Glossary

-print0: A find action that prints each file name followed by a null character instead of a newline.; xargs
\copy: The psql meta-command that moves rows between a table and a file on the machine where psql runs, here your laptop.; psql
`--`: A separator on the command line. Git reads every word after it as a path, never as an option or a branch name.; Reading history
`<rev>:<path>`: The file at <path> as it was stored in commit <rev>, for example HEAD~2:src/config.js.; Finding things in git
`A..B`: The commits reachable from B that are not reachable from A. A itself is left out.; Rebase and cherry-pick
`stash@{n}`: The name of an entry: stash@{0} is the newest, stash@{1} the one before. You can also write the plain number n.; Stash
A record: A DNS record that maps a name to an IPv4 address. dig asks for it when you give no type.; Network checks
Active: The runtime state of a unit: active while the service runs, inactive when it is stopped, failed when it exited with an error.; systemctl
Address: The part before a sed command that picks the lines it runs on: a line number like 2, a regex like /^#/, or a range like 2,4.; sed
adjacent lines: Lines that sit next to each other. uniq only merges equal lines when they are adjacent.; Sorting and counting
aggregate function: A function such as count, sum or avg that reads many rows and returns one value.; Counting in SQL
anchor: A part of a regex that matches a position instead of a character: ^ is the start of the input, $ the end and \b a word boundary.; Regex basics
Backreference: \1 or \k<name> inside a pattern. It matches the same text that a group earlier in the pattern matched.; Regex groups
bastion: A host that accepts SSH from outside and that you go through to reach the hosts behind it.; ssh and scp
bisect: A binary search over history. You mark one commit bad and one good, and Git checks out the commit halfway between them for you to test.; Finding things in git
Boot: The time between one start of the machine and the next. -b -1 means the boot before the current one, -b alone the current one.; journalctl
branch: A name that points to one commit. Each new commit on the branch moves the name forward to it.; Branches
build context: The folder you pass to docker build. COPY can only read files from it.; Dockerfile lines
Capturing group: (...) in a regex. It keeps the text it matched, numbered 1, 2, 3 in the order of the opening parentheses.; Regex groups
character class: A set of characters that matches one character, such as [0-9], its shorthand \d, or [^ ] for any character except a space.; Regex basics
cherry-pick: Git applies the change of one existing commit on top of the current branch and records it as a new commit with a new hash.; Rebase and cherry-pick
COALESCE: A function that returns the first of its arguments that is not NULL, such as COALESCE(phone, 'none').; NULL in queries
conflict target: The column list after ON CONFLICT, such as (sku). PostgreSQL uses it to pick the unique index that decides what counts as a duplicate.; Changing data
constraint: A rule on a column or a table that PostgreSQL checks on every insert and update, such as NOT NULL, UNIQUE, CHECK or a foreign key.; Tables and indexes
Container: A process started from an image, with its own file system and a name, such as web. It stays on disk after it stops, until you remove it.; docker
container: One process with its own image inside a pod. A pod with two containers needs -c to say which one you mean.; kubectl for debugging
CrashLoopBackOff: The status kubectl shows for a container that keeps crashing. The kubelet waits 10 s, 20 s, 40 s and so on between restarts, up to 5 minutes.; kubectl for debugging
CRLF: A line ending made of a carriage return (\r) and a newline (\n). cut splits lines on the newline only, so the \r stays at the end of the last field.; Cutting columns
CSR: Certificate signing request (PKCS#10): a file holding your public key and the subject you ask for, signed with your own private key.; openssl for TLS
delimiter: The character that separates fields on a line, such as : in passwd.txt. cut -d sets it; the default is a tab.; Cutting columns
Delimiter: The character that separates the parts of s/REGEXP/REPLACEMENT/. It is / by default and can be any other single character.; sed
diffstat: One line per changed file with the number of changed lines, then a summary line such as 1 file changed, 2 insertions(+).; Reading history
Enabled: A unit is enabled when systemd has created the symlinks listed in its [Install] section, so it starts at boot.; systemctl
Entry: One key and its value, written by to_entries as {"key": ..., "value": ...}.; jq transforms
EXCLUDED: A special table name inside ON CONFLICT DO UPDATE. It holds the row that the INSERT tried to add.; Changing data
exec form: An instruction written as a JSON array, like CMD ["node", "server.js"]. Docker runs the program directly, with no /bin/sh -c around it.; Dockerfile lines
