Using a bash only script, how can you provide a bash progress indicator?
For example, when I run a command from bash - while that command is executing - let the user know that something is still happening.
In this example using SCP, I'm demonstrating how to grab the process id (pid) and then do something while that process is running.
This displays a simple spinnng icon.
/usr/bin/scp me@website.com:file somewhere 2>/dev/null &
pid=$! # Process Id of the previous running command
spin[0]="-"
spin[1]="\\"
spin[2]="|"
spin[3]="/"
echo -n "[copying] ${spin[0]}"
while [ kill -0 $pid ]
do
  for i in "${spin[@]}"
  do
        echo -ne "\b$i"
        sleep 0.1
  done
done
William Pursell's solution
/usr/bin/scp me@website.com:file somewhere 2>/dev/null &
pid=$! # Process Id of the previous running command
spin='-\|/'
i=0
while kill -0 $pid 2>/dev/null
do
  i=$(( (i+1) %4 ))
  printf "\r${spin:$i:1}"
  sleep .1
done
If you have a way to estimate percentage done, such as the current number of files processed and total number, you can make a simple linear progress meter with a little math and assumptions about screen width.
count=0
total=34
pstr="[=======================================================================]"
while [ $count -lt $total ]; do
  sleep 0.5 # this is work
  count=$(( $count + 1 ))
  pd=$(( $count * 73 / $total ))
  printf "\r%3d.%1d%% %.${pd}s" $(( $count * 100 / $total )) $(( ($count * 1000 / $total) % 10 )) $pstr
done
Or instead of a linear meter you could estimate time remaining. It's about as accurate as other similar things.
count=0
total=34
start=`date +%s`
while [ $count -lt $total ]; do
  sleep 0.5 # this is work
  cur=`date +%s`
  count=$(( $count + 1 ))
  pd=$(( $count * 73 / $total ))
  runtime=$(( $cur-$start ))
  estremain=$(( ($runtime * $total / $count)-$runtime ))
  printf "\r%d.%d%% complete ($count of $total) - est %d:%0.2d remaining\e[K" $(( $count*100/$total )) $(( ($count*1000/$total)%10)) $(( $estremain/60 )) $(( $estremain%60 ))
done
printf "\ndone\n"
Referred from here is a nice spinner function (with slight modification), will help your cursor to stay in original position also.
spinner()
{
    local pid=$!
    local delay=0.75
    local spinstr='|/-\'
    while [ "$(ps a | awk '{print $1}' | grep $pid)" ]; do
        local temp=${spinstr#?}
        printf " [%c]  " "$spinstr"
        local spinstr=$temp${spinstr%"$temp"}
        sleep $delay
        printf "\b\b\b\b\b\b"
    done
    printf "    \b\b\b\b"
}
with usage:
(a_long_running_task) &
spinner
Here a simple one-liner, that I use:
while true; do for X in '-' '/' '|' '\'; do echo -en "\b$X"; sleep 0.1; done; done 
Aside from the classical spinner, you can use this progress bar
It achieves subcharacter precision by using half block characters
Code included on the link.
