Intermediate Linux and Shell Scripting Interview Questions

SIGTERM (15) politely asks a process to stop, and it can clean up first. SIGKILL (9) ends it immediately, and it cannot be caught. Send SIGTERM first.

A process that has finished, but whose parent has not read its exit status. It uses almost no resources, but it keeps its entry in the process table. It shows as Z in ps, and is cleared when the parent reads the status or exits.

start runs a service now. enable makes it start at boot. enable --now does both.

journalctl -u name, with -f to follow, -n 50 for the last lines, and --since for a time range. Many services also write to /var/log.

For quick jobs, use nohup command &, or tmux to keep a session alive. For real services, write a systemd unit, which also handles restarts and logs.

The average number of tasks running or waiting over 1, 5 and 15 minutes. Compare it with the number of CPU cores: a load of 4 on 4 cores means they are fully busy.

Minute, hour, day of month, month, and day of week. */5 * * * * means every 5 minutes, and 30 2 * * 1 means 02:30 every Monday.

cron has a minimal environment: a short PATH, no .bashrc, and a different current folder. Use full paths, set variables in the script, and log output with >> file 2>&1.

You hold a private key, and the server stores the public key in ~/.ssh/authorized_keys. The server sends a challenge, and your client proves that it has the private key by signing it, without sending the key.

sudo ss -ltnp | grep :8080, or sudo lsof -i :8080.

Check with df -h, and find the biggest folders with du -h --max-depth=1 / | sort -hr. Common causes are logs, caches and Docker data. If du shows far less than df, use lsof +L1 to find deleted files that are still open. Also check inodes with df -i.

A hard link is another name for the same inode, so the data remains until all names are removed. It cannot cross file systems or link to folders. A soft link stores a path, can cross file systems, and breaks if the target is removed.

A record that stores a file's metadata (owner, permissions, size, data location), but not its name. Names are in directories. A disk can run out of inodes while it still has free space.

awk '{print $1}' access.log | sort | uniq -c | sort -nr | head -n 10

sed -i 's/old/new/g' file. Use -i.bak to keep a backup.

uniq compares only neighbouring lines, so duplicates must be next to each other.

It is the shebang. It tells the system which interpreter should run the script.

Double quotes expand variables and command substitution. Single quotes keep the text exactly.

$? is the last exit code, $# is the number of arguments, "$@" is all the arguments as separate words, and $0 is the script's name.

-e stops on a failed command, -u treats unset variables as errors, and -o pipefail makes a pipeline fail if any part of it fails.

./script.sh runs in a new process, so its variables disappear afterwards. source runs it in the current shell, so its variables and cd changes stay.

while IFS= read -r line; do echo "$line"; done < file. Avoid for line in $(cat file), which splits on spaces.