Exit code: The number curl returns to the shell when it ends. 0 means the transfer worked; 22 means --fail saw a status of 400 or more.; curl
field: One column of a line. By default sort splits fields where blanks start; -t sets another separator.; Sorting and counting
Field: One piece of a line after awk splits it. $1 is the first field, and $0 holds the full line.; awk
Field separator: What awk splits a line on. By default it is runs of spaces and tabs; -F, sets it to a comma.; awk
file descriptor: A number the shell gives each open stream of a command: 0 is standard input, 1 is standard output, 2 is standard error.; Pipes and redirects
Filter: The jq program you pass in quotes, like .services[0]. It takes JSON in and gives JSON out.; jq basics
follow: What tail -f does: it prints the last lines, then keeps the file open and prints new lines as they are appended.; Cutting columns
foreign key: A constraint that only lets a column hold values that exist in a column of another table, here tickets.user_id and users.id.; Tables and indexes
glob: A shell pattern where * matches any characters and ? matches one, such as *.log.; Finding files
group: The set of rows that have the same values in the GROUP BY columns. The query returns one row per group.; Counting in SQL
HAVING: A condition checked on each group after the aggregates are computed. A WHERE condition runs on each row, before the rows are grouped.; Counting in SQL
HEAD: The commit you are on. On a branch, it is the newest commit of that branch, and HEAD~2 is its grandparent, following first parents.; Undoing things
Header: A Name: value line sent before the body, by the client in the request or by the server in the response.; curl
here string: The <<< operator: the shell passes one word, plus a newline, to the command on standard input.; Pipes and redirects
Image: A read-only template with a name and a tag, such as nginx:1.27. docker run creates a container from it.; docker
index: The staging area: the snapshot of files that git commit will record. git add copies a file from the working tree into it.; Seeing changes
index: The staging area: what the next commit will contain. git add copies a file into it.; Undoing things
item: One word xargs reads from standard input. By default, blanks and newlines separate items, so cache 1.tmp gives 2 items.; xargs
Iterator: .[], which outputs each element of an array as a separate result. .services[] gives 3 objects, one after the other.; jq basics
job: A command that bash started from this terminal, numbered from 1. jobs lists them and %1 refers to job 1.; Processes
join condition: The test after ON, such as o.customer_id = c.id. A pair of rows goes into the result when it is true.; Joins
Journal: The log store of systemd, written by journald. Each entry has a message plus fields; _SYSTEMD_UNIT holds the unit name.; journalctl
JSONPath: A path into an object’s fields, like .status.podIP. -o jsonpath and --sort-by take one.; kubectl get and describe
key pair: A private key that stays on your laptop and a public key, the same file name with .pub, that you copy to servers.; ssh and scp
label: A key and value stored on an object, like app=web. A label selector picks every object that has it.; kubectl get and describe
layer: The file changes one instruction adds to the image. When a layer changes, Docker rebuilds it and every layer after it.; Dockerfile lines
Lazy quantifier: A quantifier followed by ?, like *? or +?. It matches as few characters as it can.; Regex groups
LEFT JOIN: A join that keeps every row of the left table. When no right row matches, the right columns are NULL.; Joins
listening socket: A socket that waits for incoming connections on a port, like the one a database opens on 5432.; Network checks
local port forward: A port on your laptop that ssh connects, through the SSH connection, to a host and port on the far side.; ssh and scp
match: A line that contains the pattern. By default grep prints every match and nothing else.; Searching text
merge base: The newest commit that two branches share: the point where feature/pdf left main.; Seeing changes
merged: A branch is merged into main when its newest commit is reachable from main, so deleting the branch loses no commit.; Branches
meta-command: A psql command that starts with a backslash, such as \dt. psql runs it itself and never sends it to the server.; psql
Method: The verb at the start of an HTTP request: GET, HEAD, POST, DELETE. curl sends GET unless an option changes it.; curl
mode: The 9 permission bits of a file: read, write and execute for the owner, for the group, and for everyone else (others).; Permissions
namespace: A named group of objects in one cluster. kubectl looks in one namespace unless you name another with -n.; kubectl get and describe
