mjhjmtu

The user calls my script with a file path that will be either be created or overwritten at some point in the script, like foo.sh file.txt or foo.sh dir/file.txt.

The create-or-overwrite behavior is much like the requirements for putting the file on the right side of the > output redirect operator, or passing it as an argument to tee (in fact, passing it as an argument to tee is exactly what I'm doing).

Before I get into the guts of the script, I want to make a reasonable check if the file can be created/overwritten, but not actually create it. This check doesn't have to be perfect, and yes I realize that the situation can change between the check and the point where the file is actually written - but here I'm OK with a best effort type solution so I can bail out early in the case that the file path is invalid.

Examples of reasons the file couldn't created:

• the file contains a directory component, like dir/file.txt but the directory dir doesn't exist


• the user doens't have write permissions in the specified directory (or the CWD if no directory was specified

Yes, I realize that checking permissions "up front" is no the UNIX way, rather I should just try the operation and ask forgiveness later. In my particular script however, this leads to a bad user experience and I can't change the responsible component.

The obvious test would be:

if touch /path/to/file; then
 : it can be created
fi

But it does actually create the file if it's not already there. We could clean up after ourselves:

if touch /path/to/file; then
 rm /path/to/file
fi

But this would remove a file that already existed, which you probably don't want.

We do, however, have a way around this:

if mkdir /path/to/file; then
 rmdir /path/to/file
fi

You can't have a directory with the same name as another object in that directory. I can't think of a situation in which you'd be able to create a directory but not create a file. After this test, your script would be free to create a conventional /path/to/file and do whatever it pleases with it.

I think DopeGhoti's solution is better but this should also work:

file=$1
if [[ "$file:0:1" == '/' ]]; then
 dir=$file%/*
elif [[ "$file" =~ .*/.* ]]; then
 dir="$(PWD)/$file%/*"
else
 dir=$(PWD)
fi

if [[ -w "$dir" ]]; then
 echo "writable"
 #do stuff with writable file
else
 echo "not writable"
 #do stuff without writable file
fi

The first if construct checks if the argument is a full path (starts with /) and sets the dir variable to the directory path up to the last /. Otherwise if the argument does not start with a / but does contain a / (specifying a sub directory) it will set dir to the present working directory + the sub directory path. Otherwise it assumes the present working directory. It then checks if that directory is writable.

One option you might want to consider is creating the file early on but only populating it later in your script. You can use the exec command to open the file in a file descriptor (such as 3, 4, etc.) and then later use a redirection to a file descriptor (>&3, etc.) to write contents to that file.

Something like:

exec 3>dir/file.txt || 
 echo "Error creating dir/file.txt" >&2
 exit 1


# long checks here
check_ok || 
 echo "Failed checks" >&2
 # cleanup file before bailing out
 rm -f dir/file.txt
 exit 1


# now populate the file, use a redirection to write
# to the previously opened file descriptor
populate_contents >&3

You can also use a trap to clean up the file on error, that's a common practice.

This way, you get a real check for permissions that you'll be able to create the file, while at the same time being able to perform it early enough that if that fails you haven't spent time waiting for the long checks.

From what I'm gathering, you want to check that when using

tee -- "$OUT_FILE"

(note the -- or it wouldn't work for file names that start with -), tee would succeed to open the file for writing.

That is that:

• the length of the file path doesn't exceed the PATH_MAX limit


• the file exists (after symlink resolution) and is not of type directory and you have write permission to it.


• if the file doesn't exist, the dirname of the file exists (after symlink resolution) as a directory and you have write and search permission to it and the filename length doesn't exceed the NAME_MAX limit of the filesystem that directory resides in.


• or the file is a symlink that points to a file that doesn't exist and is not a symlink loop but meets the criteria just above

We'll ignore for now filesystems like vfat, ntfs or hfsplus that have limitations on what byte values file names may contain, disk quota, process limit, selinux, apparmor or other security mechanism in the way, full filesystem, no inode left, device files that can't be opened that way for a reason or another, files that are executables currently mapped in some process address space all of which could also affect the ability to open or create the file.

With zsh:

zmodload zsh/system
tee_would_likely_succeed() 
 local file=$1 ERRNO=0
 if [ -d "$file" ]; then
 return 1 # directory
 elif [ -w "$file" ]; then
 return 0 # writable non-directory
 elif [ -e "$file" ]; then
 return 1 # exists, non-writable
 elif [ "$errnos[ERRNO]" != ENOENT ]; then
 return 1 # only ENOENT error can be recovered
 else
 local dir=$file:P:h base=$file:t
 [ -d "$dir" ] && # directory
 [ -w "$dir" ] && # writable
 [ -x "$dir" ] && # and searchable
 (($#base <= $(getconf -- NAME_MAX "$dir")))
 return
 fi

What about using normal test command like outlined below?

FILE=$1

DIR=$(dirname $FILE) # $DIR now contains '.' for file names only, 'foo' for 'foo/bar'

if [ -d $DIR ] ; then
 echo "base directory $DIR for file exists"
 if [ -e $FILE ] ; then
 if [ -w $FILE ] ; then
 echo "file exists, is writeable"
 else
 echo "file exists, NOT writeable"
 fi
 elif [ -w $DIR ] ; then
 echo "directory is writeable"
 else
 echo "directory is NOT writeable"
 fi
else
 echo "can NOT create file in non-existent directory $DIR "
fi