ... construct tries running the code in {} after > .... Bash needs to know the output redirection before running the command(s). Have you tried setting stdout_file and stderr_file before the code that you want the output of redirected?

That is, your code should be:

stdout_file="/some/path/$1/timestamp.stdout"         # Creating job-id specific folders
stderr_file="/some/path/$1/timestamp.stderr"
{
   # ...content subject to redirections here...
} >"$stdout_file" 2>"$stderr_file"

You may also find >&1, 2>&2 useful, rather than passing in folders.

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4 Comments

@CharlesDuffy I've updated my answer to be authoritative. I can't comment on questions yet, and without definitively knowing that what I am saying is correct I default to guessing. Thank you for confirming :)
BTW, I'm not quite sure what you mean re: >&1 or 2>&2 -- both those operations make a file descriptor a copy of its prior value, and thus have no effect at all.
I meant >&1 2>&2 to be used for the redirection of the actual_script.sh output. So the full command could be run with, for example: ./generic_wrapper.sh > ./stdout.log 2> ./stderr.log. But I see that the OP wanted to add some additional information to the filename, like timestamp, so it may be redundant.
Sure. That said, I assume that when the OP says "timestamp.stdout", their real use case is something more like $(date +%Y%m%dT%H%M%S).stdout. They'd need a separate shell script as a wrapper to automate that, so it's reasonable to want to do it internally.

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