I was recently researching different bash prompts and stumbled upon an StackExchange answer which explains the different prompt types (
PS4). The answer explains how the PS3 prompt is used for commands like
select. I did not know about select before, so that’s the real TIL
[…] PS3 is shown when the select command is waiting for input […]
PS3 prompt is shown when a bash program is waiting for user input, and it turns out that the
select bash command is used for creating a “choice” UI
in bash. So what does that mean and how does it work?
Asking interactive questions in Bash
Let’s assume we have a shell script called
make-project which bootstraps some project stuff, for example configuration files. We could provide the user with an option to choose a format from a list of available configuration formats like
We can handle these options via shell arguments like
make-project -format xml, but we could also ask questions interactively using
The script looks like this.
#!/bin/bash # Default format format="xml" # Ask a question echo "Which configuration format do you prefer?" # Create the "UI" with a choice option select f in xml json; do format=$f # Break to end the select after something was selected break; done echo "Format: $format"
select function takes a variable name (
f above) and a list of possible values (
xml json) which is seperated by spaces.
select myvariable in value1 value2 value3; do # $myvariable will be value1, value2, or value3 selectedValue = $myvariable break; done
When our little
make-project script is run it would looks like this (in Bash 4.4 on Mac OS):
$ ./make-project.sh Which configuration format do you prefer? 1) xml 2) json #? _
The last line,
#?, is the
PS3 prompt and
_ is the cursor position. The user can now enter a number (1 for
xml or 2 for
json) and the selection would be captured and assigned to the
format variable in our script. If we add
export PS3="Your answer: " to the script it will replace the default PS3 prompt for your script as shown below.
$ ./make-project.sh Which configuration format do you prefer? 1) xml 2) json Your answer: _
select is a clean and simple way to create a interactive UI! I tried the code with the
fish shell and it looks the same.