To clear Linux’s clean filesystem caches for a test or debugging run, use sudo sh -c 'sync; echo 3 > /proc/sys/vm/drop_caches'. This asks the kernel to drop clean page-cache data and reclaimable filesystem metadata caches. It does not clear every application’s cache or free memory used by running processes.
What the command clears—and what it does not
The kernel interface /proc/sys/vm/drop_caches acts on clean page-cache data and reclaimable slab objects, including dentries and inodes. It is a non-destructive operation: it does not free dirty objects. Running sync first writes pending data, which can make more objects clean and eligible to be dropped. See the Linux Kernel Documentation for /proc/sys/vm/.
This is not a universal “clear all Linux cache” command. It does not empty application-managed caches or release memory held by processes.
Choose which cache category to drop
| Value | What it requests | When it may be useful |
|---|---|---|
1 |
Drop page cache. | A test focused on cached file contents. |
2 |
Drop reclaimable slab objects, including dentries and inodes. | A test focused on filesystem metadata caches. |
3 |
Drop both categories. | A cold-cache test that needs both documented groups cleared. |
These values are documented by the Linux kernel. For benchmarking and interface details, see the Linux man-pages project’s proc_sys_vm(5).
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Clear Linux caches safely from the terminal
- Open a terminal. You need an account permitted to use
sudo. - Run the command for the cache category you need. For both page cache and reclaimable filesystem metadata, run:
sudo sh -c 'sync; echo 3 > /proc/sys/vm/drop_caches'
To request only page cache, substitute1for3. For reclaimable dentries and inodes only, substitute2. - Enter your password if prompted. No output on success is normal.
The sudo sh -c form is important: the shell that opens the procfs file must have elevated privileges. A command such as sudo echo 3 > /proc/sys/vm/drop_caches typically fails because your unprivileged shell performs the redirection before sudo runs echo.
Should you clear the cache?
Usually, no. Linux uses cache to speed up file access and reclaims it automatically when other work needs memory. The kernel documentation discourages using drop_caches to control cache growth; discarded cache must be rebuilt, which can require substantial I/O and CPU. The man page also warns that losing caching’s benefits can degrade overall system performance.
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Use a cache drop when you need a controlled cold-cache condition for filesystem benchmarking or memory-management debugging. For ordinary memory pressure, investigate which processes or memory categories are responsible instead of repeatedly forcing cache eviction.
Troubleshooting
- “Permission denied” or “Read-only file system”: Make sure the entire shell command is elevated with
sudo sh -c, as shown above. The interface is a kernel procfs control, not a regular user-writable file. - Redirection fails even though the command uses sudo: Put the redirection inside the quoted command passed to
sh -c. Elevating onlyechodoes not elevate the shell opening the target file. - The command produces no visible confirmation: That is normal; the command does not need to print a success message.
- Memory usage looks high afterward: Cache is useful memory and can be rebuilt as files are accessed. This command neither terminates processes nor clears their private memory.
- You expected an application’s cache to be cleared: This kernel interface does not manage application-specific caches. Identify the application and use its own documented cache controls.
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