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printf

The printf builtin is a way to output text to the terminal.

It's an alternative to echo.

Why printf?

This is one of those things that people debate over which one to use. I use printf almost exclusively now, as it is much more feature-rich and portable than echo.

On different shells you will see different versions of echo, but printf is usually static in its functionality across all shells.

Furthermore, it matches printf in other languages (e.g., C), so it's a much more portable knowledge set than using echo.

Formatting Strings to Variables

In Bash 3.1+, printf has the -v VAR option.

Note: This option is not POSIX-compliant. POSIX printf has no options at all, only the standard format [args...].

This allows you to save the output of a format string into a variable rather than printing it to stdout. 

printf -v VAR_NAME ...

This is useful for formatting variables and even cutting down on subshells. 

printf -v GREETING "Hello, %s\n" "$USER"

This saves the output of the string to the GREETING variable, which can then be used later.

This option can be very powerful when combined with formats like the date format:

printf -v current_date "%(%F %T)T"
printf "Current date: %s\n" "$current_date"

Date Formatting

The printf builtin supports a date format that does not require and external call to date.

It takes the same date formats as the date command would.

  • The format string for dates is: %(DATE_FORMAT)T 
    printf "%(DATE_FORMAT)T"
    • The DATE_FORMAT inside the parentheses can be any valid format accepted by the date command.

Here's an example: 

printf "%(%F %T)T\n"
# 2025-08-25 17:02:56

  • Note that the date format did not require an argument like other formats do (e.g., %s requires an argument).

This is extremely useful.

This can be great for cutting down on the amount of child processes you spawn simply to get the current date.

Combine this powerful tool with printf's -v VAR option to save it to a variable:

printf -v current_date "%(%F %T)T"
# As opposed to:
current_date="$(date '+%F %T')"  # Old way, requires a subshell

Date Format with Other Formats

When using the date format with other formats after it, you need to pass it an argument.

For example, this will break: 

# BAD:
printf "%(%F %T)T [INFO]: Timeout limit reached: %s\n" "$idle_time"

  • The date format does not require an argument. However, if there is one present, it will ingest it.

  • So the date format is ingesting the "$idle_time" argument, and leaves nothing for the %s format.

The way around this is to pass the date format an argument.

You can simply pass it -1: 

printf "%(%F %T)T [INFO]: Timeout limit reached: %s\n" -1 "$idle_time"

If the date format comes after all other formats, it does not require an argument.

printf "[INFO]: Timeout limit reached: %s -- %(%F %T)T\n" "$idle_time"
This will work just fine. The %s format gets its argument, and the date format does not try to ingest it.