Bash script to scrape Feynman Lecture Recordings Playlist from https://www.feynmanlectures.caltech.edu/flptapes.html

scrape_feynman_lectures_playlist.sh

#!/bin/bash

# Number of concurrent downloads
num_concurrent_downloads=9

# Function to perform curl request
perform_curl() {
    local url=$1
    local filename=$2
    printf "\nDownloading: $filename from $url\n"

    curl "$url" \
        -H 'Accept: */*' \
        -H 'Accept-Language: en-GB,en-US;q=0.9,en;q=0.8' \
        -H 'Connection: keep-alive' \
        -H 'DNT: 1' \
        -H 'Referer: https://www.feynmanlectures.caltech.edu/flptapes.html' \
        -H 'Sec-Fetch-Dest: audio' \
        -H 'Sec-Fetch-Mode: no-cors' \
        -H 'Sec-Fetch-Site: same-origin' \
        -H 'User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/113.0.0.0 Safari/537.36' \
        --compressed -o "$filename"

    printf "\nCOMPLETE: $filename\n"
}


# Function to download files concurrently
download_concurrently() {
    local urls=("$@")
    local num_urls=${#urls[@]}
    local i=0
    local j=0

    printf "Downloading $num_urls files\n\n"

    # Loop through the URLs and perform concurrent downloads
    while [[ $i -lt $num_urls ]]; do
        filename=$(basename "${urls[i]}")
        perform_curl "https://www.feynmanlectures.caltech.edu/protected${urls[i]}" $filename &
        ((i++))
        ((j++))

        # Limit the number of concurrent downloads
        if [[ $j -eq $num_concurrent_downloads ]]; then
            wait -n
            ((j--))
        fi
    done

    # Wait for all remaining downloads to finish
    wait
}


# Parse JSON and extract values
parse_json() {
    local json_file=$1

    # Read the JSON file using jq
    local m4a_urls=($(jq -r '.[].m4a' "$json_file"))

    # Loop through the arrays and perform concurrent curl requests
    download_concurrently "${m4a_urls[@]}"
}

# NOTE: You can get the json object by printing console.log(recordings) in the website page https://www.feynmanlectures.caltech.edu/flptapes.html
# Save this to a file and change json_file value to it's path
json_file="source.json"
parse_json "$json_file"

 

console.log(recordings) doesn't work anymore. Here is the json text which you can paste to source.json

