The following behaves the same in Fedora and CentOS.
cd mydirectoryworks.
cd notadirectoryworks (gets an error from Bash).
However,
cd `gives me some sort of prompt:
>Nothing I've thought to enter gives any response, it merely heads to the next line to prompt again. I simply use CTRL+C to exit back to my root@localhost.
What is the purpose of this and what can one do with it?
Thanks!
24 Answers
What you've typed is a backtick - it is the start of an instruction to bash to evaluate what you type as a command. The > is displayed to indicate you are still entering the command on the next line.
If you close the backtick you'll find the whole command will run. E.g.
~$ cd `
> echo /var`
/var$JohnC's answer already explains the backtick. But what you are also wondering about is the > prompt. This a continuation prompt, and it is not only triggered by a backtick, but always when your shell clearly knows you're not done entering a command. The easiest example is putting an explicit line continuation \ at the end of an input line (which helps splitting long input):
$ echo \
> halloNote that just like PS1 controls the command prompt's look, you can also set PS2 to change the continuation prompt, e.g.
$ export PS2="(cont.) "
$ echo \
(cont.) halloThere are many reasons for the continuation to occur. A single backtick is incomplete, but you could also enter something like
ls -l `which cp`in a single line (side-note: It's recommended to use $( and ) instead, since the parentheses make it obvious where the expansion starts and ends, while single backticks make it more difficult to see where one's missing. And nesting...). Other possible reasons for a continuation prompt:
- a missing
doneafterwhileorfor - a missing
fiafter anif - a missing
esacaftercase - a missing closing parenthesis, e.g. in subshells
(cd $HOME; cat .bashrc) - a missing command after piping
|as well as conditional execution||and&&(not&though, since that's just making the command running in background) - a missing closing quote (
'or")
Curiously enough, a missing brace } after a variable expansion ${ also causes a continuation prompt, but will fail due to the inserted space:
$ echo ${
> PS2}
bash: ${
PS2}: bad substitutionIt means that your command is not complete yet. In fact, the character backtick, `, is used to delimit an inline command.
Example:
cd /tmp # Go to /tmp
pwd # Prints the current working directory
ls `pwd` # Lists the content of the current working directoryThe backtick starts/ends command substitution. The shell reads the command inside the backticks, interprets the results, and places those results into the command. E.g:. ls which cp results in the "which cp" being executed and the result of that command, i.e, /bin/cp will be used as the argument to the ls command. This behavior is, however, deprecated, in favor of the more explicit and readable $(cmd) command substitution.
When typing cd ` The shell is expecting a command to be executed to follow, and, to be close with a backtick.
In the case of the variable expansion using braces the shell presents the continuation prompt because those braces are lexically evaluated before the expansion is done.
