Diff for "BashFAQ/028"

How do I determine the location of my script? I want to read some config files from the same place.

There are two prime reasons why this issue comes up: either you want to externalize data or configuration of your script and need a way to find these external resources, or your script is intended to act upon a bundle of some sort (eg. a build script), and needs to find the resources to act upon.

It is important to realize that in the general case, this problem has no solution. Any approach you might have heard of, and any approach that will be detailed below, has flaws and will only work in specific cases. First and foremost, try to avoid the problem entirely by not depending on the location of your script!

Before we dive into solutions, let's clear up some misunderstandings. It is important to understand that:

• Your script does not actually have a location! Wherever the bytes end up coming from, there is no "one canonical path" for it. Never.

• $0 is NOT the answer to your problem. If you think it is, you can either stop reading and write more bugs, or you can accept this and read on.

Accessing data/config files

Very often, people want to make their scripts configurable. The separation principle teaches us that it's a good idea to keep configuration and code separate. The problem then ends up being: how does my script know where to find the user's configuration file for it?

Too often, people believe the configuration of a script should reside in the same directory where they put their script. This is the root of the problem.

A UNIX paradigm exists to solve this problem for you: configuration artifacts of your scripts should exist in either the user's HOME directory or /etc. That gives your script an absolute path to look for the file, solving your problem instantly: you no longer depend on the "location" of your script:

    if [[ -e ~/.myscript.conf ]]; then source ~/.myscript.conf
    elif [[ -e /etc/myscript.conf ]]; then source /etc/myscript.conf
    fi

Acting on a bundle

Sometimes scripts are part of a "bundle" and perform certain actions within or upon it. The user may unpack the bundle anywhere; ideally, the bundle's scripts should work whether that's somewhere in a home dir, or /var/tmp, or /usr/local. The files are transient, and have no fixed or predictable location.

When a script needs to act upon other files it's bundled with, independently of its absolute location, we have two options: either we rely on PWD or we rely on BASH_SOURCE. Both approaches have certain issues; here's what you need to know.

BASH_SOURCE

The BASH_SOURCE internal bash variable is actually an array of pathnames. If you expand it as a simple string, e.g. "$BASH_SOURCE", you'll get the first element, which is the pathname of the currently executing function or script. Using the BASH_SOURCE method, you access files within your bundle like this:

    cd "${BASH_SOURCE%/*}" || exit                             # cd into the bundle and use relative paths
    read somevar < etc/somefile

    read somevar < "${BASH_SOURCE%/*}/etc/somefile"            # If you want an absolute pathname

Please note that when using BASH_SOURCE, the following caveats apply:

• $BASH_SOURCE expands empty when bash does not know where the executing code comes from. Usually, this means the code is coming from standard input (e.g. ssh host 'somecode', or from an interactive session).

• $BASH_SOURCE does not follow symlinks (when you run z from /x/y, you get /x/y/z, even if that is a symlink to /p/q/r). Often, this is the desired effect. Sometimes, though, it's not. Imagine your package links its start-up script into /usr/local/bin. Now that script's BASH_SOURCE will lead you into /usr/local and not into the package.

If you're not writing a bash script, the BASH_SOURCE variable is unavailable to you. There is a common convention, however, for passing the location of the script as the process name when it is started. Most shells do this, but not all shells do so reliably, and not all of them attempt to resolve a relative path to an absolute path. Relying on this behaviour is dangerous and fragile, but can be done by looking at $0. Again, consider all your options before doing this: you are likely creating more problems than you are solving.

