Increment the title of files output by a command in a shell script

Question

I made this simple bash script to take a full-screen screenshot and save it to the pictures folder

#!/usr/bin/bash

xfce4-screenshooter -f -s /home/rgcodes/Pictures/Screenshot_$scrshotcount.png 
let "scrshotcount++"

...which runs into a problem. scrshotcount is a global variable I defined in /etc/environment to be incremented every time the script runs. However, the script fails to increment the variable globally, and causes the script to just overwrite the previous screenshot. Searches on Google and Stack Overflow revealed that the problem isn't straightforward at all (something about child shells being unable to change variables for parents), and finding some other method would be better.

Here's my question. How do we append numbers (in ascending order) to the screenshots the script throws out so that they are saved just like those taken on Windows?(Windows auto-suffixes matching filenames, rather than overwriting them, so all Screenshots have the same name 'Screenshot' and the number of times the screenshot command has been used.)

I am using @erikMD's method as a temporary stop-gap for now.

Answer 1

In addition to the excellent advice about using a date instead of a counter, here's a way to use a counter :/

dir=$HOME/Pictures

# find the current maximum value
current_max=$( 
    find "$dir" -name Screenshot_\*.png -print0 \
    | sort -z -V \
    | tail -z -n 1
)
if [[ ! $current_max =~ _([0-9]+)\.png ]]; then
    echo "can't find the screenshot with the maximum counter value" >&2
    exit 1
fi

# increment it
counter=$(( 1 + ${BASH_REMATCH[1]} ))

# and use it
xfce4-screenshooter -f -s "$dir/Screenshot_${counter}.png"

You'll have to manually create the Screenshot_1.png file.

Answer 2

@rgcodes below is a script that will capture screenshots with a numeric count indicator per your original post. (tested it on Ubuntu 20.04)

Script contents:

#!/bin/bash

set -uo pipefail

# add source file and line number to xtrace output 
# i.e. when running:  bash -x ./your_script_name.sh
export PS4='+(${BASH_SOURCE}:${LINENO}): ${FUNCNAME[0]:+${FUNCNAME[0]}(): }'

capture_screenshot() {
    local output_dir="${1:-/tmp/screenshot}"
    local img_name="${2:-Screenshot}"
    local img_ext="${3:-png}"
    
    # create output directory (if not already existing)
    mkdir -p "${output_dir}"

    # get the last image in the sorted ls output
    local latest_png=$(tail -n 1 \
        <(sort -n -t _ -k 2 \
        <(ls ${output_dir}/*.${img_ext} 2> /dev/null)))

    # use the latest image to determine img_cnt value for next image
    local img_cnt=0
    if [[ ${latest_png} =~ _([0-9]+)\.png ]]; then
        img_cnt=$((1+${BASH_REMATCH[1]}))
    elif [[ ${latest_png} =~ ${img_name}.${img_ext} ]] ; then
        img_cnt=1
    fi

    # build path to output image
    local img_path="${output_dir}/${img_name}_${img_cnt}.${img_ext}"
    
    # remove count from output image path if count == 0
    if [[ "${img_cnt}" -eq "0" ]] ; then
        img_path="${output_dir}/${img_name}.${img_ext}"
    fi        

    xfce4-screenshooter -f -s "${img_path}"
}

capture_screenshot "$@"

The uses the following as defaults, but you can change them to meet your requirements.

output directory for screenshots:
/tmp/screenshot

base screenshot image name:
Screenshot

screenshot file extension:
.png

The script will attempt to create the output directory if it does not already exist (subject to user permission for creation). Below is a sample usage.

Prior to initial script execution, the output directory does not exist:

$ ls screenshot
$

Initial execution (directory is created and Screenshot.png created:

$ ./script.sh 
$ ls /tmp/screenshot/
Screenshot.png

Subsequent executions:

$ ./script.sh 
$ ls /tmp/screenshot/
Screenshot_1.png  Screenshot.png
$ ./script.sh 
$ ls /tmp/screenshot/
Screenshot_1.png  Screenshot_2.png  Screenshot.png

Answer 3

Indeed, as suggested by @j_b in the comments, you should definitely give a try to using a timestamp with the command date +"$format".

FTR, the same idea is implemented here in this project of a gnome-screenshot bash wrapper
(disclaimer: I am the author of this repo).

Example command:

date "+%Y-%m-%d_%H-%M-%S"

↓

2021-07-29_19-13-30

So the overall script could just be something like:

#!/usr/bin/env bash

xfce4-screenshooter -f -s "$HOME/Pictures/Screenshot_$(date "+%Y-%m-%d_%H-%M-%S").png"

(Note that I added missing double-quotes, and modified your shebang, as /usr/bin/env bash is more portable than /bin/bash or /usr/bin/bash.)