Here's my attempt. I'm new to bash scripts so some of this code may be terrible :)
Example Output:
The Code:
progressBarWidth=20
# Function to draw progress bar
progressBar () {
  # Calculate number of fill/empty slots in the bar
  progress=$(echo "$progressBarWidth/$taskCount*$tasksDone" | bc -l)  
  fill=$(printf "%.0f\n" $progress)
  if [ $fill -gt $progressBarWidth ]; then
    fill=$progressBarWidth
  fi
  empty=$(($fill-$progressBarWidth))
  # Percentage Calculation
  percent=$(echo "100/$taskCount*$tasksDone" | bc -l)
  percent=$(printf "%0.2f\n" $percent)
  if [ $(echo "$percent>100" | bc) -gt 0 ]; then
    percent="100.00"
  fi
  # Output to screen
  printf "\r["
  printf "%${fill}s" '' | tr ' ' ▉
  printf "%${empty}s" '' | tr ' ' ░
  printf "] $percent%% - $text "
}
## Collect task count
taskCount=33
tasksDone=0
while [ $tasksDone -le $taskCount ]; do
  # Do your task
  (( tasksDone += 1 ))
  # Add some friendly output
  text=$(echo "somefile-$tasksDone.dat")
  # Draw the progress bar
  progressBar $taskCount $taskDone $text
  sleep 0.01
done
echo
You can see the source here: https://gist.github.com/F1LT3R/fa7f102b08a514f2c535
Here is an example of an 'activity indicator,' for an internet connection speed test via the linux 'speedtest-cli' command:
printf '\n\tInternet speed test:  '
# http://stackoverflow.com/questions/12498304/using-bash-to-display-a-progress-working-indicator
spin[0]="-"
spin[1]="\\"
spin[2]="|"
spin[3]="/"
# http://stackoverflow.com/questions/20165057/executing-bash-loop-while-command-is-running
speedtest > .st.txt &           ## & : continue running script
pid=$!                          ## PID of last command
# If this script is killed, kill 'speedtest':
trap "kill $pid 2> /dev/null" EXIT
# While 'speedtest' is running:
while kill -0 $pid 2> /dev/null; do
for i in "${spin[@]}"
do
    echo -ne "\b$i"
    sleep 0.1
done
done
# Disable the trap on a normal exit:
trap - EXIT
printf "\n\t           "
grep Download: .st.txt
printf "\t             "
grep Upload: .st.txt
echo ''
rm -f st.txt
Update - example:
I extended the answer of checksum in his answer by displaying a variable info message after the spinner:
#!/usr/bin/env bash 
function spinner() {
    local info="$1"
    local pid=$!
    local delay=0.75
    local spinstr='|/-\'
    while kill -0 $pid 2> /dev/null; do
        local temp=${spinstr#?}
        printf " [%c]  $info" "$spinstr"
        local spinstr=$temp${spinstr%"$temp"}
        sleep $delay
        local reset="\b\b\b\b\b\b"
        for ((i=1; i<=$(echo $info | wc -c); i++)); do
            reset+="\b"
        done
        printf $reset
    done
    printf "    \b\b\b\b"
}
# usage:
(a_long_running_task) &
spinner "performing long running task..."
I don't like that if the stdout output with a spinner is redirected to a file, less shows ^H for each backspace instead of avoiding them in a file output at all. Is that possible with an easy spinner like this one?
@DavidD's comments on Pez Cuckows answer, this is an example of how you can capture the output of the progress bar in a script and still see the spinner on the screen:
#!/usr/bin/env bash 
#############################################################################
###########################################################################
###
### Modified/Rewritten by A.M.Danischewski (c) 2015 v1.1
### Issues: If you find any issues emai1 me at my <first name> dot 
###         <my last name> at gmail dot com.  
###
### Based on scripts posted by Pez Cuckow, William Pursell at:  
### http://stackoverflow.com/questions/12498304/using-bash-to-display-\
###      a-progress-working-indicator
###
### This program runs a program passed in and outputs a timing of the 
### command and it exec's a new fd for stdout so you can assign a 
### variable the output of what was being run. 
### 
### This is a very new rough draft but could be expanded. 
### 
### This program is free software: you can redistribute it and/or modify
### it under the terms of the GNU General Public License as published by
### the Free Software Foundation, either version 3 of the License, or
### (at your option) any later version.
###
### This program is distributed in the hope that it will be useful,
### but WITHOUT ANY WARRANTY; without even the implied warranty of
### MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
### GNU General Public License for more details.
###
### You should have received a copy of the GNU General Public License
### along with this program.  If not, see <http://www.gnu.org/licenses/>.
###########################################################################
#############################################################################
declare    CMD="${1}"
shift      ## Clip the first value of the $@, the rest are the options. 
declare    CMD_OPTIONS="$@"
declare    CMD_OUTPUT=""
declare    TMP_OUTPUT="/tmp/_${0##*/}_$$_$(date +%Y%m%d%H%M%S%N)" 
declare -r SPIN_DELAY="0.1"
declare -i PID=
function usage() {
cat <<EOF
Description: ${0##*/}
This program runs a program passed in and outputs a timing of the 
command and it exec's a new fd for stdout so you can assign a variable 
the output of what was being run. 
