Count Down Timer in Shell

Kaushal.Modi@fakeEmailToMakeValidatorHappy.com (Kaushal Modi) — Mon, 09 Jan 2017 08:02:25 -0500

I was working on a tcsh script that did some cool stuff. But if a user ran that script not knowing the true impact of the script, it could make some bad irreversible changes.

While I could simply echo a warning statement and put a sleep 10, I wanted the wait time to be shown live.

Table of Contents

Explanation
Result
Bash implementation

So here’s what worked pretty nicely — The warning message is shown to the user, and the actual wait time countdown is also displayed.

#!/usr/bin/env tcsh
set wait_time = 10 # seconds

echo "Are you sure you meant to run this script?"
echo "This script does something drastic that you would severely regret if you happened to run this script by mistake!"
echo ""
set temp_cnt = ${wait_time}
# https://www.cyberciti.biz/faq/csh-shell-scripting-loop-example/
while ( ${temp_cnt} >= 1 )
 printf "\rYou have %2d second(s) remaining to hit Ctrl+C to cancel that operation!" ${temp_cnt}
 sleep 1
 @ temp_cnt--
end
echo ""

Explanation #

The while loop runs for $wait_time times; each time waiting for a second (sleep 1) and then decrementing the temporary counter $temp_cnt.
printf is chosen instead of echo -n because I wanted to have the seconds number always hold 2 character places (%2d).
The \r character in printf makes the magic here. It represents carriage return i.e. The cursor will return to the beginning of the line, and then print the following string, overwriting whatever there was on that line earlier.
- printf acts like echo -n i.e. a newline is not inserted automatically at the end of the printed message. In order to add a newline at the end for printf, you need to do so explicitly by adding a \n character.

Result #

Click here to see the animation on asciinema.org.

Bash implementation #

Below is a re-implementation of the above in bash.

#!/usr/bin/env bash
wait_time=10 # seconds

echo "Are you sure you meant to run this script?"
echo "This script does something drastic that you would severely regret if you happened to run this script by mistake!"
echo ""
temp_cnt=${wait_time}
while [[ ${temp_cnt} -gt 0 ]];
do
 printf "\rYou have %2d second(s) remaining to hit Ctrl+C to cancel that operation!" ${temp_cnt}
 sleep 1
 ((temp_cnt--))
done
echo ""

Check If a Command/Executable Exists from Shell Script

Kaushal.Modi@fakeEmailToMakeValidatorHappy.com (Kaushal Modi) — Wed, 23 Nov 2016 17:07:26 -0500

Shell script snippets to check if you have an executable or binary installed in PATH.

Table of Contents

Bash Shell
Tcsh Shell

I often need to check if a particular executable is present in the PATH before I can proceed with what I am doing in a shell script. Also, I need to work with both tcsh and bash scripts. Below presents the solution that has worked for these shell scripts for me.

Bash Shell #

The below solution using hash was with the help of this SO solution.

if ! hash some_exec 2>/dev/null
then
 echo "'some_exec' was not found in PATH"
fi

Here is the tl;dr from the above SO solution:

Where bash is your shell/hashbang, consistently use hash (for commands) or type (to consider built-ins & keywords). When writing a POSIX script, use command -v.

Tcsh Shell #

As it turns out, the tcsh shell does not have the same hash command as the bash shell.

But the below solution using where which I found with the help of this SO solution works fine.

if ( `where some_exec` == "" ) then
 echo "'some_exec' was not found in PATH"
endif

tcsh on A Scripter's Notes