Bash – PS1=’$(pwd)’ why this works and why is this different from PS1=$(pwd)

bashpromptpwdzsh

Why when I enter this command the prompt changes to my directory?

PS1='$(pwd)'

I am using single quotes, which means no interpolation , a.k.a echo '$(pwd)' ——→ $(pwd)

Furthermore, say that we clarified why this works… why is it functioning differently from PS1=$(pwd) ? (no quotes at all)

By different I mean that if I use the quotes then the prompt will keep changing to my current directory as I navigate through the terminal. But if I don't use quotes, then the prompt will always remain the directory that I was in when I first entered the command PS1=$(pwd)

why?

Best Answer

When you simply assign a value to a variable, the $(...) expression is evaluated unless it is enclosed in single quotes (or backslash-escaped). To understand, try and compare these two:

A=$(pwd)
echo "$A"
B='$(pwd)'
echo "$B"

The value of A immediately becomes the string /home/yourusername and obviously it's not remembered where this string comes from, it stays the same even if you change directory. The value of B, however, becomes the literal string $(pwd) without getting interpreted.

Now, in the value of PS1 something special happens: whenever the prompt is printed, certain special constructs are interpreted, e.g. the command substitution $(...) is performed exactly the way it happened above at the assignment to the A variable. Obviously if your PS1 contains the literal string of your home directory (as above with A) then there's no way it could change. But if it contains the string $(pwd) (as above with B) then it is evaluated whenever the prompt is printed and hence your actual directory is displayed.

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Bash Command – Printing and Saving the Last Bash Input Command

You can access the just-executed command line with the history built-in. (I have no idea why history 1 prints the just-executed command line but fc -nl -1 prints the previous commmand, as does fc -nl 0.)

PROMPT_COMMAND='echo -en "\033[38;5;2m"; history 1; echo -en "\033[0m\n"'

This prints a number before the command text. Here's a version that removes the number. (It may be incorrect if you go beyond 99999 history lines, I don't know how bash formats the number then.)

prompt_function () {
  local prompt_history="$(history 1)"
  prompt_history=${prompt_history:7}
  echo -En $'\033[38;5;2m'"$prompt_history"$'\033[0m\n'
}
PROMPT_COMMAND=prompt_function

(Note that echo -en ..."$prompt_history"... would expand backslashes in the command line, so I use echo -E and let the shell expansion generate the control characters with $''.).

Shell – echo literal question mark in PS1

In Bourne shell, not quoting a variable (i.e. $var instead of "$var") should be the exception. The problem here is your echo command. With echo ?, ? is substituted with every one-letter file in your directory, whereas with echo "?", the question mark is displayed as is.

Your function should then be:

parseGitBranch () {
     gitBranch=staging
     ifUntrackedFiles="$gitBranch ?"
     echo "$ifUntrackedFiles"
 }

And, since back-quotes are easy to confuse with single quotes and difficult to nest, use $(...) instead:

 PS1='$(parseGitBranch) $ '

Best Answer

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Bash Command – Printing and Saving the Last Bash Input Command

Shell – echo literal question mark in PS1

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