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
try
grep
returnstrue
if it finds the search target, andfalse
if it doesn't.So NOT
false
==true
.if
evaluation in shells are designed to be very flexible, and many times doesn't require chains of commands (as you have written).Also, looking at your code as is, your use of the
$( ... )
form of cmd-substitution is to be commended, but think about what is coming out of the process. Tryecho $(cat /etc/passwd | grep "sysa")
to see what I mean. You can take that further by using the-c
(count) option to grep and then doif ! [ $(grep -c "sysa" /etc/passwd) -eq 0 ] ; then
which works but is rather old school.BUT, you could use the newest shell features (arithmetic evaluation) like
which also gives you the benefit of using the c-lang based comparison operators,
==,<,>,>=,<=,%
and maybe a few others.In this case, per a comment by Orwellophile, the arithmetic evaluation can be pared down even further, like
OR
Finally, there is an award called the
Useless Use of Cat (UUOC)
. :-) Some people will jump up and down and cry gothca! I'll just say thatgrep
can take a file name on its cmd-line, so why invoke extra processes and pipe constructions when you don't have to? ;-)I hope this helps.