Bash – Maintain (or restore) file permissions when replacing file

bashfilespermissionsscripting

I have a command that accepts a file as an argument, modifies the file, then writes it to the file name specified in the second argument. I'll call that program modifyfile.

I wanted it to work "in place" so I wrote a shell script (bash) that modifies it to a temporary file then moves it back:

TMP=`mktemp`
modifyfile "$original" "$TMP"
mv -v "$TMP" "$original"

This has the unfortunate side effect of destroying the permissions on this file. The file gets re-created with default permissions.

Is there a way to tell the mv command to overwrite the destination without altering its permissions? Or alternately is there a way to save the user, group, and permisssions from the original and restore them?

Best Answer

Instead of using mv, just redirect cat. For example:

TMP=$(mktemp)
modifyfile "$original" "$TMP"
cat "$TMP" > "$original"

This overwrites $original with the contents of $TMP, without touching anything at the file level.

Related Solutions

Files – How to Back Up and Restore File Permissions

You can do this with the commands from the acl package (which should be available on all mainstream distributions, but might not be part of the base installation). They back up and restore ACL when ACL are present, but they also work for basic permissions even on systems that don't support ACL.

To back up permissions in the current directory and its subdirectories recursively:

getfacl -R . >permissions.facl

To restore permissions:

setfacl --restore=permissions.facl

Bash – How to check if a file can be created or truncated/overwritten in bash

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.

Best Answer

Related Solutions

Files – How to Back Up and Restore File Permissions

Bash – How to check if a file can be created or truncated/overwritten in bash

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