Command-Line Notes

A short English companion to the Chinese pages: the handful of rules that explain most shell surprises, plus two lookup tables I keep coming back to. The full notes are in Chinese; these sections are the parts that read better in English.

1. Quoting, in three rules

Nearly every "why did my script do that" question comes down to how the shell split the line into words.

Single quotes are literal: no expansion at all. You cannot embed a single quote inside them.
Double quotes are literal except for $, backticks and \, which still expand.
No quotes means word splitting and globbing. Runs of spaces collapse, and a filename with a space becomes two arguments.

name="two  spaces"
printf '%s\n' $name      # "two spaces"   - collapsed
printf '%s\n' "$name"    # "two  spaces"  - preserved

The practical rule: always quote variable expansions unless you have a specific reason not to. The classic failure is a loop over files that breaks the first time someone uploads my report.txt.

2. Exit codes and strict mode

Zero means success, anything else is a failure. $? holds the status of the previous command, but every command overwrites it — including echo — so capture it immediately.

curl -fsS -o /dev/null http://example.com/health
rc=$?
[ "$rc" -eq 0 ] || echo "health check failed: $rc" >&2

Start non-trivial scripts with:

set -euo pipefail

Flag	Effect
`-e`	Exit when a command fails (with exceptions: conditions, `&&` lists, `!`).
`-u`	Error on an unset variable instead of expanding it to an empty string.
`-o pipefail`	A pipeline fails if any stage fails, not just the last one.

Without pipefail, curl … | head reports success whenever head succeeds, which hides a broken download completely.

3. Pipeline patterns worth memorising

# Count lines by a key
awk '{print $1}' access.log | sort | uniq -c | sort -rn

# Keep the header, filter the rest
awk 'NR == 1 || $3 > 100' data.tsv

# Replace in place (GNU sed; BSD sed needs -i '')
sed -i 's/old/new/g' file

# Walk files safely, including names with spaces
find . -type f -name '*.log' -print0 | xargs -0 gzip

# Fail loudly if a variable is empty
: "${TOKEN:?TOKEN must be set}"

Two habits that prevent most data loss: run find … -print before adding -delete, and never parse ls output — use globs or find.

4. Undoing things in Git

Situation	Command	What survives
Edited, not staged	`git restore <file>`	Nothing — the change is discarded
Staged, not committed	`git restore --staged <file>`	The edit stays in the working tree
Committed, not pushed	`git commit --amend`	Same changes, new commit object
Committed, not pushed	`git reset --soft HEAD~1`	Changes return to the index
Already pushed	`git revert <sha>`	History unchanged, a new inverse commit is added
Anything, within 90 days	`git reflog`	Recover a lost tip by hash

Never rewrite a branch other people have pulled. git push --force-with-lease is the least dangerous way to force, because it refuses when the remote has commits you have not seen.

5. HTTP status codes at a glance

Code	Meaning	When to use it
`200`	OK	Successful read or update with a body
`201`	Created	After `POST`, with a `Location` header
`204`	No Content	Success with nothing to return
`301` / `308`	Moved permanently	Permanent; browsers cache it for a long time
`302` / `307`	Found / Temporary redirect	Use 307 if the method and body must be preserved
`304`	Not Modified	Answer to a conditional request; no body
`401`	Unauthorized	Missing or invalid credentials
`403`	Forbidden	Authenticated but not allowed
`404`	Not Found	The resource does not exist
`409`	Conflict	Concurrent edit or state clash
`429`	Too Many Requests	Rate limited; include `Retry-After`
`502` / `504`	Bad Gateway / Gateway Timeout	Upstream returned garbage / did not answer in time

Returning 200 with an error payload inside the body breaks caching, monitoring and retry logic all at once. Status codes are for infrastructure; business error codes belong in the body.

6. Where to go next

The Chinese pages go into much more detail, with worked examples and platform differences:

Git 命令速查 — branches, rebase, bisect, common traps
Shell 文本处理 — grep, sed, awk, a log-analysis walkthrough
正则表达式 — dialect differences and catastrophic backtracking
Markdown 语法 — the parts that render inconsistently
HTTP 基础 — caching, conditional requests, curl recipes
字符编码 — UTF-8 byte structure, mojibake, NFC/NFD