Ubuntu – bash: set -x logs to file

bashcommand linedebugscripts

I have a shell script with set -x to have verbose/debug output:

#!/bin/bash

set -x
command1
command2
...

The output looks like this:

+ command1
whatever output from command1
+ command2
whatever output from command2

My problem is, the shell output (caused by set -x) goes to the stderr, mixed with the output of the commands (command1, command2, …). I would be happy to have the "normal" output on the screen (like the script woud run without set -x) and the "extra" output of bash separately in a file.

So I would like to have this on the screen:

whatever output from command1
whatever output from command2

and this in a log file:

+ command1
+ command2

(also fine if the log file has everything together)

The set -x 2> file obviously doens't take the right effect, because it's not the output of the set command, but it change the behaviour of the bash.

Using bash 2> file for the entire script also doesn't do the right thing, because it redirects the stderr of every command which run in this shell as well, so I don't see the error message of the commands.

Best Answer

  • Based on this ServerFault answer Send bash -x output to logfile without interupting standard output, modern versions of bash include a BASH_XTRACEFD specifically for specifying an alternate file descriptor for the output of set -x

    So for example you can do

    #!/bin/bash
    
    exec 19>logfile
    BASH_XTRACEFD=19
    
    set -x
    command1
    command2
    ...
    

    to send the output of set -x to file logfile while preserving regular standard output and standard error streams for the following commands.

    Note the use of fd 19 is arbitrary - it just needs to be an available descriptor (i.e. not 0, 1, 2 or another number that you have already allocated).

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