Pipes, redirection and scripts

📘What it really is

• Pipes (|) send one command's output into the next:

grep "error" /var/log/syslog | tail -n 20
cat access.log | cut -d' ' -f1 | sort | uniq -c | sort -rn | head

The second counts requests per IP address and shows the top ten.

• Redirection:
• > file writes output to a file (replacing it); >> file adds to the end;
• 2> errors.txt sends error messages to a file;

• command > out.log 2>&1 sends both output and errors to one file.

• Shell scripts put commands in a file to repeat them:

#!/usr/bin/env bash
set -euo pipefail          # stop on errors, unset variables, failed pipes

BACKUP_DIR="/srv/backups"
STAMP=$(date +%Y-%m-%d_%H%M)
mkdir -p "$BACKUP_DIR"
cp /srv/crm/data/crm.db "$BACKUP_DIR/crm-$STAMP.db"
echo "Backup saved: crm-$STAMP.db"

Make it runnable (chmod +x backup.sh) and run it (./backup.sh).

• Good habits: set -euo pipefail at the top; quote variables ("$BACKUP_DIR") so spaces don't break things; test on your lab first.

• Scheduling: crontab -e runs scripts on a schedule; 0 2 * * * /srv/crm/backup.sh runs at 2 a.m. daily. (systemd timers are a modern alternative.)

• With AI: AI writes shell scripts well, and dangerous ones convincingly. Read every line, especially rm, > and anything with sudo, and run it in your lab first.