I cannot use cd anymore when using it in a bash script

[~/Downloads] # cat cd-backward
#!/bin/bash
cd ..
[~/Downloads] # ./cd-backward
[~/Downloads] #

I should move to ~ at the last line.

cd works perfectly in terminal strangely. Nothing happens also when I run bash -c 'cd Download for instance.

1

3 Answers

Every script is executed in its own subshell, that is a separate process that cannot modify its parent working directory. The only way to change a working directory using a script written to a file is sourcing it using . or source (they are equivalent) like this:

$ . cd-backward

or

$ source cd-backward

Note that in such case you don't even need shebang (#!/bin/bash) at the top of your script.

When you launch a script, it runs in its own shell, as Arkadiusz already mentioned. In this case, you have instance of bash. You can see it if you modify the script:

#!/bin/bash
cd ..
pwd

Sample run on my system gives :

bash-4.3$ pwd
/home/xieerqi/Downloads
bash-4.3$ ./cd-backward
/home/xieerqi
bash-4.3$ pwd
/home/xieerqi/Downloads

Appropriately enough, within the script subshell, you navigate to home directory. Parent shell's current working directory however remains unchanged

Your process (called $$) has a "Current directory", ~/Downloads.

When you ./cd-backward, that Bash script is run in its own process, which has its own "Current Directory", which starts off as ~/Downloads.

The cd .. affects the "Current Directory" of the ./cd-backward process, changing it to ~, but NOT affecting the "Current Directory" of your original process.

./cd-backward finishes, its process exits, and the cd .. is forgotten about.

Here are two ways I affect my own "Current Directory", kept in my '~/.bashrc`:

First, a couple of aliases:

alias ..='cd ..'
alias ...='cd .. ; cd ..'

Here's how I keep my current directory in my Window title (through intercepting cd):

# from the "xttitle(1)" man page - put info in window title
update_title()
{ [[ $TERM = xterm ]] || [[ $TERM = xterm-color ]] && xttitle "[$$] ${USER}@${HOSTNAME}:$PWD"
}
cd()
{ [[ -z "$*" ]] && builtin cd $HOME [[ -n "$*" ]] && builtin cd "$*" update_title
}