Shell fundamentals

Play it

Night Shift: Answer the pager on a simulated server: find the cause with real shell commands, then fix it.

The game links back to this page from: Permissions, Processes, Writing scripts that don't surprise you.

Permissions

ls -l file.sh              # -rwxr-xr-x  owner group  size  date  name
chmod +x file.sh            # add execute for everyone
chmod 644 file.txt          # rw- r-- r--  (owner rw, group/other read)
chown user:group file.txt   # change owner and group

Permission triplets are owner / group / other, each r (4) w (2) x (1) summed — 755 = rwxr-xr-x. x on a directory means "can cd into / list contents with a known name", not "executable".

Processes

ps aux | grep nginx     # list processes, filter by name
ps aux --sort=-%cpu | head  # the biggest CPU users first
top / htop               # live process viewer
kill -TERM <pid>         # ask a process to exit cleanly (default signal)
kill -9 <pid>             # SIGKILL — last resort, the process can't clean up
pkill -f "node server"   # kill by matching the command line
jobs / fg / bg / Ctrl+Z  # manage jobs in the current shell session
nohup long-cmd &          # keep running after the shell exits

Pipes, redirection, and exit codes

cmd1 | cmd2               # pipe stdout of cmd1 into stdin of cmd2
cmd > out.txt              # redirect stdout, overwrite
cmd >> out.txt             # redirect stdout, append
cmd 2> err.txt              # redirect stderr only
cmd > out.txt 2>&1          # redirect both to the same file
cmd1 && cmd2                # run cmd2 only if cmd1 succeeded (exit code 0)
cmd1 || cmd2                # run cmd2 only if cmd1 failed
echo $?                     # exit code of the last command

Writing scripts that don't surprise you

Every script under examples/scripts/ starts with:

#!/usr/bin/env bash
set -euo pipefail

• set -e — exit immediately if any command fails, instead of continuing with a stale or missing result.
• set -u — treat an unset variable as an error, instead of silently expanding to "".
• set -o pipefail — a pipeline's exit code is the first non-zero one, not just the last command's (false | true would otherwise "succeed").
• Quote variable expansions ("$var", "$@") — unquoted, the shell word-splits on whitespace and glob-expands, which breaks on paths with spaces.

Try it: examples/scripts/backup.sh

cd topics/linux/examples/scripts
./backup.sh /etc /tmp/backups     # tar+gzip /etc into /tmp/backups/etc-<timestamp>.tar.gz
./backup.sh                        # missing arg -> ${1:?...} prints a usage error and exits 1

Read the script alongside this note — ${1:?message} is the idiomatic way to require an argument under set -u, and the whole file is kept shellcheck-clean as a CI check.