[{"title":"<span>#1</span> Atoms in motion (9/26/61)","m4a":"audio/mp4/FLP_1_01.mp4","oga":"audio/ogg/FLP_1_01.ogg"},{"title":"<span>#2</span> Basic physics (9/29/61)","m4a":"audio/mp4/FLP_2_01.mp4","oga":"audio/ogg/FLP_2_01.ogg"},{"title":"<span>#3</span> The relation of physics to other sciences (10/3/61)","m4a":"audio/mp4/FLP_3_01.mp4","oga":"audio/ogg/FLP_3_01.ogg"},{"title":"<span>#4</span> Conservation of energy (10/6/61)","m4a":"audio/mp4/FLP_4_01.mp4","oga":"audio/ogg/FLP_4_01.ogg"},{"title":"<span>#5</span> Time and distance (10/10/61)","m4a":"audio/mp4/FLP_5_01.mp4","oga":"audio/ogg/FLP_5_01.ogg"},{"title":"<span>#6</span> Probability (10/13/61)","m4a":"audio/mp4/FLP_6_01.mp4","oga":"audio/ogg/FLP_6_01.ogg"},{"title":"<span>#7</span> The theory of gravitation (10/17/61)","m4a":"audio/mp4/FLP_7_01.mp4","oga":"audio/ogg/FLP_7_01.ogg"},{"title":"<span>#8</span> Motion (10/20/61)","m4a":"audio/mp4/FLP_8_01.mp4","oga":"audio/ogg/FLP_8_01.ogg"},{"title":"<span>#9</span> Newton's laws of dynamics (10/24/61)","m4a":"audio/mp4/FLP_9_01.mp4","oga":"audio/ogg/FLP_9_01.ogg"},{"title":"<span>#10</span> Conservation of momentum (10/28/61)","m4a":"audio/mp4/FLP_10_01.mp4","oga":"audio/ogg/FLP_10_01.ogg"},{"title":"<span>#11</span> Vectors(11/3/61)","m4a":"audio/mp4/FLP_11_01.mp4","oga":"audio/ogg/FLP_11_01.ogg"},{"title":"<span>#12</span> On force(11/7/61)","m4a":"audio/mp4/FLP_12_01.mp4","oga":"audio/ogg/FLP_12_01.ogg"},{"title":"<span>#13</span> Work and potential energy (11/10/61)","m4a":"audio/mp4/FLP_13_01.mp4","oga":"audio/ogg/FLP_13_01.ogg"},{"title":"<span>#14</span> Work and potential energy, second try (11/14/61)","m4a":"audio/mp4/FLP_14_01.mp4","oga":"audio/ogg/FLP_14_01.ogg"},{"title":"<span>#15</span> Relativity (11/17/61)","m4a":"audio/mp4/FLP_15_01.mp4","oga":"audio/ogg/FLP_15_01.ogg"},{"title":"<span>#16</span> Relativistic energy and momentum (11/21/61)","m4a":"audio/mp4/FLP_16_01.mp4","oga":"audio/ogg/FLP_16_01.ogg"},{"title":"<span>#17</span> Space-time (11/28/61)","m4a":"audio/mp4/FLP_17_01.mp4","oga":"audio/ogg/FLP_17_01.ogg"},{"title":"<span class='nonum'></span> Review Lecture A (12/1/61)","m4a":"audio/mp4/FLP_RevA_01.mp4","oga":"audio/ogg/FLP_RevA_01.ogg"},{"title":"<span class='nonum'></span> Review Lecture B (12/5/61)","m4a":"audio/mp4/FLP_RevB_01.mp4","oga":"audio/ogg/FLP_RevB_01.ogg"},{"title":"<span class='nonum'></span> Review Lecture C (12/8/61)","m4a":"audio/mp4/FLP_RevC_01.mp4","oga":"audio/ogg/FLP_RevC_01.ogg"},{"title":"<span>#18</span> Rotation in two dimensions (1/5/62)","m4a":"audio/mp4/FLP_18_01.mp4","oga":"audio/ogg/FLP_18_01.ogg"},{"title":"<span>#19</span> Center of mass; moment of inertia (1/9/62)","m4a":"audio/mp4/FLP_19_01.mp4","oga":"audio/ogg/FLP_19_01.ogg"},{"title":"<span>#20</span> Rotation in space (1/12/62)","m4a":"audio/mp4/FLP_20_01.mp4","oga":"audio/ogg/FLP_20_01.ogg"},{"title":"<span>#21</span> Dynamical Effects and Their Applications (1/16/62)","m4a":"audio/mp4/FLP_21_01.mp4","oga":"audio/ogg/FLP_21_01.ogg"},{"title":"<span>#22</span> The harmonic oscillator (1/19/62)","m4a":"audio/mp4/FLP_22_01.mp4","oga":"audio/ogg/FLP_22_01.ogg"},{"title":"<span>#23</span> Algebra (1/23/62)","m4a":"audio/mp4/FLP_23_01.mp4","oga":"audio/ogg/FLP_23_01.ogg"},{"title":"<span>#24</span> Resonance(1/26/62)","m4a":"audio/mp4/FLP_24_01.mp4","oga":"audio/ogg/FLP_24_01.ogg"},{"title":"<span>#25</span> Transients(2/2/62)","m4a":"audio/mp4/FLP_25_01.mp4","oga":"audio/ogg/FLP_25_01.ogg"},{"title":"<span>#26</span> Linear systems (2/6/62)","m4a":"audio/mp4/FLP_26_01.mp4","oga":"audio/ogg/FLP_26_01.ogg"},{"title":"<span>#27</span> Optics: The principle of least time (2/9/62)","m4a":"audio/mp4/FLP_27_01.mp4","oga":"audio/ogg/FLP_27_01.ogg"},{"title":"<span>#28</span> Geometrical optics (2/13/62)","m4a":"audio/mp4/FLP_28_01.mp4","oga":"audio/ogg/FLP_28_01.ogg"},{"title":"<span>#29</span> Electromagnetic radiation (2/16/62)","m4a":"audio/mp4/FLP_29_01.mp4","oga":"audio/ogg/FLP_29_01.ogg"},{"title":"<span>#30</span> Interference (2/20/62)","m4a":"audio/mp4/FLP_30_01.mp4","oga":"audio/ogg/FLP_30_01.ogg"},{"title":"<span>#31</span> Diffraction (2/23/62)","m4a":"audio/mp4/FLP_31_01.mp4","oga":"audio/ogg/FLP_31_01.ogg"},{"title":"<span>#32</span> The origin of the refractive index (2/27/62)","m4a":"audio/mp4/FLP_32_01.mp4","oga":"audio/ogg/FLP_32_01.ogg"},{"title":"<span>#33</span> Radiation damping. Light scattering (3/2/62)","m4a":"audio/mp4/FLP_33_01.mp4","oga":"audio/ogg/FLP_33_01.ogg"},{"title":"<span>#34</span> Color vision (3/6/62)","m4a":"audio/mp4/FLP_34_01.mp4","oga":"audio/ogg/FLP_34_01.ogg"},{"title":"<span>#34A</span> Mechanisms of seeing (3/9/62)","m4a":"audio/mp4/FLP_34A_01.mp4","oga":"audio/ogg/FLP_34A_01.ogg"},{"title":"<span>#35</span> Polarization (3/27/62)","m4a":"audio/mp4/FLP_35_01.mp4","oga":"audio/ogg/FLP_35_01.ogg"},{"title":"<span>#36</span> Relativistic effects in radiation (3/30/62)","m4a":"audio/mp4/FLP_36_01.mp4","oga":"audio/ogg/FLP_36_01.ogg"},{"title":"<span>#37</span> Quantum behavior (4/3/62)","m4a":"audio/mp4/FLP_37_01.mp4","oga":"audio/ogg/FLP_37_01.ogg"},{"title":"<span>#38</span> The relation of wave and particle viewpoints (4/6/62)","m4a":"audio/mp4/FLP_38_01.mp4","oga":"audio/ogg/FLP_38_01.ogg"},{"title":"<span>#39</span> The relation of wave and particle viewpoints, second try (4/10/62)","m4a":"audio/mp4/FLP_39_01.mp4","oga":"audio/ogg/FLP_39_01.ogg"},{"title":"<span>#40</span> The kinetic theory of gases (4/13/62)","m4a":"audio/mp4/FLP_40_01.mp4","oga":"audio/ogg/FLP_40_01.ogg"},{"title":"<span>#41</span> The principles of statistical mechanics (4/17/62)","m4a":"audio/mp4/FLP_41_01.mp4","oga":"audio/ogg/FLP_41_01.ogg"},{"title":"<span>#41A</span> Brownian motion (4/20/62)","m4a":"audio/mp4/FLP_41A_01.mp4","oga":"audio/ogg/FLP_41A_01.ogg"},{"title":"<span>#42</span> Applications of kinetic theory (4/27/62)","m4a":"audio/mp4/FLP_42_01.mp4","oga":"audio/ogg/FLP_42_01.ogg"},{"title":"<span>#43</span> Diffusion (5/1/62)","m4a":"audio/mp4/FLP_43_01.mp4","oga":"audio/ogg/FLP_43_01.ogg"},{"title":"<span>#44</span> The laws of thermodynamics (5/4/62)","m4a":"audio/mp4/FLP_44_01.mp4","oga":"audio/ogg/FLP_44_01.ogg"},{"title":"<span>#45</span> Illustrations of thermodynamics (5/8/62)","m4a":"audio/mp4/FLP_45_01.mp4","oga":"audio/ogg/FLP_45_01.ogg"},{"title":"<span>#46</span> Ratchet and pawl (5/11/62)","m4a":"audio/mp4/FLP_46_01.mp4","oga":"audio/ogg/FLP_46_01.ogg"},{"title":"<span>#47</span> Sound. The