Usage: ${0##*/} <command> [command options]
 E.g.  
    >$ ${0##*/} sleep 5 \&\& echo "hello" \| figlet
     Running: sleep 5 && echo hello | figlet, PID 2587:/
     real   0m5.003s
     user   0m0.000s
     sys    0m0.002s
      _          _ _       
     | |__   ___| | | ___  
     | '_ \ / _ \ | |/ _ \ 
     | | | |  __/ | | (_) |
     |_| |_|\___|_|_|\___/ 
     Done..
    >$ var=\$(${0##*/} sleep 5 \&\& echo hi)
     Running: sleep 5 && echo hi, PID 32229:-
     real   0m5.003s
     user   0m0.000s
     sys    0m0.001s
     Done..
     >$ echo \$var
     hi
EOF
} 
function spin_wait() { 
 local -a spin 
 spin[0]="-"
 spin[1]="\\"
 spin[2]="|"
 spin[3]="/"
 echo -en "Running: ${CMD} ${CMD_OPTIONS}, PID ${PID}: " >&3
 while kill -0 ${PID} 2>/dev/random; do
   for i in "${spin[@]}"; do
     echo -ne "\b$i" >&3
     sleep ${SPIN_DELAY}
   done
 done
} 
function run_cmd() { 
 exec 3>$(tty)
 eval "time ${CMD} ${CMD_OPTIONS}" 2>>"${TMP_OUTPUT}" | tee "${TMP_OUTPUT}" & 
 PID=$! # Set global PID to process id of the command we just ran. 
 spin_wait
 echo -en "\n$(< "${TMP_OUTPUT}")\n" >&3 
 echo -en "Done..\n" >&3
 rm "${TMP_OUTPUT}"
 exec 3>&-
} 
if [[ -z "${CMD}" || "${CMD}" =~ ^-. ]]; then 
 usage | more && exit 0 
else 
 run_cmd  
fi 
exit 0 
Psychedelic progress bar for bash scripting. Call by command line as './progressbar x y' where 'x' is a time in seconds and 'y' is a message to display. The inner function progressbar() works standalone as well and takes 'x' as a percentage and 'y' as a message.
#!/bin/bash
if [ "$#" -eq 0 ]; then echo "x is \"time in seconds\" and z is \"message\""; echo "Usage: progressbar x z"; exit; fi
progressbar() {
        local loca=$1; local loca2=$2;
        declare -a bgcolors; declare -a fgcolors;
        for i in {40..46} {100..106}; do
                bgcolors+=("$i")
        done
        for i in {30..36} {90..96}; do
                fgcolors+=("$i")
        done
        local u=$(( 50 - loca ));
        local y; local t;
        local z; z=$(printf '%*s' "$u");
        local w=$(( loca * 2 ));
        local bouncer=".oO°Oo.";
        for ((i=0;i<loca;i++)); do
                t="${bouncer:((i%${#bouncer})):1}"
                bgcolor="\\E[${bgcolors[RANDOM % 14]}m \\033[m"
                y+="$bgcolor";
        done
        fgcolor="\\E[${fgcolors[RANDOM % 14]}m"
        echo -ne " $fgcolor$t$y$z$fgcolor$t \\E[96m(\\E[36m$w%\\E[96m)\\E[92m $fgcolor$loca2\\033[m\r"
};
timeprogress() {
        local loca="$1"; local loca2="$2";
        loca=$(bc -l <<< scale=2\;"$loca/50")
        for i in {1..50}; do
                progressbar "$i" "$loca2";
                sleep "$loca";
        done
        echo -e "\n"
};
timeprogress "$1" "$2"