How can I make a bash command run periodically?

Question

I want to execute a script and have it run a command every x minutes.

Also any general advice on any resources for learning bash scripting could be really cool. I us

Answer 1

macOS users: here's a partial implementation of the GNU watch command (as of version 0.3.0) for interactive periodic invocations for primarily visual inspection:

It is syntax-compatible with the GNU version and fails with a specific error message if an unimplemented feature is used.

Notable limitations:

• The output is not limited to one screenful.
• Displaying output differences is not supported.
• Using precise timing is not supported.
• Colored output is always passed through (--color is implied).

Also implements a few non-standard features, such as waiting for success (-E) to complement waiting for error (-e) and showing the time of day of the last invocation as well as the total time elapsed so far.

Run watch -h for details.

Examples:

watch -n 1 ls # list current dir every second
watch -e 'ls *.lockfile' # list lock files and exit once none exist anymore.

Source code (paste into a script file named watch, make it executable, and place in a directory in your $PATH; note that syntax highlighting here is broken, but the code works):

#!/usr/bin/env bash

THIS_NAME=$(basename "$BASH_SOURCE")

VERSION='0.1'

# Helper function for exiting with error message due to runtime error.
#   die [errMsg [exitCode]]
# Default error message states context and indicates that execution is aborted. Default exit code is 1.
# Prefix for context is always prepended.
# Note: An error message is *always* printed; if you just want to exit with a specific code silently, use `exit n` directly.
die() {
  echo "$THIS_NAME: ERROR: ${1:-"ABORTING due to unexpected error."}" 1>&2
  exit ${2:-1} # Note: If the argument is non-numeric, the shell prints a warning and uses exit code 255.
}

# Helper function for exiting with error message due to invalid parameters.
#   dieSyntax [errMsg]
# Default error message is provided, as is prefix and suffix; exit code is always 2.
dieSyntax() {
  echo "$THIS_NAME: PARAMETER ERROR: ${1:-"Invalid parameter(s) specified."} Use -h for help." 1>&2
  exit 2
}

# Get the elapsed time since the specified epoch time in format HH:MM:SS.
# Granularity: whole seconds.
# Example:
#   tsStart=$(date +'%s')
#   ...
#   getElapsedTime $tsStart 
getElapsedTime() {
  date -j -u -f '%s' $(( $(date +'%s') - $1 ))  +'%H:%M:%S' 
}

# Command-line help.
if [[ "$1" == '--help' || "$1" == '-h' ]]; then
  cat < 0") )) || dieSyntax "$errMsg"
      [[ $interval == *.* ]] || interval+='.0'
      ;;
    -t|--no-title)
      noHeader=1
      ;;
    -q|--quiet)
      quietStdOut=1
      ;;
    -Q|--quiet-both)
      quietStdOutAndStdErr=1
      ;;
    -?|--?*) # An unrecognized switch.
      dieSyntax "Unrecognized option: '$1'. To force interpretation as non-option, precede with '--'."
      ;;
    *)  # 1st data parameter reached; proceed with *argument* analysis below.
      break
      ;;
  esac
  shift
done

# Make sure we have at least a command name
[[ -n "$1" ]] || dieSyntax "Too few parameters specified."

# Suppress output streams, if requested.
# Duplicate stdout and stderr first.
# This allows us to produce output to stdout (>&3) and stderr (>&4) even when suppressed.
exec 3<&1 4<&2  
if (( quietStdOutAndStdErr )); then
  exec &> /dev/null
elif (( quietStdOut )); then
  exec 1> /dev/null
fi

# Set an exit trap to ensure that the duplicated file descriptors are closed.
trap 'exec 3>&- 4>&-' EXIT

# Start loop with periodic invocation.
# Note: We use `eval` so that compound commands - e.g. 'ls; bash --version' - can be passed.
tsStart=$(date +'%s')
while :; do
  (( dontClear )) || clear
  (( noHeader )) || echo "Every ${interval}s. [$(date +'%H:%M:%S') - elapsed: $(getElapsedTime $tsStart)]: $@"$'\n' >&3
  if (( useExec )); then
    (exec "$@")  # run in *subshell*, otherwise *this* script will be replaced by the process invoked
  else
    if [[ $* == *' '* ]]; then
      # A single argument with interior spaces was provided -> we must use `bash -c` to evaluate it properly.
      bash -c "$*"
    else
      # A command name only or a command name + arguments were specified as separate arguments -> let bash run it directly.
      "$@"
    fi
  fi
  ec=$?
  (( ec != 0 && beepOnErr )) && printf '\a'
  (( ec == 0 && runUntilSuccess )) && { echo $'\n'"[$(date +'%H:%M:%S') - elapsed: $(getElapsedTime $tsStart)] Exiting as requested: exit code 0 reported." >&3; exit 0; }
  (( ec != 0 && runUntilFailure )) && { echo $'\n'"[$(date +'%H:%M:%S') - elapsed: $(getElapsedTime $tsStart)] Exiting as requested: non-zero exit code ($ec) reported." >&3; exit 0; }
  sleep $interval
done