"Up" script for moving up directories quickly

Question

A long time ago I created a script for moving up directories very quickly in the command line using the command up. You can find usage notes here.

It's a very simple script with just 8 lines of source code, as follows:

if [ -z "$1" ]; then
    cd ..
else
    for i in `seq 1 $1`;
    do
        cd ..
    done
fi

I've never personally had any problems with it since I made it and started using it myself — but are there any accidental or malicious inputs (particularly with the blind injection of $1) that might cause this to do something bad that I'm not aware of?

Given that the up command isn't used for anything in any command line I'm aware of, I'd like to promote more widespread usage of this script file so that people can type up instead of cd .. all the time, saving three keystrokes (or more if they want to move up more directories) for a very common operation.

Answer 1

Your recommendation is to define alias up=". path/to/up" so that when you type up 3, it expands to . up 3. However, since you want to take an optional argument and affect the state of the current shell, I think you would be better off defining a shell function instead.

As it turns out, the [ -z "$1" ] special case is not necessary, since seq 1 just expands to 1.

You end up executing n separate cd .. commands for up n. This leads to a usability bug: cd - or cd $OLDPWD, which normally take you back to the previous directory, don't work the way I expect.

Suggested solution:

up() {
    cd $(for i in $(seq 1 $1) ; do echo -n ../ ; done)
}

Answer 2

This is a decent concept, but by looping on the cd you loose some of the value of the $OLDPWD function in the shell. For example, I often use the special construct cd - in a shell, and that changes directory to the one you were in before.

Your code will make that impossible.

I would instead recommend that you instead build up a chain of ../ string values, like ../../../../ for 4 directories, and then just call cd once, which will preserve the cd - function, and the $OLDPWD.

Additionally, this would be a good feature to include as a function in your code, rather than a script. Bash shell likes functions, and they make life easier.

Finally, if someone supplies a non-number as an argument, it will do odd things.

I played with your code, and came up with:

up () {
    local count=$1
    if [ -z "$count" ]; then
        cd ..
        return
    fi

    test "$count" -eq "$count" || return 1

    local todir=""
    for i in `seq 1 $count`;
    do
      todir="../$todir"
    done
    cd $todir
}

The features of the above code I like are:

• it is a function of the shell, so there's no additional script called.
• it checks the value is a number, by doing a numeric comparison on the value: test "$count" -eq "$count" (That will throw an error if the inputs are not integers)
• it only does a single 'cd', so things like cd - still work.

I would add that to my ~/.bashrc file, or source it in to my current shell.

Answer 3

200_success's answer uses the evil eval which in his context was unnecessary.

Update: Oh, what, bash? Let's use some brace expansion + evil things:

# This time we have to be evil. All code in this answer is CC0.
up() { [ "$1" -eq "$1" ] &>/dev/null || set -- 1; "cd \$(printf '../%.s' {1..$1})"; }

---

local is not a that nice solution since POSIX doesn't contain local. After some searching, I found out that it's possible to use printf to duplicate strings. After replacing the brace expansion with seq, here is what I got:

up() { cd "$(printf '../%.s' $(seq 1 $1))"; }

P.S.: The use of seq for simple looping is sometimes considered harmful, especially when used with for var in. To make you feel less guilty:

# integer seq, @copyright CC0.
iseq() (
    : ${iseq_fmt='%s\n'}
    case "$#" in
        (1)  i=1  last=$1 incr=1;;
        (2)  i=$1 last=$2 incr=1;;
        (3)  i=$1 last=$3 incr=$2;;
        (*)  return 2;;
    esac
    [ "$i" -eq "$i" ] && [ "$incr" -eq "$incr" ] && [ "$last" -eq "$last" ] || return 2
    while [ "$i" -le "$last" ]; do
        printf "$iseq_fmt" "$i"
        : $((i = i + incr))
    done
)

Answer 4

You probably want to forward any options passed to cd, particularly -L, -P, -e and -@. Luckily, none of those take any arguments, so it's straightforward to catch them:

up() {
    local options=()
    for i in "$@"
    do
        case "$i" in
          -*) options+=("$i") ;;
          [1-9]*)
             test "$i" -eq "$i" &&
             cd "${options[@]}" $(perl -e "print'../'x$i;")
             return ;;
        esac
    done

    # If we got here, there were no non-option arguments
    cd "${options[@]}" ..
}

I might name it .. rather than up.