Is there, in bash, a builtin command to get the absolute path of a relative file path?

Question

I can use the external command realpath to get the absolute path of a file:

realpath tmp/toto

returns

/home/john/tmp/toto

Can I use a bash built-in to get the same effect?

Answer 1

The bash shell does not have a built-in way to explicitly get an absolute path given a relative pathname.

In the zsh shell, one could do

pathname=../../home/kk/.zshenv
print -r -- $pathname:P

and get /home/kk/.zshenv back (note that it also resolves all symbolic links wherever possible is a similar way as realpath() would).

In bash, you could, for a pathname designating a non-directory and assuming you have search access to its parent directory, use

$ pathname=../../home/kk/.zshenv
$ ( OLDPWD=- CDPATH= cd -P -- "${pathname%/*}" && printf '%s/%s\n' "$PWD" "${pathname##*/}" )
/home/kk/.zshenv

That is, temporarily (in a sub-shell) cd to the directory holding the file, and then construct the absolute pathname using the value $PWD and the filename component of the pathname. The -P is needed to avoid cd's special treatment of .. components. It has the side effect of resolving symlinks components in the directory itself. If the file itself is a symlink however, it won't be resolved (that's a difference from zsh's :P modifier).

We set OLDPWD to - to work around the fact that cd -P -- - does a chdir($OLDPWD) instead of chdir("-") (that trick works in bash but not all other sh implementations).

For a directory path it's easier:

$ pathname=../../home/kk
$ ( OLDPWD=- CDPATH= cd -P -- "$pathname" && pwd )
/home/kk

This could obviously be put into a shell function for convenience:

myrealpath () (
    if [[ -d $1 ]]; then
        OLDPWD=- CDPATH= cd -P -- "$1" && pwd
    else
        OLDPWD=- CDPATH= cd -P -- "${1%/*}" && printf '%s/%s\n' "$PWD" "${1##*/}"
    fi
)

(Note that this whole function is defined in a sub-shell to avoid changing the working directory for the calling shell.)

Testing this function:

$ myrealpath ../../home/kk
/home/kk

$ myrealpath ../../home/kk/.zshenv
/home/kk/.zshenv

Depending on the OS, using the function with no argument will return the absolute path of the current directory or give an error. In future versions of bash, that's likely to be an error on every system, as cd '' to fail will become a POSIX requirement.

Answer 2

Bash comes with a realpath loadable extension. Since bash-4.4 this should be installed by default¹, so you should be able to do:

BASH_LOADABLES_PATH=${BASH_LOADABLES_PATH:-/usr/local/lib/bash:/usr/lib/bash}
enable -f realpath realpath
realpath ..
help realpath

Once enabled, each loadable command becomes a "real" builtin with no additional fork/exec overhead, and with the ability to manipulate the bash environment (where offered, e.g. the mypid loadable)..

The -f option specifies the path to the loadable extension binary, if it's not in the expected place or your distro didn't set it correctly, you will need to set the variable BASH_LOADABLES_PATH to the directory containing extensions (or, use -f with the full path to the loadable). If it's set correctly, you won't need the first line above.

When writing portable scripts that prefer, but do not require such extensions, I will use a wrapper function anyway — along the lines of @Kusalananda's — so that it can fall back to an external (such as readlink or realpath from coreutils).

---

1. This extension is available since bash-2.05, but prior to bash-4.4 extensions were not compiled and installed by default, though it is straightforward to do so if you have a working build environment (C compiler etc.). Distributions may or may not include the extensions, sadly RHEL 8 (and hence CentOS 8) build bash-4.4 with the loadable extensions deliberately removed.

Answer 3

I suggest the following, which have a high probability of working on non-Linux OSes (including AIX). To compute an absolute path (i.e. a path starting at /):

abspath() {
  [[ $1 = /* ]] && printf "%s\n" "$1" || printf "%s\n" "$PWD/$1"
}

For a canonical path (i.e. one with all symlinks expanded):

canonical_path() {
  perl -mCwd -le ' print Cwd::abs_path $ARGV[0] ' -- "$1"
}

When testing against other possible solutions (including a call to realpath), you'll notice a significant difference depending on whether or not the last path component exists, and whether or not the last path component is a symlink.

Answer 4

Some systems possess "realpath" command, e.g. Ubuntu. You can try realpath ~/foo.txt and it could give something like /home/user/foo.txt, then you can use basedir to cut off file name.