Analyzing file metadata in the shell

Taking Stock

© Lead Image © Sebastian Duda, 123RF.com

© Lead Image © Sebastian Duda, 123RF.com

Article from Issue 233/2020
Author(s):

Armed with the right shell commands, you can quickly identify and evaluate file and directory metadata.

Imagine you have a directory with hundreds or even thousands of files (without uniform extensions) that you want to organize. Or maybe you want to know the last access date of a file for backup, forensics, or version management purposes.

Instead of tediously clicking your way through the files in a graphical file manager, a shell script with the test command can help identify filesystem objects as well as provide additional information about the files.

Determining File Type

The file command provides information about a file's contents (Figure 1). Because it tests for patterns in the content, file cannot be misled by file extensions (Figure 2).

Figure 1: By adding the -i option, file determines that a file is a plain text file.
Figure 2: file reliably identifies files whose extensions do not match the actual content.

file gets its pattern information from the magic file /usr/share/misc/magic.mgc. For special cases, you can create your own magic file and pass it in to file by calling it with the option -m <filename>. If you are particularly interested in MIME files, use the --mime-type option.

You can even access and evaluate device files with file. Additionally, it also outputs the major and minor numbers (Figure 3) – the major number specifies the kernel's device driver, while the minor number specifies the individual device managed by the device driver.

Figure 3: If you call up the information for a device file, file shows the major and minor numbers if possible.

Given the appropriate privileges, you can obtain information about the filesystem (Figure 4). To do so, use the -s <device file> option. If you only enter the device for the hard disk without the partition number, the output contains block size details, among other things.

Figure 4: Because devices in Linux act like files, file can also output information on them.

file usually outputs the information in the form <filename>: <data>. You can take advantage of this when using the tool in shell scripts. If you use a for loop in the script, you need unique file names. You can obtain these by typing ls -1. This gives you one file name per line. The subshell in the loop header of the for loop thus provides reliable arguments until a space occurs in the file name. To avoid this, you have to convert or quote the name.

Listing 1 shows a sample script that specifically searches for PDF files and displays them for selection (Figure 5). The dynamic selection menu is created with the help of Smenu.

Listing 1

pdflist.sh

01 #! /bin/bash
02
03 # Default quit menu
04 menu="E-N-D"
05
06 cd $HOME/Data
07
08 # Search for PDF files
09 for i in $(ls -1); do
10   file $i | cut -d \: -f2 | grep -q PDF
11   if [ $? -eq 0 ]; then
12     # Show selection
13     menu=$(echo $menu $i)
14   fi
15 done
16
17 # Selection menu with Smenu and PDF display
18 while true; do
19   choice=$(echo $menu | smenu -n 10 -t1 )
20   if [ "$choice" = "E-N-D" ]; then
21     exit
22   fi
23   atril $choice
24   clear
25 done
Figure 5: With just a couple of lines of shell code and Smenu, you can create a simple selection menu for a specific file type.

Status Information

Similar to ls, the stat command provides file and directory details. Without specifying any other options, stat outputs a full set of data for the listed files (Figure 6). Figure 6 also shows the effect of read access – note the Access line with the date and time information. However, this feature does not work for filesystems mounted with the noatime option. noatime speeds up data access, because the filesystem does not have to create an entry whenever something is read.

Figure 6: Without specifying any options, stat's output contains a large amount of information.

Using stat -c <format> you can read specific information about a file (or a filesystem) and evaluate it in a script. Table 1 shows formatting information for stat. Figure 7 shows some calls, including querying access rights in numerical form. This information could be useful for an installation script.

Table 1

Stat Format Information

Syntax

Meaning

%a

Access rights in numerical format

%A

Access rights in detailed format

%d

Device number (decimal) for a device file

%t

Major number device file (hexadecimal)

%T

Minor number device file (hexadecimal)

%F

File type

%m

Filesystem where the file resides

%u

Owner UID

%U

Owner username

%g

GID

%G

Group name

%x

Last read access (plain text)

%X

Last read access (Unix seconds)

%y

Last change (plain text)

%Y

Last change (Unix seconds)

%z

Last access (plain text)

%Z

Last access (Unix seconds)

Figure 7: Examples of formatted stat queries.

Note that stat's output is a bit ambiguous: For example, Access means the last read access, but you should note that mount options like noatime influence this value. Modify refers to the contents of the file, so it may contain the creation date, but always includes the last write access. Change shows you information about changing access rights (the owner or similar). The value for Birth is currently not determined by stat on Linux due to a program error.

Listing 2 shows a small script that reads the access rights of a file and then changes them if they are too permissive. It calls the command to change permissions with the -v option so you can see what it is doing in the terminal. Figure 8 shows the database for this; Figure 9 shows the script running.

Listing 2

restrictive.sh

01 #! /bin/bash
02
03 # Define $1 as directory,
04 # else cancel
05 if [ -z $1 ]; then
06   exit
07 fi
08
09 # Change to directory
10 cd $1
11
12 for i in $(ls -1); do
13   # Evaluate access permissions
14   stat -c %a $i | grep -q 75
15   if [ $? -eq 0 ]; then
16     # Change if group
17     # or anyone can execute
18     # the file.
19     chmod -v 700 $i
20   fi
21 done
Figure 8: Access rights that are too permissive can give unauthorized persons access to the data under certain circumstances.
Figure 9: The shell script from Listing 2 showing the changed files.

Changing Timestamps

Applications that work with a file will typically modify the timestamp information. You can do this manually with the touch command (see Table 2). If you run touch for a nonexistent file name, the system creates a corresponding entry in the filesystem (i.e., it creates a file without any content). If you call touch <file> without any options, the program updates all the timestamps in the file to the system time.

Table 2

Touch Options

Option

Action

-a

Change access time

-m

Change last change time

-t <time>

Use <time> instead of system time

-r <file>

Provides a reference file from which touch takes the timestamp

The time specification for the -t option takes the form of <MMDDhhmm>. You can also add the calendar year and seconds to the specification: <YYYYMMDDhhmm.ss>. Figure 10 shows how to change the access time using touch. Figure 11 shows an example of referencing an existing file for the timestamp. The stat command's resulting output shows the special access date set by the command in Figure 10.

Figure 10: Using touch, you can modify a file's timestamp.
Figure 11: A timestamp taken from a reference file.

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