How can I make a bash command run periodically?

In addition to @sputnick's answer, there is also watch. From the man page:

Execute a program periodically, showing output full screen

By default this is every 2 seconds. watch is useful for tailing logs, for example.

• If you need to visually monitor a command which gives a static output, use watch [options] command. For example, for monitoring free memory, run:

  watch -n 1 free -m
  

  where the -n 1 option sets update interval to 1 second (default is 2 seconds).
  Check man watch or the online manual for details.

• If you need to visually monitor changes in a log file, tail is your command of choice, for example:

  tail -f /path/to/logs/file.log
  

  where the -f (for “follow”) option tells the program to output appended data as the file grows.
  Check man tail or the online manual for details.

If you want to run a command periodically, there's 3 ways :

• using the crontab command ex. * * * * * command (run every minutes)
• using a loop like : while true; do ./my_script.sh; sleep 60; done (not precise)
• using systemd timer

See cron

Some pointers for best bash scripting practices :

http://mywiki.wooledge.org/BashFAQ
Guide: http://mywiki.wooledge.org/BashGuide
ref: http://www.gnu.org/software/bash/manual/bash.html
http://wiki.bash-hackers.org/
USE MORE QUOTES!: http://www.grymoire.com/Unix/Quote.html
Scripts and more: http://www.shelldorado.com/

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 <<EOF

SYNOPSIS
  $THIS_NAME [-n seconds] [opts] cmd [arg ...]

DESCRIPTION
  Executes a command periodically and displays its output for visual inspection.

  NOTE: This is a PARTIAL implementation of the GNU \`watch\` command, for OS X.
  Notably, the output is not limited to one screenful, and displaying
  output differences and using precise timing are not supported.
  Also, colored output is always passed through (--color is implied).
  Unimplemented features are marked as [NOT IMPLEMENTED] below.
  Conversely, features specific to this implementation are marked as [NONSTD].
  Reference version is GNU watch 0.3.0.

  CMD may be a simple command with separately specified
  arguments, if any, or a single string containing one or more
  ;-separated commands (including arguments) - in the former case the command
  is directly executed by bash, in the latter the string is passed to \`bash -c\`.
  Note that GNU watch uses sh, not bash.
  To use \`exec\` instead, specify -x (see below).

  By default, CMD is re-invoked indefinitely; terminate with ^-C or
  exit based on conditions:
  -e, --errexit
    exits once CMD indicates an error, i.e., returns a non-zero exit code.
  -E, --okexit [NONSTD] 
    is the inverse of -e: runs until CMD returns exit code 0.

  By default, all output is passed through; the following options modify this
  behavior; note that suppressing output only relates to CMD's output, not the
  messages output by this utility itself:
  -q, --quiet [NONSTD]
    suppresses stdout output from the command invoked;
  -Q, --quiet-both [NONSTD] 
    suppresses both stdout and stderr output.

  -l, --list [NONSTD]
    list-style display; i.e., suppresses clearing of the screen 
    before every invocation of CMD.

  -n secs, --interval secs
    interval in seconds between the end of the previous invocation of CMD
    and the next invocation - 2 seconds by default, fractional values permitted;
    thus, the interval between successive invocations is the specified interval
    *plus* the last CMD's invocation's execution duration.

  -x, --exec
    uses \`exec\` rather than bash to execute CMD; this requires
    arguments to be passed to CMD to be specified as separate arguments 
    to this utility and prevents any shell expansions of these arguments
    at invocation time.

  -t, --no-title
    suppresses the default title (header) that displays the interval, 
    and (NONSTD) a time stamp, the time elapsed so far, and the command executed.

  -b, --beep
    beeps on error (bell signal), i.e., when CMD reports a non-zero exit code.

  -c, --color
    IMPLIED AND ALWAYS ON: colored command output is invariably passed through.

  -p, --precise [NOT IMPLEMENTED]

  -d, --difference [NOT IMPLEMENTED]

EXAMPLES
    # List files in home folder every second.
  $THIS_NAME -n 1 ls ~
    # Wait until all *.lockfile files disappear from the current dir, checking every 2 secs.
  $THIS_NAME -e 'ls *.lockfile'

EOF
    exit 0
fi

  # Make sure that we're running on OSX.
[[ $(uname) == 'Darwin' ]] || die "This script is designed to run on OS X only."

# Preprocess parameters: expand compressed options to individual options; e.g., '-ab' to '-a -b'
params=() decompressed=0 argsReached=0
for p in "$@"; do
  if [[ $argsReached -eq 0 && $p =~ ^-[a-zA-Z0-9]+$ ]]; then # compressed options?
    decompressed=1
    params+=(${p:0:2})
    for (( i = 2; i < ${#p}; i++ )); do
        params+=("-${p:$i:1}")
    done
  else
    (( argsReached && ! decompressed )) && break
    [[ $p == '--' || ${p:0:1} != '-' ]] && argsReached=1
    params+=("$p")
  fi
done
(( decompressed )) && set -- "${params[@]}"; unset params decompressed argsReached p # Replace "$@" with the expanded parameter set.

# Option-parameters loop.
interval=2  # default interval
runUntilFailure=0
runUntilSuccess=0
quietStdOut=0
quietStdOutAndStdErr=0
dontClear=0
noHeader=0
beepOnErr=0
useExec=0
while (( $# )); do
  case "$1" in
    --) # Explicit end-of-options marker.
      shift   # Move to next param and proceed with data-parameter analysis below.
      break
      ;;
    -p|--precise|-d|--differences|--differences=*)
      dieSyntax "Sadly, option $1 is NOT IMPLEMENTED."
      ;;
    -v|--version)
      echo "$VERSION"; exit 0
      ;;
    -x|--exec)
      useExec=1
      ;;
    -c|--color)
      # a no-op: unlike the GNU version, we always - and invariably - pass color codes through.
      ;;
    -b|--beep)
      beepOnErr=1
      ;;
    -l|--list)
      dontClear=1
      ;;
    -e|--errexit)
      runUntilFailure=1
      ;;
    -E|--okexit)
      runUntilSuccess=1
      ;;
    -n|--interval)
      shift; interval=$1;
      errMsg="Please specify a positive number of seconds as the interval."
      interval=$(bc <<<"$1") || dieSyntax "$errMsg"
      (( 1 == $(bc <<<"$interval > 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