Elegant way for verbose mode in scripts?

Question

When I write bash scripts I usually get the verbose mode this way (simplified):

_V=0

while getopts "v" OPTION
do
  case $OPTION in
    v) _V=1
       ;;
  esac
done

and then every time I want a "verbose output" I type this:

[ $_V -eq 1 ] && echo "verbose mode on" || echo "verbose mode off"

or for example this:

[ $_V -eq 1 ] && command -v || command

Is there a way to do it more elegant? I was thinking about defining a function named "verbose" and type it instead of [ $_V -eq 1 ], but this would only be a tiny improvement.

I'm sure, there is more common way to do it…

Answer 1

Almost all answers are about how to create a log function, so I will cover only the second part: how to pass the '-v' option to a command only when verbose mode is on.

The question suggests this:

[ $_V -eq 1 ] && command -v || command

If _V has only 2 possible values (non-empty or empty), and if you use bash, then you can bash parameter expansion Alternate Value :+. Here an example:

command ${_V:+-v}

From Bash manual:

${parameter:+word}
   Use Alternate Value.  If parameter is null or unset, nothing is substituted,
   otherwise the expansion of word is substituted.

Answer 2

I would propose a modified version of @fentas's answer:

# set verbose level to info
__VERBOSE=6

declare -A LOG_LEVELS
# https://en.wikipedia.org/wiki/Syslog#Severity_level
LOG_LEVELS=([0]="emerg" [1]="alert" [2]="crit" [3]="err" [4]="warning" [5]="notice" [6]="info" [7]="debug")
function .log () {
  local LEVEL=${1}
  shift
  if [ ${__VERBOSE} -ge ${LEVEL} ]; then
    if [ -t 0 ]; then
      # seems we are in an interactive shell
      echo "[${LOG_LEVELS[$LEVEL]}]" "$@" >&2
    else
      # seems we are in a cron job
      logger -p "${LOG_LEVELS[$LEVEL]}" -t "$0[$$]" -- "$*"
    fi
  fi
}

Answer 3

To avoid using multiple if statements or using a variable to hold a function name how about declaring different functions based on the verbosity!

This works for ALL bourne shell derivatives not just bash!

#verbose=verbose_true       # uncomment to make script verbose
if [ "$verbose" ]; then
  log() { echo "$@"; }
else
  log() { :; }
fi

log This Script is Verbose

NOTE: the use of "verbose=verbose_true" makes script tracing a lot nicer but you can make that one if however you like.

Answer 4

After reading all other posts I came up with this

# set verbose level to info
__VERBOSE=6

declare -A LOG_LEVELS
# https://en.wikipedia.org/wiki/Syslog#Severity_level
LOG_LEVELS=([0]="emerg" [1]="alert" [2]="crit" [3]="err" [4]="warning" [5]="notice" [6]="info" [7]="debug")
function .log () {
  local LEVEL=${1}
  shift
  if [ ${__VERBOSE} -ge ${LEVEL} ]; then
    echo "[${LOG_LEVELS[$LEVEL]}]" "$@"
  fi
}

Then you can simply use it like this

# verbose error
.log 3 "Something is wrong here"

Which will output

[error] Something is wrong here

Answer 5

If you want to avoid doing an "if" statement every single time you want to log something, you can try this approach (which is how I do it).

The idea is that instead of calling log, you call $echoLog instead. So, if you are in verbose mode, $echoLog will just be echo, but in non-verbose mode, it is a function that prints nothing and just ignores the arguments.

Here's some code you can copy.

# Use `$echoLog` everywhere you print verbose logging messages to console
# By default, it is disabled and will be enabled with the `-v` or `--verbose` flags
declare echoLog='silentEcho'
function silentEcho() {
    :
}

# Somewhere else in your script's setup, do something like this
while [[ $# > 0 ]]; do
    case "$1" in
        -v|--verbose) echoLog='echo'; ;;
    esac
    shift;
done

Now, you can just drop lines like $echoLog "Doing something verbose log worthy" anywhere you want.

