Bash – Log Background Job Execution Time

background-processbashtime

I have a script I execute in the background and capture all output with a command like this:

nohup bash -x test.sh < /dev/null > log.txt 2>&1 &

How can I find out how long it took to complete? If I try to use time after bash -x I get an error:

/usr/bin/time: /usr/bin/time: cannot execute binary file

If I try to use acct it doesn't seem to log the process or I can't find it.

Best Answer

[this assumes that you want the output of time to also go to log.txt, something which is not clear from your question]

Just don't use nohup.

nohup does nothing special but ignoring the SIGHUP signal, redirecting stdout and stderr to a file and --only in some variants, like the nohup from GNU coreutils-- redirecting the stdin from /dev/null opened in write-only mode.

You can easily do all that from the shell:

{ trap '' HUP; time bash -x your_script args ... ; } > log.txt 2>&1 0>/dev/null &

Or safer, if started from a script or if you don't plan to ever bring back the command to the foreground with fg:

(trap '' HUP; time bash -x your_script args ... 0>/dev/null &) > log.txt 2>&1

(You can omit the 0>/dev/null if you don't care to emulate that feature of GNU nohup).

Of course, you can use the syntax trick from the first example with nohup too if, for some unfathomable reason, you ascribe magical properties to nohup:

{ time nohup bash -x your_script args ... ; } > log.txt 2>&1 &

Related Solutions

Bash – Make Bash Use External Time Command Rather Than Shell Built-in

You can use the command shell built-in to bypass the normal lookup process and run the given command as an external command regardless of any other possibilities (shell built-ins, aliases, etc.). This is often done in scripts which need to be portable across systems, although probably more commonly using the shorthand \ (as in \rm rather than command rm or rm, as especially the latter may be aliased to something not known like rm -i).

$ time

real    0m0.000s
user    0m0.000s
sys 0m0.000s
$ command time
Usage: time [-apvV] [-f format] [-o file] [--append] [--verbose]
       [--portability] [--format=format] [--output=file] [--version]
       [--quiet] [--help] command [arg...]
$

This can be used with an alias, like so:

$ alias time='command time'
$ time
Usage: time [-apvV] [-f format] [-o file] [--append] [--verbose]
       [--portability] [--format=format] [--output=file] [--version]
       [--quiet] [--help] command [arg...]
$

The advantage of this over e.g. alias time=/usr/bin/time is that you aren't specifying the full path to the time binary, but instead falling back to the usual path search mechanism.

The alias command itself can go into e.g. ~/.bashrc or /etc/bash.bashrc (the latter is global for all users on the system).

For the opposite case (forcing use of the shell built-in in case there's an alias defined), you'd use something like builtin time, which again overrides the usual search process and runs the named shell built-in. The bash man page mentions that this is often used in order to provide custom cd functionality with a function named cd, which in turn uses the builtin cd to do the real thing.

Bash – Why does “bash -x” break this script

The problem is this line:

TIMESEC=$(echo blah | ( /usr/bin/time -f %e grep blah >/dev/null ) 2>&1 | awk -F. '{print $1}')

where you are redirecting the standard error to match the standard output. bash is writing its trace-messages to the standard error, and is (for instance) using its built-in echo along with other shell constructs all in the bash process.

If you change it to something like

TIMESEC=$(echo blah | sh -c "( /usr/bin/time -f %e grep blah >/dev/null )" 2>&1 | awk -F. '{print $1}')

it will work around that problem, and perhaps be an acceptable compromise between tracing and working:

++ awk -F. '{print $1}'
++ sh -c '( /usr/bin/time -f %e grep blah >/dev/null )'
++ echo blah
+ TIMESEC=0                 
+ echo ABC--0--DEF
ABC--0--DEF
+ '[' 0 -eq 0 ']'
+ echo 'we are here!'
we are here!

Best Answer

Related Solutions

Bash – Make Bash Use External Time Command Rather Than Shell Built-in

Bash – Why does “bash -x” break this script

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