Linux

Operate Linux systems avoiding permission traps, silent failures, and common admin mistakes.

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Last updated: April 23, 2026

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Linux Gotchas

Permission Traps

```
chmod 777
```
fixes nothing, breaks everything — find the actual owner/group issue
Setuid on scripts is ignored for security — only works on binaries
```
chown -R
```
follows symlinks outside target directory — use
```
--no-dereference
```
Default umask 022 makes files world-readable — set 077 for sensitive systems
ACLs override traditional permissions silently — check with
```
getfacl
```

Process Gotchas

```
kill
```
sends SIGTERM by default, not SIGKILL — process can ignore it
```
nohup
```
doesn't work if process already running — use
```
disown
```
instead
Background job with
```
&
```
still dies on terminal close without
```
disown
```
or
```
nohup
```
Zombie processes can't be killed — parent must call wait() or be killed
```
kill -9
```
skips cleanup handlers — data loss possible, use SIGTERM first

Filesystem Traps

Deleting open file doesn't free space until process closes it — check
```
lsof +L1
```
```
rm -rf /path /
```
with accidental space = disaster — use
```
rm -rf /path/
```
trailing slash
Inodes exhausted while disk shows space free — many small files problem
Symlink loops cause infinite recursion —
```
find -L
```
follows them
```
/tmp
```
cleared on reboot — don't store persistent data there

Disk Space Mysteries

Deleted files held open by processes —
```
lsof +L1
```
shows them, restart process to free
Reserved blocks (5% default) only for root —
```
tune2fs -m 1
```
to reduce
Journal eating space —
```
journalctl --vacuum-size=500M
```
Docker overlay eating space —
```
docker system prune -a
```
Snapshots consuming space — check LVM, ZFS, or cloud provider snapshots

Networking

```
localhost
```
and
```
127.0.0.1
```
may resolve differently — check
```
/etc/hosts
```
Firewall rules flushed on reboot unless saved —
```
iptables-save
```
or use firewalld/ufw persistence
```
netstat
```
deprecated — use
```
ss
```
instead
Port below 1024 requires root — use
```
setcap
```
for capability instead
TCP TIME_WAIT exhaustion under load — tune
```
net.ipv4.tcp_tw_reuse
```

SSH Traps

Wrong permissions on ~/.ssh = silent auth failure — 700 for dir, 600 for keys
Agent forwarding exposes your keys to remote admins — avoid on untrusted servers
Known hosts hash doesn't match after server rebuild — remove old entry with
```
ssh-keygen -R
```
SSH config Host blocks: first match wins — put specific hosts before wildcards
Connection timeout on idle — add
```
ServerAliveInterval 60
```
to config

Systemd

```
systemctl enable
```
doesn't start service — also need
```
start
```
```
restart
```
vs
```
reload
```
: restart drops connections, reload doesn't (if supported)
Journal logs lost on reboot by default — set
```
Storage=persistent
```
in journald.conf
Failed service doesn't retry by default — add
```
Restart=on-failure
```
to unit
Dependency on network:
```
After=network.target
```
isn't enough — use
```
network-online.target
```

Cron Pitfalls

Cron has minimal PATH — use absolute paths or set PATH in crontab
Output goes to mail by default — redirect to file or
```
/dev/null
```
Cron uses system timezone, not user's — set TZ in crontab if needed
Crontab lost if edited incorrectly —
```
crontab -l > backup
```
before editing
@reboot runs on daemon restart too, not just system reboot

Memory and OOM

OOM killer picks "best" victim, often not the offender — check dmesg for kills
Swap thrashing worse than OOM — monitor with
```
vmstat
```
Memory usage in
```
free
```
includes cache — "available" is what matters
Process memory in
```
/proc/[pid]/status
```
— VmRSS is actual usage
cgroups limit respected before system OOM — containers die first

Commands That Lie

```
df
```
shows filesystem capacity, not physical disk — check underlying device
```
du
```
doesn't count sparse files correctly — file appears smaller than disk usage
```
ps aux
```
memory percentage can exceed 100% (shared memory counted multiple times)
```
uptime
```
load average includes uninterruptible I/O wait — not just CPU
```
top
```
CPU percentage is per-core — 400% means 4 cores maxed

Linux

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Manual Installation

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Linux Gotchas

Permission Traps

Process Gotchas

Filesystem Traps

Disk Space Mysteries

Networking

SSH Traps

Systemd

Cron Pitfalls

Memory and OOM

Commands That Lie

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