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I have a simple download script and I use set -x which works great; I can see each step it performs, and I can identify errors in the script or in the download:

#!/bin/bash set -x #short_date=$(/bin/date +%m%d%y) short_date=$(/bin/date -d "8 day ago" +%m%d%y) #long_date=$(/bin/date +%Y%m%d) long_date=$(/bin/date -d "8 day ago" +%Y%m%d) scp -v -P 1332 -i /home/casper/.ssh/id_rsa_BANK [email protected]:/home/friendly/transfer/out/EXCHANGE_$short_date.csv /local/casper3/dailymetrics/BANK_$long_date.csv

I would like to automate this job. Is there a way I could save the set -x output to a log file? Maybe to one log file - or a different log file for each day. I don't know what would work best.

Below is sample set -x output from the above script. 

++ /bin/date +%m%d%y + short_date=102814 ++ /bin/date +%Y%m%d + long_date=20141028 + scp -v -P 1332 -i /home/casper/.ssh/id_rsa_BANK [email protected]:/home/friendly/transfer/out/EXCHANGE_102814.csv /local/casper3/dailymetrics/BANK_20141028.csv Executing: program /usr/bin/ssh host 192.168.1.10, user friendly, command scp -v -f /home/friendly/transfer/out/EXCHANGE_102814.csv OpenSSH_5.3p1, OpenSSL 1.0.0-fips 29 Mar 2010 debug1: Reading configuration data /home/casper/.ssh/config debug1: Reading configuration data /etc/ssh/ssh_config debug1: Applying options for * debug1: Connecting to 192.168.1.10 [192.168.1.10] port 7777. debug1: Connection established.
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    Which version? With bash 4 you can do this without redirecting all of stderr; with old versions, it's all the same stream. Commented Oct 28, 2014 at 14:32
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    By the way, you might consider something like PS4=':$BASH_SOURCE:$LINENO+' to log the file and line for each command. Commented Oct 28, 2014 at 14:36
  • Take a look at: unix.stackexchange.com/a/155553/74329 Commented Oct 28, 2014 at 14:40

1 Answer 1

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Assuming bash 4, BASH_XTRACEFD can be set to override the file descriptor (by default 2, stderr) to which set -x output is written:

short_date=$(/bin/date +%m%d%y) exec {BASH_XTRACEFD}>>"$short_date".log set -x

If running bash 4.0 rather than 4.1 or newer, you have BASH_XTRACEFD but not automatic file descriptor allocation, meaning you'll need to assign one yourself; in the below example, I'm picking file descriptor 100:

short_date=$(/bin/date +%m%d%y) exec 100>>"$short_date".log BASH_XTRACEFD=100 set -x

For older releases, your only choice is to redirect all of stderr, rather than only the xtrace stream:

short_date=$(/bin/date +%m%d%y) exec 2>>"$short_date.log" set -x
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11 Comments

Should that be ${BASH_XTRACEFD} or is there a peculiar feature I need to read about?
@Alfe, yes, there's a feature at play here -- the automatic file descriptor allocation previously mentioned. Quoting from the redirection section of the bash manual: "Each redirection that may be preceded by a file descriptor number may instead be preceded by a word of the form {varname}. In this case, for each redirection operator except >&- and <&-, the shell will allocate a file descriptor greater than 10 and assign it to {varname}. If >&- or <&- is preceded by {varname}, the value of varname defines the file descriptor to close."
It's also possible to use tee to keep a copy on STDERR, as per stackoverflow.com/questions/3173131/….
@AdamSpiers, you lose sync between the copy going through tee and the content written directly if you log the traced output to different sinks even if later rejoining it to stderr; thus, using exec {BASH_XTRACEFD}> >(tee xtrace.log >&2) or any equivalent is not something I'd advise. (If you mean to log all stderr output -- both from xtrace and otherwise -- to both a file and console, you don't have that problem). Anyhow, the OP here is asking about logging only xtrace, which is why that's the question I answered.
@AdamSpiers, ...POSIX guarantees ordering between writes only if they're to copies of the same FD; as soon as you redirect one but not the other, things get messy. You have the same problem if you try to log stdout and stderr to two different files.
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