Bash – Why Does [ -d $dirname ] Match When Variable is Empty or Unset?

bashshell-script

Well I've this code

dirname=

if [ -d $dirname ];
 then
 cd $dirname && rm *
fi

as you see I've this empty variable, what I want to know is why when using thing like this empty variable with the single square brackets it removes all the user's home directory files

And if I used the double square brackets it does not remove the user's home directory files

Like this

dirname=

if [[ -d $dirname ]];
 then
 cd $dirname && rm *
fi

I've read the difference syntax when using both Single Square Brackets and Double Square Brackets

May I know why this happens ?

Best Answer

When cd is not passed an argument, it will change to the default directory (in most cases, the user's home directory, $HOME).

The interesting part of this question is that when you pass nothing to the -d operator of the bash test or [ built-in, bash appears to exit with 0 (in my opinion, this is unexpected).

When you set dirname to an empty variable, you're essentially running this:

if [ -d ]; then
    cd && rm *
fi

To me, it's surprising that the code inside this if block is executed, but I can confirm on my machine that it is. As a comment above explains, test is not interpreting the -d as an operator anymore and is simply returning 0 because it's a non-null string. One way to help guard against this kind of behavior is to make sure you quote your variables:

if [ -d "$dirname" ]; then
    cd "$dirname" && rm *
fi

In this case, if $dirname is empty, you'll be passing "" to the -d operator which correctly evaluates a non-zero exit code so the code inside the block is not executed.

Related Solutions

Shell – Grep command in shell script

This is the error you get if you try to search in an empty directory:

$ tree
.
├── dir
│   └── file
└── empty_dir

2 directories, 1 file
$ grep foo dir/*  ## works, no error
$ grep foo empty_dir/*
grep: empty_dir/*: No such file or directory

Basically, when you run something like grep ./*, the glob (*) is interpreted by the shell which will expand it to the contents of the directory you gave. If the directory is empty, that expands to nothing and the shell returns an error. You will get the same error irrespective of which program you use:

$ ls empty_dir/*
ls: cannot access empty_dir/*: No such file or directory

So, I'm guessing that ~/5as-darbas/inputs/multiple_dir/ is empty. This is not a big deal and you can just ignore the error. If you want to deal with it more gracefully, you could give the directory name (no glob) and run a recursive grep:

$ grep -R foo empty_dir/

To do this with your current setup, change `~/5as-darbas/inputs/multiple_dir/* to ~/5as-darbas/inputs/multiple_dir/ and give grep -R as the first argument to your script:

$ myscript.sh 'grep -R' regExp/regExp tests/MULTIPLE.inp outputs/MULTIPLE.out

Shell Metacharacters – How to Escape Shell Metacharacters Automatically with Find Command

It's so much easier with zsh globs here:

for f (**/*.xml(.)) (mv -v -- $f **/$f:r:t(/[1]))

Or if you want to include hidden xml files and look inside hidden directories like find would:

for f (**/*.xml(.D)) (mv -v -- $f **/$f:r:t(D/[1]))

But beware that files called .xml, ..xml or ...xml would become a problem, so you may want to exclude them:

setopt extendedglob
for f (**/(^(|.|..)).xml(.D)) (mv -v -- $f **/$f:r:t(D/[1]))

With GNU tools, another approach to avoid having to scan the whole directory tree for each file would be to scan it once and look for all directories and xml files, record where they are and do the moving in the end:

(export LC_ALL=C
find . -mindepth 1 -name '*.xml' ! -name .xml ! \
  -name ..xml ! -name ...xml -type f -printf 'F/%P\0' -o \
  -type d -printf 'D/%P\0' | awk -v RS='\0' -F / '
  {
    if ($1 == "F") {
      root = $NF
      sub(/\.xml$/, "", root)
      F[root] = substr($0, 3)
    } else D[$NF] = substr($0, 3)
  }
  END {
    for (f in F)
      if (f in D) 
        printf "%s\0%s\0", F[f], D[f]
  }' | xargs -r0n2 mv -v --
)

Your approach has a number of problems if you want to allow any arbitrary file name:

embedding {} in the shell code is always wrong. What if there's a file called $(rm -rf "$HOME").xml for instance? The correct way is to pass those {} as argument to the in-line shell script (-exec sh -c 'use as "$1"...' sh {} \;).
With GNU find (implied here as you're using -quit), *.xml would only match files consisting of a sequence of valid characters followed by .xml, so that excludes file names that contain invalid characters in the current locale (for instance file names in the wrong charset). The fix for that is to fix the locale to C where every byte is a valid character (that means error messages will be displayed in English though).
If any of those xml files are of type directory or symlink, that would cause problems (affect the scanning of directories, or break symlinks when moved). You may want to add a -type f to only move regular files.
Command substitution ($(...)) strips all trailing newline characters. That would cause problems with a file called foo␤.xml for instance. Working around that is possible but a pain: base=$(basename "$1" .xml; echo .); base=${base%??}. You can at least replace basename with the ${var#pattern} operators. And avoid command substitution if possible.
your problem with file names containing wildcard characters (?, [, * and backslash; they are not special to the shell, but to the pattern matching (fnmatch()) done by find which happens to be very similar to shell pattern matching). You'd need to escape them with a backslash.
the problem with .xml, ..xml, ...xml mentioned above.

So, if we address all of the above, we end up with something like:

LC_ALL=C find . -type f -name '*.xml' ! -name .xml ! -name ..xml \
  ! -name ...xml -exec sh -c '
  for file do
    base=${file##*/}
    base=${base%.xml}
    escaped_base=$(printf "%s\n" "$base" |
      sed "s/[[*?\\\\]/\\\\&/g"; echo .)
    escaped_base=${escaped_base%??}
    find . -name "$escaped_base" -type d -exec mv -v "$file" {\} \; -quit
  done' sh {} +

Phew...

Now, it's not all. With -exec ... {} +, we run as few sh as possible. If we're lucky, we'll run only one, but if not, after the first sh invocation, we'll have moved a number of xml files around, and then find will continue looking for more, and may very well find the files we have moved in the first round again (and most probably try to move them where they are).

Other than that, it's basically the same approach as the zsh ones. A few other notable differences:

with the zsh one, the file list is sorted (by directory name and file name), so the destination directory is more or less consistent and predictable. With find, it's based on the raw order of files in directories.
with zsh, you'll get an error message if no matching directory to move the file to is found, not with the find approach above.
With find, you'll get error messages if some directories cannot be traversed, not with the zsh one.

A last note of warning. If the reason you get some files with dodgy file names is because the directory tree is writable by an adversary, then beware than none of the solutions above are safe if the adversary may rename files under the feet of that command.

For instance, if you're using LXDE, the attacker could make a malicious foo/lxde-rc.xml, create a lxde-rc folder, detect when you're running your command and replace that lxde-rc with a symlink to your ~/.config/openbox/ during the race window (which can be made as large as necessary in many ways) between find finding that lxde-rc and mv doing the rename("foo/lxde-rc.xml", "lxde-rc/lxde-rc.xml") (foo could also be changed to that symlink making you move your lxde-rc.xml elsewhere).

Working around that is probably impossible using standard or even GNU utilities, you'd need to write it in a proper programming language, doing some safe directory traversal and using renameat() system calls.

All the solutions above will also fail if the directory tree is deep enough that the limit on the length of the paths given to the rename() system call done by mv is reached (causing rename() to fail with ENAMETOOLONG). A solution using renameat() would also work around the problem.

Best Answer

Related Solutions

Shell – Grep command in shell script

Shell Metacharacters – How to Escape Shell Metacharacters Automatically with Find Command

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