Bash vs. Dash - a simple speed test

Somtimes when you sit down to write a shell script, efficiency is not at the front of your mind. As long as it does what it needs to do in a reasonable amount of time, there's usually no problem. This is especially true of personal scripts, that may not be used more than a couple of times. However, it is a good practice to try and make scripts as efficient as reasonably possible.

A simple way to improve the efficiency of a script is to use Dash instead of Bash. Dash is quicker than Bash, we will go through a basic example that shows that.

The script we want to test

We'll go through a program that tests two scripts, with the only difference being the path to the shell on the shebang line - #!/bin/bash or #!/bin/dash.

The scripts file names begin the same, but have -bash or -dash depending on the shebang in the file.

• bibtextcreator-bash.sh
• bibtextcreator-dash.sh

The script does basic argument handling, then uses pdftotext to save a neatly formatted string of the input PDF's first pages. Finally it echoes the string to /dev/null.

Searching through a PDF as if it were a plaintext file is a typical reason one might write a shell script; for example you can use pdftotext to grep for an ISBN or DOI and programatically do citations.

Note that I'm using Ubuntu 20.04, and the the paths to these shells may be different on your system.

Again, the only difference is shebang line #!/bin/bash or #!/bin/dash.

Now the actual script, without shebang line.

set -eu 

# --------------------------------------------------------------------------------------------------------------------------------------------------

if [ $# -eq 0 ]; then
    echo "The first argument should be a valid PDF"
    echo "PDFs in current dir:"
    ls -1 *.pdf
    exit 1
fi

if [ ! -f $1 ]; then
    echo "$1 does not exist"
    exit 1
fi
# --------------------------------------------------------------------------------------------------------------------------------------------------

# Save formatted text output of input PDF, using pdftotext.
#   Using "-" specifies output as std out. 
IDENTIFIER_SEARCH_AREA="$(pdftotext -layout -l 25 $1 -)"

# For the speedtest, just print to /dev/null.
echo "${IDENTIFIER_SEARCH_AREA}" >> /dev/null

# --------------------------------------------------------------------------------------------------------------------------------------------------

The runner script

Now that we have a script ready to compare the efficiency Bash to Dash, let's think about how we want to go about devising a script that acts as a runner.

We want to end up with a script where you can just run something like shell_speedtest bash and shell_speedtest dash

We could just run a simple Bash one liner to see the difference.

• time (for i in $(seq 1 100); do ./bibtextcreator-bash.sh testarticle.pdf; done)
• time (for i in $(seq 1 100); do ./bibtextcreator-dash.sh testarticle.pdf; done)

But we want an environment that can easily be changed to fit our needs.

#!/bin/dash

set -eu

RUNS="1000"

shell_speedtest () {
    test_shell="$1"
    script_base_fname="bibtextcreator"
    script_fname="${script_base_fname}-${1}.sh" # i.e bibtextcreator-bash.sh or bibtextcreator-dash.sh
    script_shebang="$(head -n 1 ${script_fname})"
    
    printf "shell:${test_shell}\t${script_fname}\t${script_shebang}\t"
    
    # Run script x amount of times in loop.
    start_time="$(date -u +%s%N)"
    for i in $(seq 1 ${RUNS}); do  "./${script_fname}" testarticle.pdf; done
    end_time="$(date -u +%s%N)"
    
    time_delta_nanoseconds="$(expr ${end_time} - ${start_time})"
    time_delta_seconds="$(echo "${time_delta_nanoseconds} / (10^9)" | bc -l)"

    printf "%.3f seconds\t" ${time_delta_seconds}
    printf "(${time_delta_nanoseconds} nanoseconds)\n"
}

echo "testing shells with ${RUNS} runs"
shell_speedtest bash
shell_speedtest dash

Conclusion

The results show that Dash is not just trivially faster, it was consistently ~10% faster than Bash.

Even with changing up the order of the shells, (meaning shell_speedtest dash gets executed first), dash is faster.