How to print current bash prompt?

Question

The question is simple. I want to evaluate current value of PS1 in my bash script.

All materials on google point to tutorials on pimping it up, but I want to evaluate to see how would it be rendered by my current terminal, or at least by some terminal.

Is there any software/function that would help me achieve that? Of course I'd like to have all escaped characters evaluated, so echo $PS1 is not that useful in my case.

Answer 1

Bash 4.4+ solution using parameter transformation for a prompt string: echo "${PS1@P}"

[adamhotep@tabasco ~]$ echo "the prompt is '${PS1@P}'"
the prompt is '[adamhotep@tabasco ~]$'
[adamhotep@tabasco ~]$ TEST_STRING='\u is dining at \t using \s \V'
[adamhotep@tabasco ~]$ echo "${TEST_STRING}"
\u is dining at \t using \s \V
[adamhotep@tabasco ~]$ echo "${TEST_STRING@P}"
adamhotep is dining at 21:45:10 using bash 5.0.3
[adamhotep@tabasco ~]$ 

From the Bash Reference Manual page on Shell Parameter Expansion:

${parameter@operator}

Parameter transformation. The expansion is either a transformation of the value of parameter or information about parameter itself, depending on the value of operator.
Each operator is a single letter:

Q  The expansion is a string that is the value of parameter quoted in a
   format that can be reused as input.
E  The expansion is a string that is the value of parameter with backslash
   escape sequences expanded as with the $'…' quoting mechanism.
P  The expansion is a string that is the result of expanding the value of
   parameter as if it were a prompt string (see PROMPTING below).
A  The expansion is a string in the form of an assignment statement or
   declare command that, if evaluated, will recreate parameter with its
   attributes and value.
a  The expansion is a string consisting of flag values representing
   parameter's attributes.

If parameter is @ or *, the operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with @ or *, the operation is applied to each member of the array in turn, and the expansion is the resultant list.

(See also this answer from duplicate question Echo expanded PS1.)

 

Z Shell (zsh) can do this with ${(%%)PS1} or with its print builtin's -P flag:

[adamhotep@tabasco ~]% echo "the prompt is '${(%%)PS1}'"
the prompt is '[adamhotep@tabasco ~]%'
[adamhotep@tabasco ~]% print -P "the prompt is '$PS1'"
the prompt is '[adamhotep@tabasco ~]%'
[adamhotep@tabasco ~]% TEST_STRING="%n is dining at %* using %N $ZSH_VERSION"
[adamhotep@tabasco ~]% echo "$TEST_STRING"
%n is dining at %* using %N 5.7.1
[adamhotep@tabasco ~]% echo "${(%%)TEST_STRING}"
adamhotep is dining at 11:49:01 using zsh 5.7.1
[adamhotep@tabasco ~]% print -P "$TEST_STRING"
adamhotep is dining at 11:49:07 using zsh 5.7.1
[adamhotep@tabasco ~]% 

The Zsh Expansion and Subsitution manual tells us:

Parameter Expansion Flags. If the opening brace is directly followed by an opening parenthesis, the string up to the matching closing parenthesis will be taken as a list of flags. In cases where repeating a flag is meaningful, the repetitions need not be consecutive; for example, (q%q%q) means the same thing as the more readable (%%qqq). The following flags are supported:
…

%    Expand all % escapes in the resulting words in the same way as in prompts (see Prompt Expansion). If this flag is given twice, full prompt expansion is done on the resulting words, depending on the setting of the PROMPT_PERCENT, PROMPT_SUBST and PROMPT_BANG options.

From the Zsh Builtins documentation for print:

-P    Perform prompt expansion (see Prompt Expansion). In combination with -f, prompt escape sequences are parsed only within interpolated arguments, not within the format string.

Answer 2

One more possibility, using script utility (part of bsdutils package on ubuntu):

$ TEST_PS1="\e[31;1m\u@\h:\n\e[0;1m\$ \e[0m"
$ RANDOM_STRING=some_random_string_here_that_is_not_part_of_PS1
$ script /dev/null <<-EOF | awk 'NR==2' RS=$RANDOM_STRING
PS1="$TEST_PS1"; HISTFILE=/dev/null
echo -n $RANDOM_STRING
echo -n $RANDOM_STRING
exit
EOF
<prints the formatted prompt properly here>

script command generates a file specified & the output is also shown on stdout. If filename is omitted, it generates a file called typescript.

Since we are not interested in the log file in this case, filename is specified as /dev/null. Instead the stdout of the script command is passed to awk for further processing.

1. The entire code can also be encapsulated into a function.
2. Also, the output prompt can also be assigned to a variable.
3. This approach also supports parsing of PROMPT_COMMAND...

EDIT:
It appears that the new version of script echoes the piped stdin in the typescript. To handle that, the above mechanism can be changed to:

$ TEST_PS1="\e[31;1m\u@\h:\n\e[0;1m\$ \e[0m"
$ RANDOM_STRING=some_random_string_here_that_is_not_part_of_PS1
$ script /dev/null <<-EOF | awk '{old=current; current=$0;} END{print old}' RS=$RANDOM_STRING
PS1="$TEST_PS1"; HISTFILE=/dev/null
alias $RANDOM_STRING=true
$RANDOM_STRING
$RANDOM_STRING
EOF

<prints the formatted prompt properly here>

Explanation:

Try entering these commands manually on the terminal. Copy these commands under the heredoc as they are and paste with mouse middle click. The script command's stdout would contain something very similar.

e.g. With above case, the output of the script command gives this:

PS1="\e[31;1m\u@\h:\n\e[0;1m$ \e[0m"; HISTFILE=/dev/null
alias some_random_string_here_that_is_not_part_of_PS1=true
some_random_string_here_that_is_not_part_of_PS1
some_random_string_here_that_is_not_part_of_PS1
 \e[0m"; HISTFILE=/dev/nullhsane-dev : ~/Desktop $ PS1="\e[31;1m\u@\h:\n\e[0;1m$ 
anishsane@anishsane-dev:
$ alias some_random_string_here_that_is_not_part_of_PS1=true
anishsane@anishsane-dev:
$ some_random_string_here_that_is_not_part_of_PS1
anishsane@anishsane-dev:
$ some_random_string_here_that_is_not_part_of_PS1
anishsane@anishsane-dev:
$ exit

Split that stdout with "some_random_string_here_that_is_not_part_of_PS1" as delimiter (record separator of awk) and print the last but one record.

EDIT2:

Another mechanism (using bash source code and gdb):

$ gdb -batch -p $$ -ex 'call bind_variable("expanded_PS1", decode_prompt_string (get_string_value ("PS1")), 0)'
$ echo "$expanded_PS1"
<prints the formatted prompt properly here>

1. There is a tiny issue here though. The \[ or \] strings in PS1 will get printed as \1/\2 respectively. You can remove those with tr -d '\1\2' <<< "$expanded_PS1"
2. If you get error like gdb failed to attach to the process (seems to happen in ubuntu :-\ ), run gdb with sudo.