wave equation (5/15/62)","m4a":"audio/mp4/FLP_47_01.mp4","oga":"audio/ogg/FLP_47_01.ogg"},{"title":"<span>#48</span> Beats (5/18/62)","m4a":"audio/mp4/FLP_48_01.mp4","oga":"audio/ogg/FLP_48_01.ogg"},{"title":"<span>#49</span> Modes (5/22/62)","m4a":"audio/mp4/FLP_49_01.mp4","oga":"audio/ogg/FLP_49_01.ogg"},{"title":"<span>#50</span> Harmonics (5/25/62)","m4a":"audio/mp4/FLP_50_01.mp4","oga":"audio/ogg/FLP_50_01.ogg"},{"title":"<span>#51</span> Waves (5/29/62)","m4a":"audio/mp4/FLP_51_01.mp4","oga":"audio/ogg/FLP_51_01.ogg"},{"title":"<span>#52</span> Symmetry in physical laws (6/1/62)","m4a":"audio/mp4/FLP_52_01.mp4","oga":"audio/ogg/FLP_52_01.ogg"},{"title":"<span>#S1</span> Electromagnetism (9/27/62)","m4a":"audio/mp4/FLP_S1_01.mp4","oga":"audio/ogg/FLP_S1_01.ogg"},{"title":"<span>#S2</span> Differential calculus of vector fields (10/1/62)","m4a":"audio/mp4/FLP_S2_01.mp4","oga":"audio/ogg/FLP_S2_01.ogg"},{"title":"<span>#S3</span> Integral vector calculus (10/4/62)","m4a":"audio/mp4/FLP_S3_01.mp4","oga":"audio/ogg/FLP_S3_01.ogg"},{"title":"<span>#S4</span> Electrostatics (10/8/62)","m4a":"audio/mp4/FLP_S4_01.mp4","oga":"audio/ogg/FLP_S4_01.ogg"},{"title":"<span>#S5</span> Application of Gauss' law (10/11/62)","m4a":"audio/mp4/FLP_S5_01.mp4","oga":"audio/ogg/FLP_S5_01.ogg"},{"title":"<span>#S6</span> The electric field in various circumstances (10/15/62)","m4a":"audio/mp4/FLP_S6_01.mp4","oga":"audio/ogg/FLP_S6_01.ogg"},{"title":"<span>#S7</span> The electric field in various circumstances II (10/18/62)","m4a":"audio/mp4/FLP_S7_01.mp4","oga":"audio/ogg/FLP_S7_01.ogg"},{"title":"<span>#S8</span> Electrostatic energy (10/22/62)","m4a":"audio/mp4/FLP_S8_01.mp4","oga":"audio/ogg/FLP_S8_01.ogg"},{"title":"<span>#S9</span> Electricity in the atmosphere (10/25/62)","m4a":"audio/mp4/FLP_S9_01.mp4","oga":"audio/ogg/FLP_S9_01.ogg"},{"title":"<span>#S10</span> Dielectrics (11/1/62)","m4a":"audio/mp4/FLP_S10_01.mp4","oga":"audio/ogg/FLP_S10_01.ogg"},{"title":"<span>#S11</span> Inside dielectrics (11/5/62)","m4a":"audio/mp4/FLP_S11_01.mp4","oga":"audio/ogg/FLP_S11_01.ogg"},{"title":"<span>#S12</span> Electrostatic analogs (11/8/62)","m4a":"audio/mp4/FLP_S12_01.mp4","oga":"audio/ogg/FLP_S12_01.ogg"},{"title":"<span>#S13</span> Magnetostatics (11/12/62)","m4a":"audio/mp4/FLP_S13_01.mp4","oga":"audio/ogg/FLP_S13_01.ogg"},{"title":"<span>#S14</span> The magnetic field in various circumstances (11/15/62)","m4a":"audio/mp4/FLP_S14_01.mp4","oga":"audio/ogg/FLP_S14_01.ogg"},{"title":"<span>#S15</span> The vector potential (11/19/62)","m4a":"audio/mp4/FLP_S15_01.mp4","oga":"audio/ogg/FLP_S15_01.ogg"},{"title":"<span>#S16</span> Induction (11/26/62)","m4a":"audio/mp4/FLP_S16_01.mp4","oga":"audio/ogg/FLP_S16_01.ogg"},{"title":"<span>#S17</span> Induction II (11/29/62)","m4a":"audio/mp4/FLP_S17_01.mp4","oga":"audio/ogg/FLP_S17_01.ogg"},{"title":"<span>#S18</span> The Maxwell equations (12/3/62)","m4a":"audio/mp4/FLP_S18_01.mp4","oga":"audio/ogg/FLP_S18_01.ogg"},{"title":"<span>#S19</span> The principle of least action (12/6/62)","m4a":"audio/mp4/FLP_S19_01.mp4","oga":"audio/ogg/FLP_S19_01.ogg"},{"title":"<span>#S20</span> Solutions of Maxwell's equations (in free