Fish Shell:
Comparing two dates

How to:

With Fish Shell, we can compare two dates using the date command. Below are examples.

# Getting the current date in seconds since epoch
set current_date (date +%s)

# Converting a specific date to seconds since epoch
set specific_date (date -d "2023-04-01" +%s)

# Comparing the dates
if test $specific_date -lt $current_date
    echo "Specific date is earlier than the current date."
else if test $specific_date -eq $current_date
    echo "Dates are the same."
else
    echo "Specific date is later than the current date."
end

Sample output if the current date is past April 1st, 2023:

Specific date is earlier than the current date.

Deep Dive

Historically, comparing dates in programming has been a bit of a hassle due to various date formats and time zones. Fish Shell simplifies this task with its built-in date function, converting dates to seconds since the Unix epoch (January 1, 1970). This gives us a universal point in time to compare against.

Alternatives to Fish Shell for comparing dates include scripting languages like Python or using date manipulation tools available in Unix-based systems, like dateutil in GNU core utilities (coreutils). Implementation-wise, when we use date +%s, Fish internally calls the system date command, which is why it’s so effective cross-platform.

Comparing dates is also essential for cron jobs, backup scripts, and time-based access control. Being comfy with date comparisons means smoother automation and fewer temporal bugs.

See Also

• Fish Shell Documentation
• GNU Coreutils: Date
• The Unix Epoch Time