Pfthb

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.

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=$1 ERRNO=0 LC_ALL=C
 if [ -d "$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

In bash or any Bourne-like shell, just replace the

zmodload zsh/system
tee_would_likely_succeed() 
 <zsh-code>

with:

tee_would_likely_succeed() 
 zsh -s -- "$@" << 'EOF'
 zmodload zsh/system
 <zsh-code>
EOF

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:

#!/bin/bash

# Open the file for read/write, so it doesn't get
# truncated just yet (to preserve the contents in
# case the initial checks fail.)
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


# We're ready to write, first truncate the file.
# Use "truncate(1)" from coreutils, and pass it
# /dev/fd/3 so the file doesn't need to be reopened.
truncate -s 0 /dev/fd/3

# 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.

UPDATED: In order to avoid clobbering the file in case the checks fail, use bash's fd<>file redirection which does not truncate the file right away. (We don't care about reading from the file, this is just a workaround so we don't truncate it. Appending with >> would probably just work too, but I tend to find this one a bit more elegant, keeping the O_APPEND flag out of the picture.)

When we're ready to replace the contents, we need to truncate the file first (otherwise if we're writing fewer bytes than there were in the file before, the trailing bytes would stay there.) We can use the truncate(1) command from coreutils for that purpose, and we can pass it the open file descriptor we have (using the /dev/fd/3 pseudo-file) so it doesn't need to reopen the file. (Again, technically something simpler like : >dir/file.txt would probably work, but not having to reopen the file is a more elegant solution.)

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.

You mentioned user experience was driving your question. I'll answer from a UX angle, since you've got good answers on the technical side.

Rather than performing the check up-front, how about writing the results into a temporary file then at the very end, placing the results into the user's desired file? Like:

userfile=$1:?Where would you like the file written?
tmpfile=$(mktemp)

# ... all the complicated stuff, writing into "$tmpfile"

# fill user's file, keeping existing permissions or creating anew
# while respecting umask
cat "$tmpfile" > "$userfile"
if [ 0 -eq $? ]; then
 rm "$tmpfile"
else
 echo "Couldn't write results into $userfile." >&2
 echo "Results available in $tmpfile." >&2
 exit 1
fi

The good with this approach: it produces the desired operation in the normal happy path scenario, side-steps the test-and-set atomicity issue, preserves permissions of the target file while creating if necessary, and is dead simple to implement.

Note: had we used mv, we'd be keeping the permissions of the temporary file -- we don't want that, I think: we want to keep the permissions as set on the target file.

Now, the bad: it requires twice the space (cat .. > construct), forces the user to do some manual work if the target file wasn't writable at the time it needed to be, and leaves the temporary file laying around (which might have security or maintenance issues).

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

TL;DR:

: >> "$userfile"

From the OP:

I want to make a reasonable check if the file can be created/overwritten, but not actually create it.

And from your comment to my answer from a UX perspective:

The overwhelming majority of the time this fails is because the path provided is not valid. I can't reasonably prevent some rogue user some concurrently modifying the FS to break the job in the middle of the operation, but I certainly can check the #1 failure cause of an invalid or not writable path.

The only reliable test is to open(2) the file, because only that resolves every question about the writeability: path, ownership, filesystem, network, security context, etc. Any other test will address some part of writeability, but not others. If you want a subset of tests, you'll ultimately have to choose what's important to you.

But here's another thought. From what I understand:

1. the content creation process is long-running, and


2. the target file should be left in a consistent state.

You're wanting to do this pre-check because of #1, and you don't want to overwrite an existing file because of #2. So why don't you just ask the shell to open the file for appending, but don't actually append anything?

$ tree -ps
.
├── [dr-x------ 4096] dir_r
├── [drwx------ 4096] dir_w
├── [-r-------- 0] file_r
└── [-rw------- 0] file_w

$ for p in file_r dir_r/foo file_w dir_w/foo; do : >> $p; done
-bash: file_r: Permission denied
-bash: dir_r/foo: Permission denied

$ tree -ps
.
├── [dr-x------ 4096] dir_r
├── [drwx------ 4096] dir_w
│   └── [-rw-rw-r-- 0] foo
├── [-r-------- 0] file_r
└── [-rw------- 0] file_w

Under the hood, this resolves the writeability question exactly as wanted:

open("dir_w/foo", O_WRONLY|O_CREAT|O_APPEND, 0666) = 3

but without modifying the file's contents. Now, yes, this approach:

• adjusts the file's modification time: you could mitigate that by storing the current value (from stat) then re-applying (via touch).


• doesn't tell you if the file is append only, which might be a problem when you go about updating it at the end of your content creation. You can detect this, to a degree, with lsattr and react accordingly.


• creates a file that didn't previously exist, if such is the case: mitigate this with a selective rm.

While I contend (in my other answer) that the most user-friendly approach is to create a temporary file the user has to move, I think this is the least user-hostile approach to fully vet their input.