space) (1/3/63)","m4a":"audio/mp4/FLP_S20_01.mp4","oga":"audio/ogg/FLP_S20_01.ogg"},{"title":"<span>#S21</span> Solutions of Maxwell's equations (with currents and charges) (1/7/63)","m4a":"audio/mp4/FLP_S21_01.mp4","oga":"audio/ogg/FLP_S21_01.ogg"},{"title":"<span>#S22</span> A.C. circuits (1/10/63)","m4a":"audio/mp4/FLP_S22_01.mp4","oga":"audio/ogg/FLP_S22_01.ogg"},{"title":"<span>#S23</span> Cavity resonators (1/14/63)","m4a":"audio/mp4/FLP_S23_01.mp4","oga":"audio/ogg/FLP_S23_01.ogg"},{"title":"<span>#S24</span> Wave guides (1/17/63)","m4a":"audio/mp4/FLP_S24_01.mp4","oga":"audio/ogg/FLP_S24_01.ogg"},{"title":"<span>#S25</span> Electrodynamics in relativistic notation (1/21/63)","m4a":"audio/mp4/FLP_S25_01.mp4","oga":"audio/ogg/FLP_S25_01.ogg"},{"title":"<span>#S26</span> Lorentz transformation of the fields (1/24/63)","m4a":"audio/mp4/FLP_S26_01.mp4","oga":"audio/ogg/FLP_S26_01.ogg"},{"title":"<span>#S27</span> Field energy and field momentum (1/31/63)","m4a":"audio/mp4/FLP_S27_01.mp4","oga":"audio/ogg/FLP_S27_01.ogg"},{"title":"<span>#S28</span> Electromagnetic mass (2/4/63)","m4a":"audio/mp4/FLP_S28_01.mp4","oga":"audio/ogg/FLP_S28_01.ogg"},{"title":"<span>#S29</span> The motion of charges in electric and magnetic fields (2/7/63)","m4a":"audio/mp4/FLP_S29_01.mp4","oga":"audio/ogg/FLP_S29_01.ogg"},{"title":"<span>#S30</span> Crystals (2/10/63)","m4a":"audio/mp4/FLP_S30_01.mp4","oga":"audio/ogg/FLP_S30_01.ogg"},{"title":"<span>#S31</span> Tensors (2/14/63)","m4a":"audio/mp4/FLP_S31_01.mp4","oga":"audio/ogg/FLP_S31_01.ogg"},{"title":"<span>#S32</span> Refractive index of dense materials (2/18/63)","m4a":"audio/mp4/FLP_S32_01.mp4","oga":"audio/ogg/FLP_S32_01.ogg"},{"title":"<span>#S33</span> Reflection from surfaces (2/21/63)","m4a":"audio/mp4/FLP_S33_01.mp4","oga":"audio/ogg/FLP_S33_01.ogg"},{"title":"<span>#S34</span> Diamagnetism and Paramagnetism (2/25/63)","m4a":"audio/mp4/FLP_S34_01.mp4","oga":"audio/ogg/FLP_S34_01.ogg"},{"title":"<span>#S35</span> Magnetic resonance (2/28/63)","m4a":"audio/mp4/FLP_S35_01.mp4","oga":"audio/ogg/FLP_S35_01.ogg"},{"title":"<span>#S36</span> Ferromagnetism (3/4/63)","m4a":"audio/mp4/FLP_S36_01.mp4","oga":"audio/ogg/FLP_S36_01.ogg"},{"title":"<span>#S37</span> Magnetic materials (3/7/63)","m4a":"audio/mp4/FLP_S37_01.mp4","oga":"audio/ogg/FLP_S37_01.ogg"},{"title":"<span>#S38</span> Elasticity (3/28/63)","m4a":"audio/mp4/FLP_S38_01.mp4","oga":"audio/ogg/FLP_S38_01.ogg"},{"title":"<span>#S39</span> Elastic materials (4/1/63)","m4a":"audio/mp4/FLP_S39_01.mp4","oga":"audio/ogg/FLP_S39_01.ogg"},{"title":"<span>#S40</span> Flow of dry water (4/4/63)","m4a":"audio/mp4/FLP_S40_01.mp4","oga":"audio/ogg/FLP_S40_01.ogg"},{"title":"<span>#S41</span> Flow of wet water (4/8/63)","m4a":"audio/mp4/FLP_S41_01.mp4","oga":"audio/ogg/FLP_S41_01.ogg"},{"title":"<span>#S42</span> Probability amplitudes (4/11/63)","m4a":"audio/mp4/FLP_S42_01.mp4","oga":"audio/ogg/FLP_S42_01.ogg"},{"title":"<span>#S43</span> Identical particles (4/15/63)","m4a":"audio/mp4/FLP_S43_01.mp4","oga":"audio/ogg/FLP_S43_01.ogg"},{"title":"<span>#S44</span> Spin one (4/18/63)","m4a":"audio/mp4/FLP_S44_01.mp4","oga":"audio/ogg/FLP_S44_01.ogg"},{"title":"<span>#S45</span> Spin one-half (4/22/63)","m4a":"audio/mp4/FLP_S45_01.mp4","oga":"audio/ogg/FLP_S45_01.ogg"},{"title":"<span>#S46</span> The dependence of amplitudes on time (4/29/63)","m4a":"audio/mp4/FLP_S46_01.mp4","oga":"audio/ogg/FLP_S46_01.ogg"},{"title":"<span>#S47</span> The Hamiltonian matrix (5/2/63)","m4a":"audio/mp4/FLP_S47_01.mp4","oga":"audio/ogg/FLP_S47_01.ogg"},{"title":"<span>#S48</span> The ammonia maser (5/6/63)","m4a":"audio/mp4/FLP_S48_01.mp4","oga":"audio/ogg/FLP_S48_01.ogg"},{"title":"<span>#S49</span> Other two-state systems (5/9/63)","m4a":"audio/mp4/FLP_S49_01.mp4","oga":"audio/ogg/FLP_S49_01.ogg"},{"title":"<span>#S50</span> Two-state systems, cont. (5/13/63)","m4a":"audio/mp4/FLP_S50_01.mp4","oga":"audio/ogg/FLP_S50_01.ogg"},{"title":"<span>#S51</span> Hyperfine splitting in hydrogen (5/16/63)","m4a":"audio/mp4/FLP_S51_01.mp4","oga":"audio/ogg/FLP_S51_01.ogg"},{"title":"<span>#S52</span> Conservation of angular momentum (5/20/63)","m4a":"audio/mp4/FLP_S52_01.mp4","oga":"audio/ogg/FLP_S52_01.ogg"},{"title":"<span>#S53</span> Angular momentum continued (5/23/63)","m4a":"audio/mp4/FLP_S53_01.mp4","oga":"audio/ogg/FLP_S53_01.ogg"},{"title":"<span>#S53A</span> Special Lecture:Curved space (5/25/63)","m4a":"audio/mp4/FLP_S53A_01.mp4","oga":"audio/ogg/FLP_S53A_01.ogg"},{"title":"<span>#S54</span> Dependence of space amplitudes (5/27/63)","m4a":"audio/mp4/FLP_S54_01.mp4","oga":"audio/ogg/FLP_S54_01.ogg"},{"title":"<span>#S56</span> Propagation in a crystal lattice (5/7/64)","m4a":"audio/mp4/FLP_S56_01.mp4","oga":"audio/ogg/FLP_S56_01.ogg"},{"title":"<span>#S57</span> Semiconductors (5/11/64)","m4a":"audio/mp4/FLP_S57_01.mp4","oga":"audio/ogg/FLP_S57_01.ogg"},{"title":"<span>#S58</span> The independent particle approximation (5/14/64)","m4a":"audio/mp4/FLP_S58_01.mp4","oga":"audio/ogg/FLP_S58_01.ogg"},{"title":"<span>#S59</span> The dependence of amplitudes on positions in space (5/18/64)","m4a":"audio/mp4/FLP_S59_01.mp4","oga":"audio/ogg/FLP_S59_01.ogg"},{"title":"<span>#S60</span> Symmetry and conservation laws (5/21/64)","m4a":"audio/mp4/FLP_S60_01.mp4","oga":"audio/ogg/FLP_S60_01.ogg"},{"title":"<span>#S61</span> Angular momentum (5/25/64)","m4a":"audio/mp4/FLP_S61_01.mp4","oga":"audio/ogg/FLP_S61_01.ogg"},{"title":"<span>#S62</span> The hydrogen atom and the periodic table (5/28/64)","m4a":"audio/mp4/FLP_S62_01.mp4","oga":"audio/ogg/FLP_S62_01.ogg"},{"title":"<span>#S63</span> Operators (6/1/64)","m4a":"audio/mp4/FLP_S63_01.mp4","oga":"audio/ogg/FLP_S63_01.ogg"},{"title":"<span>#S64</span> The Schrödinger equation in a classical context: superconductivity (6/4/64)","m4a":"audio/mp4/FLP_S64_01.mp4","oga":"audio/ogg/FLP_S64_01.ogg"}]

When I run this on Ubuntu 20.04 in August of 2023, it runs and generates a bunch of files with all the right names. But every file contains an error message in HTML, basically saying "file not found" for each of the mp4 files.

Somewhere in my searches I saw one of the filenames and noticed that CalTech changed /audio/ to /protectedaudio/ and I was able to get the script to work again by changing one line to add 'proteted' to the name:
perform_curl "https://www.feynmanlectures.caltech.edu/protected${urls[i]}" $filename &

@KayakerMagic Thanks a lot! I have updated the script to include your suggestion