PWD

Another option is to rely on PWD, the current working directory. In this case, you can assume the user has first cd'ed into your bundle and make all your pathnames relative. Using the PWD method, you access files within your bundle like this:

    read somevar < etc/somefile                 # Using pathname relative to PWD
    read somevar < "$PWD/etc/somefile"          # Expand PWD if you want an absolute pathname
    bundleDir=$PWD                              # Store PWD if you expect to cd in your script.
    read somefile < "$bundleDir/etc/somefile"

To reduce fragility, you could even test whether, for example, the relative path to the script name is correct, to make sure the user has indeed cd'ed into the bundle:

    [[ -e bin/myscript ]] || { echo >&2 "Please cd into the bundle before running this script."; exit 1; }

You can also try some heuristics, just in case the user is sitting one directory above the bundle:

    if [[ ! -e bin/myscript ]]; then
        if [[ -d mybundle-1.2.5 ]]; then
            cd mybundle-1.2.5 || { echo >&2 "Bundle directory exists but I can't cd there."; exit 1; }
        else
            echo >&2 "Please cd into the bundle before running this script."; exit 1;
        fi
    fi

If you ever do need an absolute path, you can always get one by prefixing the relative path with $PWD: echo "Saved to: $PWD/result.csv"

The only difficulty here is that you're forcing your user to change into your bundle's directory before your script can function. Regardless, this may well be your best option!

Config

If neither the BASH_SOURCE or the PWD option sound interesting, you might want to consider going the route of configuration files instead (see the previous section). In this case, you require that your user set the location of your bundle in a configuration file, and have him put that configuration file in a location you can easily find. For example:

    [[ -e ~/.myscript.conf ]] || { echo >&2 "First configure the product in ~/.myscript.conf"; exit 1; }
    source ~/.myscript.conf      # ~/.myscript.conf defines something like bundleDir=/x/y
    [[ ! $bundleDir ]]        || { echo >&2 "Please define bundleDir='/some/path' in ~/.myscript.conf"; exit 1; }
    cd "$bundleDir"           || { echo >&2 "Could not cd to <$bundleDir>"; exit 1; }

    # Now you can use the PWD method: use relative paths.

Why you can't just use $0

It's not possible to find the location of a script reliably in all cases. Common ways of finding a script's location depend on the name of the script, as seen in the predefined variable $0. But providing the script name in $0 is only a (very common) convention, not a requirement.

The suspect answer is "in some shells, $0 is always an absolute path, even if you invoke the script using a relative path, or no path at all". But this isn't reliable across shells; some of them (including BASH) return the actual command typed in by the user instead of the fully qualified path. And this is just the tip of the iceberg!

Your script may not actually be on a locally accessible disk at all. Consider this:

  ssh remotehost bash < ./myscript

The shell running on remotehost is getting its commands from a pipe. There's no script anywhere on any disk that bash can see.

Moreover, even if your script is stored on a local disk and executed, it could move. Someone could mv the script to another location in between the time you type the command and the time your script checks $0. Or someone could have unlinked the script during that same time window, so that it doesn't actually have a link within a file system any more.

(That may sound fanciful, but it's actually very common. Consider a script installed in /opt/foobar/bin, which is running at the time someone upgrades foobar to a new version. They may delete the entire /opt/foobar/ hierarchy, or they may move the /opt/foobar/bin/foobar script to a temporary name before putting a new version in place. For these reasons, even approaches like "use lsof to find the file which the shell is using as standard input" will still fail.)

Even in the cases where the script is in a fixed location on a local disk, the $0 approach still has some major drawbacks. The most important is that the script name (as seen in $0) may not be relative to the current working directory, but relative to a directory from the program search path $PATH (this is often seen with KornShell). Or (and this is most likely problem by far...) there might be multiple links to the script from multiple locations, one of them being a simple symlink from a common PATH directory like /usr/local/bin, which is how it's being invoked. Your script might be in /opt/foobar/bin/script but the naive approach of reading $0 won't tell you that -- it may say /usr/local/bin/script instead.

Some people will try to work around the symlink issue with readlink -f "$0". Again, this may work in some cases, but it's not bulletproof. Nothing that reads $0 will ever be bulletproof, because $0 itself is unreliable. Furthermore, readlink is nonstandard, and won't be available on all platforms.

For a more general discussion of the Unix file system and how symbolic links affect your ability to know where you are at any given moment, see this Plan 9 paper.

CategoryShell