Relative paths based on file location instead of current working directory
BashShellBash Problem Overview
Given:
some.txt
dir
|-cat.sh
With cat.sh having the content:
cat ../some.txt
Then running ./cat.sh
inside dir
works fine while running ./dir/cat.sh
on the same level as dir
does not. I expect this to be due to the different working directories. Is there an easy way to make the path ../some.txt
relative to the location of cat.sh
?
Bash Solutions
Solution 1 - Bash
What you want to do is get the absolute path of the script (available via ${BASH_SOURCE[0]}
) and then use this to get the parent directory and cd
to it at the beginning of the script.
#!/bin/bash
parent_path=$( cd "$(dirname "${BASH_SOURCE[0]}")" ; pwd -P )
cd "$parent_path"
cat ../some.text
This will make your shell script work independent of where you invoke it from. Each time you run it, it will be as if you were running ./cat.sh
inside dir
.
Note that this script only works if you're invoking the script directly (i.e. not via a symlink), otherwise the finding the current location of the script gets a little more tricky)
Solution 2 - Bash
@Martin Konecny's answer provides the correct answer, but - as he mentions - it only works if the actual script is not invoked through a symlink residing in a different directory.
This answer covers that case: a solution that also works when the script is invoked through a symlink or even a chain of symlinks:
Linux / GNU readlink
solution:
If your script needs to run on Linux only or you know that GNU readlink
is in the $PATH
, use readlink -f
, which conveniently resolves a symlink to its ultimate target:
scriptDir=$(dirname -- "$(readlink -f -- "$BASH_SOURCE")")
Note that GNU readlink
has 3 related options for resolving a symlink to its ultimate target's full path: -f
(--canonicalize
), -e
(--canonicalize-existing
), and -m
(--canonicalize-missing
) - see man readlink
.
Since the target by definition exists in this scenario, any of the 3 options can be used; I've chosen -f
here, because it is the most well-known one.
Multi-(Unix-like-)platform solution (including platforms with a POSIX-only set of utilities):
If your script must run on any platform that:
-
has a
readlink
utility, but lacks the-f
option (in the GNU sense of resolving a symlink to its ultimate target) - e.g., macOS.- macOS uses an older version of the BSD implementation of
readlink
; note that recent versions of FreeBSD/PC-BSD do support-f
.
- macOS uses an older version of the BSD implementation of
-
does not even have
readlink
, but has POSIX-compatible utilities - e.g., HP-UX (thanks, @Charles Duffy).
The following solution, inspired by https://stackoverflow.com/a/1116890/45375,
defines helper shell function, rreadlink()
, which resolves a given symlink to its ultimate target in a loop - this function is in effect a POSIX-compliant implementation of GNU readlink
's -e
option, which is similar to the -f
option, except that the ultimate target must exist.
Note: The function is a bash
function, and is POSIX-compliant only in the sense that only POSIX utilities with POSIX-compliant options are used. For a version of this function that is itself written in POSIX-compliant shell code (for /bin/sh
), see here.
-
If
readlink
is available, it is used (without options) - true on most modern platforms. -
Otherwise, the output from
ls -l
is parsed, which is the only POSIX-compliant way to determine a symlink's target.
Caveat: this will break if a filename or path contains the literal substring->
- which is unlikely, however.
(Note that platforms that lackreadlink
may still provide other, non-POSIX methods for resolving a symlink; e.g., @Charles Duffy mentions HP-UX'sfind
utility supporting the%l
format char. with its-printf
primary; in the interest of brevity the function does NOT try to detect such cases.) -
An installable utility (script) form of the function below (with additional functionality) can be found as
rreadlink
in the npm registry; on Linux and macOS, install it with[sudo] npm install -g rreadlink
; on other platforms (assuming they havebash
), follow the manual installation instructions.
If the argument is a symlink, the ultimate target's canonical path is returned; otherwise, the argument's own canonical path is returned.
#!/usr/bin/env bash
# Helper function.
rreadlink() ( # execute function in a *subshell* to localize the effect of `cd`, ...
local target=$1 fname targetDir readlinkexe=$(command -v readlink) CDPATH=
# Since we'll be using `command` below for a predictable execution
# environment, we make sure that it has its original meaning.
{ \unalias command; \unset -f command; } &>/dev/null
while :; do # Resolve potential symlinks until the ultimate target is found.
[[ -L $target || -e $target ]] || { command printf '%s\n' "$FUNCNAME: ERROR: '$target' does not exist." >&2; return 1; }
command cd "$(command dirname -- "$target")" # Change to target dir; necessary for correct resolution of target path.
fname=$(command basename -- "$target") # Extract filename.
[[ $fname == '/' ]] && fname='' # !! curiously, `basename /` returns '/'
if [[ -L $fname ]]; then
# Extract [next] target path, which is defined
# relative to the symlink's own directory.
if [[ -n $readlinkexe ]]; then # Use `readlink`.
target=$("$readlinkexe" -- "$fname")
else # `readlink` utility not available.
# Parse `ls -l` output, which, unfortunately, is the only POSIX-compliant
# way to determine a symlink's target. Hypothetically, this can break with
# filenames containig literal ' -> ' and embedded newlines.
target=$(command ls -l -- "$fname")
target=${target#* -> }
fi
continue # Resolve [next] symlink target.
fi
break # Ultimate target reached.
done
targetDir=$(command pwd -P) # Get canonical dir. path
# Output the ultimate target's canonical path.
# Note that we manually resolve paths ending in /. and /.. to make sure we
# have a normalized path.
if [[ $fname == '.' ]]; then
command printf '%s\n' "${targetDir%/}"
elif [[ $fname == '..' ]]; then
# Caveat: something like /var/.. will resolve to /private (assuming
# /var@ -> /private/var), i.e. the '..' is applied AFTER canonicalization.
command printf '%s\n' "$(command dirname -- "${targetDir}")"
else
command printf '%s\n' "${targetDir%/}/$fname"
fi
)
# Determine ultimate script dir. using the helper function.
# Note that the helper function returns a canonical path.
scriptDir=$(dirname -- "$(rreadlink "$BASH_SOURCE")")
Solution 3 - Bash
Just one line will be OK.
cat "`dirname $0`"/../some.txt