How do I manage log verbosity inside a shell script?

Question

I have a pretty long bash script that invokes quite a few external commands (git clone, wget, apt-get and others) that print a lot of

Answer 1

Improving on @Fred's idea a little bit more, we could build a small logging library this way:

declare -A _log_levels=([FATAL]=0 [ERROR]=1 [WARN]=2 [INFO]=3 [DEBUG]=4 [VERBOSE]=5)
declare -i _log_level=3
set_log_level() {
  level="${1:-INFO}"
  _log_level="${_log_levels[$level]}"
}

log_execute() {
  level=${1:-INFO}
  if (( $1 >= ${_log_levels[$level]} )); then
    "${@:2}" >/dev/null
  else
    "${@:2}"
  fi
}

log_fatal()   { (( _log_level >= ${_log_levels[FATAL]} ))   && echo "$(date) FATAL  $*";  }
log_error()   { (( _log_level >= ${_log_levels[ERROR]} ))   && echo "$(date) ERROR  $*";  }
log_warning() { (( _log_level >= ${_log_levels[WARNING]} )) && echo "$(date) WARNING  $*";  }
log_info()    { (( _log_level >= ${_log_levels[INFO]} ))    && echo "$(date) INFO   $*";  }
log_debug()   { (( _log_level >= ${_log_levels[DEBUG]} ))   && echo "$(date) DEBUG  $*";  }
log_verbose() { (( _log_level >= ${_log_levels[VERBOSE]} )) && echo "$(date) VERBOSE $*"; }

# functions for logging command output
log_debug_file()   { (( _log_level >= ${_log_levels[DEBUG]} ))   && [[ -f $1 ]] && echo "=== command output start ===" && cat "$1" && echo "=== command output end ==="; }
log_verbose_file() { (( _log_level >= ${_log_levels[VERBOSE]} )) && [[ -f $1 ]] && echo "=== command output start ===" && cat "$1" && echo "=== command output end ==="; }

Let's say the above source is in a library file called logging_lib.sh, we could use it in a regular shell script this way:

#!/bin/bash

source /path/to/lib/logging_lib.sh

set_log_level DEBUG

log_info  "Starting the script..."

# method 1 of controlling a command's output based on log level
log_execute INFO date

# method 2 of controlling the output based on log level
date &> date.out
log_debug_file date.out

log_debug "This is a debug statement"
...
log_error "This is an error"
...
log_warning "This is a warning"
...
log_fatal "This is a fatal error"
...
log_verbose "This is a verbose log!"

Will result in this output:

Fri Feb 24 06:48:18 UTC 2017 INFO    Starting the script...
Fri Feb 24 06:48:18 UTC 2017
=== command output start ===
Fri Feb 24 06:48:18 UTC 2017
=== command output end ===
Fri Feb 24 06:48:18 UTC 2017 DEBUG   This is a debug statement
Fri Feb 24 06:48:18 UTC 2017 ERROR   This is an error
Fri Feb 24 06:48:18 UTC 2017 ERROR   This is a warning
Fri Feb 24 06:48:18 UTC 2017 FATAL   This is a fatal error

As we can see, log_verbose didn't produce any output since the log level is at DEBUG, one level below VERBOSE. However, log_debug_file date.out did produce the output and so did log_execute INFO, since log level is set to DEBUG, which is >= INFO.

Using this as the base, we could also write command wrappers if we need even more fine tuning:

git_wrapper() {
  # run git command and print the output based on log level
}

With these in place, the script could be enhanced to take an argument --log-level level that can determine the log verbosity it should run with.

---

Here is a complete implementation of logging for Bash, rich with multiple loggers:

https://github.com/codeforester/base/blob/master/lib/stdlib.sh

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If anyone is curious about why some variables are named with a leading underscore in the code above, see this post:

• Correct Bash and shell script variable capitalization