Can I hook into the cd command?

Question

I'm trying to set something up on a server I run, when ever I cd into a public_html folder 95% of the time there's a few commands I will always run to check certain things.

Is there anyway I can hook into cd so if the directory is a public_html, it will automatically run the commands for me?

If I can't hook into the cd command, are there any other things I could do to achieve the outcome I'm after?

I'm running CentOS 5.8.

Answer 1

With ksh or bash (or zsh):

cd() {
  builtin cd "$@" || return
  [ "$OLDPWD" = "$PWD" ] || case $PWD in
      (*/public_html) echo do something
    esac
}

With zsh:

chpwd()
  case $PWD in
    (*/public_html) echo do something
  esac

(chpwd is a hook function that is called whenever the current working directory changes (by way of cd, pushd, popd...)).

Answer 2

You could add this function to your .bashrc or other startup file (depending on your shell).

cd() {      
   if [ "$1" = "public_html" ]; then
      echo "current dir is my dir"
   fi
   builtin cd "$1"
}

Answer 3

Wrapping cd, an existing command, is not recommended.

A more universal solution would be defining a custom chpwd hook in Bash. (According to tags of this question, I assume you're using Bash)

There's not a complete hook system designed in Bash when compared with other modern shells. PROMPT_COMMAND variable is used as a hook function, which is equivalent to precmd hook in ZSH, fish_prompt in Fish. For the time being, ZSH is the only shell I've known that has a chpwd hook builtin.

PROMPT_COMMAND

If set, the value is interpreted as a command to execute before the printing of each primary prompt ($PS1).

https://www.gnu.org/savannah-checkouts/gnu/bash/manual/bash.html#Bash-Variables

chpwd Hook in Bash

A trick is provided to setup a chpwd equivalent hook in Bash based on PROMPT_COMMAND.

# create a PROPMT_COMMAND equivalent to store chpwd functions
typeset -g CHPWD_COMMAND=""

_chpwd_hook() {
  shopt -s nullglob

  local f

  # run commands in CHPWD_COMMAND variable on dir change
  if [[ "$PREVPWD" != "$PWD" ]]; then
    local IFS=$';'
    for f in $CHPWD_COMMAND; do
      "$f"
    done
    unset IFS
  fi
  # refresh last working dir record
  export PREVPWD="$PWD"
}

# add `;` after _chpwd_hook if PROMPT_COMMAND is not empty
PROMPT_COMMAND="_chpwd_hook${PROMPT_COMMAND:+;$PROMPT_COMMAND}"

Since we're detecting PWD change directly, the solution works with cd, pushd, and popd.

Note: The main difference between our chpwd implementaion in Bash and the chpwd in ZSH is, PROMPT_COMMAND is not supported in a non-interactive Bash shell.

Usage

_public_html_action() {
  if [[ $PWD == */public_html ]]; then
    # actions
  fi
}

# append the command into CHPWD_COMMAND
CHPWD_COMMAND="${CHPWD_COMMAND:+$CHPWD_COMMAND;}_public_html_action"

Source: Create chpwd Equivalent Hook in Bash from my gist.

For anyone want an answer for ZSH. Use chpwd hook in ZSH. Don't define chpwd() function directly. More detail here.

Answer 4

I am not a bash expert but I would take @UVV's answer and modify it just a little so that it does this:

• instead of checking for public_html, I'd just check for some hook-script file in the target dir $1, say, cd_hook.sh.
• if that hook-script exists, run it, and move on with the cd

This seems more generic because you'd then be able to apply the cd-hook to any directory if you so choose, by just adding a cd_hook.sh in the said directory.

Answer 5

Using powerful zsh approach in bash:

First an easy way to extend bash:

~/.runscripts

#load all scripts inside $1 dir

run_scripts()
{
    for script in $1/*; do

        # skip non-executable snippets
        [ -f "$script" ] && [ -x "$script" ] || continue

        # execute $script in the context of the current shell
        . $script
    done
}

Include in to .bashrc:

. ~/.run_scripts

run_scripts ~/.bashrc.d

You can create ~/.bashrc.d/cdhook with:

#!/bin/bash

chpwd() {
  : #no action
}

cd() {      
   builtin cd $1
   chpwd $1
}

Now is up to you to replace the function:

#list files after cd
chpwd() {
  ls -lah --color
}