Setting File Modification Time

The stat command displays detailed timestamp information for a file:

stat /opt/test.conf

To create a file or update its modification time, use touch with the -m and -d options:

# Create or update modification time to July 7, 2020
touch -m -d "2020-07-07 00:00" /opt/abc.txt

If the file doesn’t exist, it will be created as a empty file with the specified modification time. If it exists, only the modification timestamp is updated.

Verification via stat shows:

最近更改：2020-07-07 00:00:00.000000000 +0800

Finding Files by Modification Time

Use find with the -mtime option to locate files based on their last modification time:

• -mtime +N: files modified more than N days ago
• -mtime -N: files modified within the last N days (including to day)

Example: find all .txt file modified more than 3 days ago:

find /path/to/search -name "*.txt" -mtime +3

To find .txt files modified within the last 3 days:

find /path/to/search -name "*.txt" -mtime -3

Searching Files by Size

The find command also supports size-based filtering using -size:

• -size Nk: exactly N kilobytes
• -size -Nm: less than N megabytes
• -size +Ng: greater than N gigabytes

Valid units include k (kilobytes), M (megabytes), and G (gigabytes).

Example: find files larger than 5MB in /opt:

find /opt -size +5M

Creating Files of Specific Sizes with dd

The dd utility can generate files filled with zeros of a precise size:

# Create a 1MB file
dd if=/dev/zero of=a.txt bs=1M count=1

# Create a 5MB file
dd if=/dev/zero of=b.txt bs=5M count=1

# Create a 10MB file
dd if=/dev/zero of=c.txt bs=10M count=1

Here:

• if=/dev/zero reads from a special device that outputs null bytes
• of=filename specifies the output file
• bs sets the block size
• count defines how many blocks to copy

File sizes can be verified with:

ls -l    # shows size in bytes
ls -lh   # shows human-readable sizes (e.g., 1.0M, 5.0M)