CODEWITHSUNDEEP
bashshellstderr
null
null

Reputation: 7594

Redirecting STDERR in a Bash file results in a file created even when there is no error. Why?

I've created a small Bash script that does a MySQL data dump. Because the dump can be fairly large I put the process in the background and then wait for either an error or the log to show up in the file system. I have the following code:

mysqldump main_db > /loc/to/dmp/file.sql 2>/loc/to/error/log/file.log &

The problem is that I get a '/loc/to/error/log/file.log' file the size of 0 (which I presume means no real error) sometimes when this command is run, which kills the process, even though there is no error.

I'm not sure why the STDERR would write a file when there was no data to write. Is this because of the & background process?

Upvotes: 1

Views: 497

Answers (3)

Mtl Dev
Mtl Dev

Reputation: 1622

A possible easy workaround:

mysqldump main_db > file.sql 2> errors.log ; [ -s errors.log ] || rm -f errors.log

OR
(short readable easily adjustable script with timestamp)

OUTPUT="/loc/to/dmp/`date +%F.%T`.sql"
ERRORS="$OUTPUT.ERRORS"
mysqldump main_db > $OUTPUT 2> $ERRORS
[ -s $ERRORS ] || rm -f $ERRORS

Upvotes: 0

nos
nos

Reputation: 229108

The redirected files are set up before your script is executed by the executing shell.

I.e. after having parsed your command which includes redirected stdout/stderr, the shell forks, opens(creates the files if they don't exists). attach the opened filedescriptors to filedescriptor 1 and 2 (stdout/err respectivly) and then executes the actual command.

Upvotes: 5

JSBձոգչ
JSBձոգչ

Reputation: 41378

The redirection file is created whether or not any data is ever written to it. Whichever process is watching the error log should check for non-zero filesize, not existence.

Upvotes: 0

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