Answer 6

#!/bin/bash
# A flexible verbosity redirection function
# John C. Petrucci (http://johncpetrucci.com)
# 2013-10-19
# Allows your script to accept varying levels of verbosity flags and give appropriate feedback via file descriptors.
# Example usage: ./this [-v[v[v]]]

verbosity=2 #Start counting at 2 so that any increase to this will result in a minimum of file descriptor 3.  You should leave this alone.
maxverbosity=5 #The highest verbosity we use / allow to be displayed.  Feel free to adjust.

while getopts ":v" opt; do
    case $opt in
        v) (( verbosity=verbosity+1 ))
        ;;
    esac
done
printf "%s %d\n" "Verbosity level set to:" "$verbosity"

for v in $(seq 3 $verbosity) #Start counting from 3 since 1 and 2 are standards (stdout/stderr).
do
    (( "$v" <= "$maxverbosity" )) && echo This would display $v 
    (( "$v" <= "$maxverbosity" )) && eval exec "$v>&2"  #Don't change anything higher than the maximum verbosity allowed.
done

for v in $(seq $(( verbosity+1 )) $maxverbosity ) #From the verbosity level one higher than requested, through the maximum;
do
    (( "$v" > "2" )) && echo This would not display $v 
    (( "$v" > "2" )) && eval exec "$v>/dev/null" #Redirect these to bitbucket, provided that they don't match stdout and stderr.
done

# Some confirmations:
printf "%s\n" "This message is seen at verbosity level 3 and above." >&3
printf "%s\n" "This message is seen at verbosity level 4 and above." >&4
printf "%s\n" "This message is seen at verbosity level 5 and above." >&5

Answer 7

I also came up with this function to do a quick ifelse:

function verbose () {
    [[ $_V -eq 1 ]] && return 0 || return 1
}

This executes a command if $_V is set to 1. Use it like this:

verbose && command #command will be executed if $_V == 1

or

verbose && command -v || command # execute 'command -v' if $_V==1, else execute 'command'

Answer 8

A first try at a more flexible system with verbosity levels (Bash 4):

# CONFIG SECTION
# verbosity level definitions
config[verb_levels]='debug info status warning error critical fatal'

# verbosity levels that are to be user-selectable (0-this value)
config[verb_override]=3

# user-selected verbosity levels (0=none, 1=warnings, 2=warnings+info, 3=warning+info+debug)
config[verbosity]=2

# FUNCTION DEFINITIONS SECTION
_messages() {
    # shortcut functions for messages
    # non overridable levels exit with errlevel
    # safe eval, it only uses two (namespaced) values, and a few builtins
    local verbosity macro level=0
    for verbosity in ${config[verb_levels]}; do
        IFS="" read -rd'' macro <<MACRO
        _$verbosity() {
            $( (( $level <= ${config[verb_override]} )) && echo "(( \${config[verbosity]} + $level > ${config[verb_override]} )) &&" ) echo "${verbosity}: \$@";
            $( (( $level > ${config[verb_override]} )) && echo "exit $(( level - ${config[verb_override]} ));" )
        }
MACRO
        eval "$macro"
        (( level++ ))
    done
}

# INITIALIZATION SECTION
_messages

After initialization, anywhere in your code you can use things like:

! (( $# )) && _error "parameter expected"

[[ -f somefile ]] && _warning "file $somefile already exists"

_info "some info"

_status "running command"
if (( ${config[verbosity]} <= 1 )); then
    command
else
    command -v
fi

# explicitly changing verbosity at run time
old_verbosity=${config[verbosity]}
config[verbosity]=1

etc.

Answer 9

verbose=false

while getopts "v" OPTION
do
  case $OPTION in
    v) verbose=true
       ;;
  esac
done

Then

$verbose && echo "Verbose mode on" || echo "Verbose mode off"

This will execute /bin/true or /bin/false, returning 0 or 1 respectively.

Answer 10

As you noticed, you can define some log functions like log, log_debug, log_error, etc.

function log () {
    if [[ $_V -eq 1 ]]; then
        echo "$@"
    fi
}

It can help increasing your main code readability and hide show\nonshow logic into logging function.

log "some text"

If _V(global variable) is equal 1 "some text" will be printed, in other case it will not.