To check if a directory exists in a shell script, you can use the following:
if [ -d "$DIRECTORY" ]; then
# Control will enter here if $DIRECTORY exists.
fi
Or to check if a directory doesn't exist:
if [ ! -d "$DIRECTORY" ]; then
# Control will enter here if $DIRECTORY doesn't exist.
fi
However, as Jon Ericson points out, subsequent commands may not work as intended if you do not take into account that a symbolic link to a directory will also pass this check.
E.g. running this:
ln -s "$ACTUAL_DIR" "$SYMLINK"
if [ -d "$SYMLINK" ]; then
rmdir "$SYMLINK"
fi
Will produce the error message:
rmdir: failed to remove `symlink': Not a directory
So symbolic links may have to be treated differently, if subsequent commands expect directories:
if [ -d "$LINK_OR_DIR" ]; then
if [ -L "$LINK_OR_DIR" ]; then
# It is a symlink!
# Symbolic link specific commands go here.
rm "$LINK_OR_DIR"
else
# It's a directory!
# Directory command goes here.
rmdir "$LINK_OR_DIR"
fi
fi
Take particular note of the double-quotes used to wrap the variables. The reason for this is explained by 8jean in another answer.
If the variables contain spaces or other unusual characters it will probably cause the script to fail.
#!/usr/bin/env bash
SCRIPT_DIR="$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )"
is a useful one-liner which will give you the full directory name of the script no matter where it is being called from.
It will work as long as the last component of the path used to find the script is not a symlink (directory links are OK). If you also want to resolve any links to the script itself, you need a multi-line solution:
#!/usr/bin/env bash
SOURCE="${BASH_SOURCE[0]}"
while [ -h "$SOURCE" ]; do # resolve $SOURCE until the file is no longer a symlink
DIR="$( cd -P "$( dirname "$SOURCE" )" >/dev/null 2>&1 && pwd )"
SOURCE="$(readlink "$SOURCE")"
[[ $SOURCE != /* ]] && SOURCE="$DIR/$SOURCE" # if $SOURCE was a relative symlink, we need to resolve it relative to the path where the symlink file was located
done
DIR="$( cd -P "$( dirname "$SOURCE" )" >/dev/null 2>&1 && pwd )"
This last one will work with any combination of aliases, source
, bash -c
, symlinks, etc.
Beware: if you cd
to a different directory before running this snippet, the result may be incorrect!
Also, watch out for $CDPATH
gotchas, and stderr output side effects if the user has smartly overridden cd to redirect output to stderr instead (including escape sequences, such as when calling update_terminal_cwd >&2
on Mac). Adding >/dev/null 2>&1
at the end of your cd
command will take care of both possibilities.
To understand how it works, try running this more verbose form:
#!/usr/bin/env bash
SOURCE="${BASH_SOURCE[0]}"
while [ -h "$SOURCE" ]; do # resolve $SOURCE until the file is no longer a symlink
TARGET="$(readlink "$SOURCE")"
if [[ $TARGET == /* ]]; then
echo "SOURCE '$SOURCE' is an absolute symlink to '$TARGET'"
SOURCE="$TARGET"
else
DIR="$( dirname "$SOURCE" )"
echo "SOURCE '$SOURCE' is a relative symlink to '$TARGET' (relative to '$DIR')"
SOURCE="$DIR/$TARGET" # if $SOURCE was a relative symlink, we need to resolve it relative to the path where the symlink file was located
fi
done
echo "SOURCE is '$SOURCE'"
RDIR="$( dirname "$SOURCE" )"
DIR="$( cd -P "$( dirname "$SOURCE" )" >/dev/null 2>&1 && pwd )"
if [ "$DIR" != "$RDIR" ]; then
echo "DIR '$RDIR' resolves to '$DIR'"
fi
echo "DIR is '$DIR'"
And it will print something like:
SOURCE './scriptdir.sh' is a relative symlink to 'sym2/scriptdir.sh' (relative to '.')
SOURCE is './sym2/scriptdir.sh'
DIR './sym2' resolves to '/home/ubuntu/dotfiles/fo fo/real/real1/real2'
DIR is '/home/ubuntu/dotfiles/fo fo/real/real1/real2'
Best Answer
Answer
POSIX compatible:
Example use:
For Bash specific environments:
Explanation
Avoid
which
. Not only is it an external process you're launching for doing very little (meaning builtins likehash
,type
orcommand
are way cheaper), you can also rely on the builtins to actually do what you want, while the effects of external commands can easily vary from system to system.Why care?
which
that doesn't even set an exit status, meaning theif which foo
won't even work there and will always report thatfoo
exists, even if it doesn't (note that some POSIX shells appear to do this forhash
too).which
do custom and evil stuff like change the output or even hook into the package manager.So, don't use
which
. Instead use one of these:(Minor side-note: some will suggest
2>&-
is the same2>/dev/null
but shorter – this is untrue.2>&-
closes FD 2 which causes an error in the program when it tries to write to stderr, which is very different from successfully writing to it and discarding the output (and dangerous!))If your hash bang is
/bin/sh
then you should care about what POSIX says.type
andhash
's exit codes aren't terribly well defined by POSIX, andhash
is seen to exit successfully when the command doesn't exist (haven't seen this withtype
yet).command
's exit status is well defined by POSIX, so that one is probably the safest to use.If your script uses
bash
though, POSIX rules don't really matter anymore and bothtype
andhash
become perfectly safe to use.type
now has a-P
to search just thePATH
andhash
has the side-effect that the command's location will be hashed (for faster lookup next time you use it), which is usually a good thing since you probably check for its existence in order to actually use it.As a simple example, here's a function that runs
gdate
if it exists, otherwisedate
:Alternative with a complete feature set
You can use scripts-common to reach your need.
To check if something is installed, you can do: