1. Missing Duplicate Tags
2. Over-use of *.*
3. Over-scripting
4. Writing Multiple Tags
5. Redirecting Tags

# Common Mistakes when using ExifTool

### 1. Missing Duplicate Tags

> By default, ExifTool will suppress duplicate tags in the output. Often this is desireable but sometimes one can be fooled into thinking that information doesn't exist when it is just hidden by another tag with the same name. For instance, the following command won't necessarily return all of the EXIF tags:

>     exiftool -exif:all image.jpg

> To avoid this problem, use the `-a` option.

### 2. Over-use of Wildcards in File Names (eg. "`*.*`")

> It is often preferable to specify a directory name (eg. "`.`") instead of using wildcards (eg. "`*.*`") for a number of reasons:
> 1. "`*.*`" will process any file with a "." in the name. This includes files that ExifTool should not normally process (like the "_original" backup files for example). By specifying a directory name instead, ExifTool will process only supported file types. Or the `-ext` option may be used to process specific file types.
> 2. "`*.*`" will process any sub-directories which contain "." in the name. This may be unexpected.
> 3. The `-r` option (to recursively process sub-directories) is only effective when a directory name is specified, so it doesn't work when "`*.*`" is specified (unless the first-level directories have a "." in the name, as mentioned in point b above).
> 4. Arguments like "`*.jpg`" are a problem on systems with case-sensitive file names (like OS X and Linux) because JPG images with uppercase extensions will be missed. It is better to avoid this problem and use "`-ext jpg .`" to process all JPG images in the current directory because this technique is case-insensitive.
> 5. This can be a security problem on systems where the shell automatically expands wildcards (eg. Mac and Linux) because a malicious arrangement of file names could potentially have unwanted effects since they may be interpreted as ExifTool options (see Security Issues).
> 6. There are problems using wildcards to match files with Unicode characters in their names on Windows systems.

### 3. Over-scripting

> Often users write shell scripts to do some specific batch processing when the exiftool application already has the ability to do this either without scripting or with a greatly simplified script. This includes the ability to recursively scan sub-directories for a specific file extension (case insensitive), rename files from metadata values, and move files to different directories.

> For example, this Unix script (from here):

>     find -name '*.jpg' | while read PIC; do
>     DATE=$(exiftool -p '$DateTimeOriginal' $PIC |
>     sed 's/[: ]//g')
>     touch -t $(echo $DATE | sed 's/\(..$\)/\.\1/') $PIC
>     mv -i $PIC $(dirname $PIC)/$DATE.jpg
>     done

> may be replaced with this single command:

>     exiftool -d %Y%m%d "-filename<datetimeoriginal" "-filemodifydate<datetimeoriginal#" -ext jpg -r .

### 4. Writing/Copying Multiple Tags

> Any number of tags may be specified on a single command line, but often people execute a separate command to write or copy each tag, which is very inefficient. Combining all of the tag assignments into a single command avoids the significant overhead of launching exiftool for the subsequent commands. For example:

>     exiftool -artist=phil -modifydate=now -tagsfromfile %d%f.xmp -xmp:title -xmp:description -ext jpg c:\images

> The `-@` option may be used to read command-line arguments from a file, and may be useful in situations where there are a large number of arguments.

### 5. Errors and Inefficiencies when Redirecting Tags

> The syntax to redirect tags when copying is `"-DSTTAG<SRCTAG"`, but the syntax also allows a string containing tag names (prefixed by `$`) to be used in place of `SRCTAG`. Three common mistakes when using this syntax are:
> 1. Adding a leading "`-`" before the `SRCTAG`. (eg. `"-EXIF:Artist<-XMP:Creator"`) This is wrong, and the tag will not be copied. (It should be `"-EXIF:Artist<XMP:Creator"`.)
> 2. Adding a leading "`$`" when copying a simple tag. (eg. `"-comment<$filename"`). This is usually not necessary (see exception below) and it is less efficient for ExifTool to process the source string than it is to copy the tag directly. Also, values of list-type and shortcut tags are concatenated in the string rather than being copied individually, and wildcards are not allowed. Another difference is that a minor warning is generated if a tag doesn't exist when interpolating its value in a string (with "`$`"), but isn't when copying the tag directly.
> 3. Using "`=`" instead of "`<`" to copy a tag, or using "`<`" instead of "`=`" to assign a value.

>    "`<`" is used for copying, in which case the source (right-hand-side) operand is interpreted as either a tag name (if the operand does not contain a "`$`" symbol), or a string containing tag names prefixed by "`$`" symbols (if the operand contains a "`$`" symbol). "`=`" is used to assign a simple tag value, and the source operand is a string that is written directly to the destination tag. (And the combination "`<=`" is used to assign a tag value from the contents of a file.)

> An exception to rule "b" above occurs when trying to copy the value of one tag to a group of differnt tags, for example:

>     exiftool "-time:all<datetimeoriginal" FILE  (WRONG!)

> The above command doesn't work because the destination tag name of "All" writes to the tag with the same name as the source (ie. only "DateTimeOriginal"). However, when interpolated in a string the identity of the source tag is lost, so the following command will write to all tags in the Time group:

>     exiftool "-time:all<$datetimeoriginal" FILE

> (Note that single quotes would be necessary in the above command under Mac/Linux.)

Last revised Dec. 11, 2019

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