Non-capturing group: (?:...) in a regex. It groups a part of the pattern for | or a quantifier and keeps nothing.; Regex groups
NULL: The marker for a missing value. =, <> and the other comparison operators return NULL when one side is NULL, and WHERE drops a row whose condition is NULL.; NULL in queries
null character: The byte with value 0. find -print0 and xargs -0 use it to separate file names, so a space or a newline inside a name stays part of the name.; xargs
NULLIF: A function that returns NULL when its 2 arguments are equal, and its first argument otherwise.; NULL in queries
Object construction: {...} in a filter. It builds a new object, like {name: .service}, from fields of the input.; jq transforms
octal mode: The mode written as digits, one per class, each the sum of read (4), write (2) and execute (1). 640 is rw-r-----.; Permissions
OFFSET: The number of sorted rows to skip before LIMIT starts counting the rows it returns.; SELECT basics
OVER: The clause right after a function that makes it a window function. Inside it, PARTITION BY and ORDER BY say which rows the function sees and in what order.; Window functions
partition: The group of rows that share the same PARTITION BY value, here one per region. Without PARTITION BY, the whole result is one partition.; Window functions
patch: The lines a commit added (starting with +) and removed (starting with -), file by file.; Reading history
pattern: The text or regular expression grep looks for in each line, such as ERROR or WARN|ERROR.; Searching text
pattern: The text on the right of LIKE. % matches any run of characters, even none, and _ matches exactly one character.; SELECT basics
Pattern: A condition before {...}, like $4 == 500. awk runs the action only on the lines where it is true.; awk
Pattern space: The buffer that holds the current line while sed runs the script on it. sed prints it at the end of each line unless you pass -n.; sed
peer: A row with the same ORDER BY value as the current row, such as ids 2 and 3 with 250 each.; Window functions
PEM: The text form of a certificate or key: base64 between lines like -----BEGIN CERTIFICATE----- and -----END CERTIFICATE-----.; openssl for TLS
pickaxe: The -S and -G options of git log. They keep only the commits whose diff touches a given string or regular expression.; Finding things in git
PID: The process ID: the number the kernel gives each running process, such as 4242.; Processes
pipe: The | operator: it connects the standard output of the command on its left to the standard input of the command on its right.; Pipes and redirects
Pipe: | inside a filter. It feeds each result of the left side into the right side, like .services[] | .name.; jq basics
plan: The steps PostgreSQL picks to run a query, such as a sequential scan of orders or an index scan. EXPLAIN prints it.; psql
Priority: The syslog level of an entry, from 0 (emerg) to 7 (debug). err is 3 and warning is 4.; journalctl
Published port: A host port that Docker forwards to a container port. -p 8080:80 sends host port 8080 to container port 80.; docker
quantifier: A suffix that says how many times the item before it repeats: + for 1 or more, * for 0 or more, {4} for exactly 4.; Regex basics
rebase: Git takes the commits of a branch, applies them one by one on top of another commit, and moves the branch to the last new commit.; Rebase and cherry-pick
reflog: A local log of every commit HEAD pointed to, newest first, numbered HEAD@{0}, HEAD@{1} and so on.; Undoing things
remote: A name stored in your clone for another copy of the repo, with its URL. origin is the remote that git clone creates.; Remotes
remote-tracking branch: Your local copy of a branch on a remote, such as origin/fix/tax-tests. git fetch updates it.; Branches
remote-tracking branch: A local copy of a branch on a remote, such as origin/main. git fetch updates it; you do not commit on it.; Remotes
resolver: The DNS server that answers your lookups. dig uses the one in /etc/resolv.conf unless you name another with @.; Network checks
RETURNING: A clause at the end of INSERT, UPDATE or DELETE that sends back columns of the rows the statement changed.; Changing data
revision: A numbered version of a Deployment’s pod template. A change to the template, such as a new image, creates one; scaling does not.; kubectl for debugging
SAN: Subject Alternative Name: a certificate extension that holds a list of names, such as DNS:www.shop.example.org.; openssl for TLS
select list: The expressions between SELECT and FROM. Each one gives one column of the result.; SELECT basics
short hash: The first few characters of a commit’s 40-character id, enough to name that commit uniquely in this repo.; Reading history
signal: A numbered message the kernel delivers to a process. kill sends one; SIGTERM is 15 and SIGKILL is 9.; Processes
SNI: Server Name Indication: the host name the client puts in its ClientHello, the first message of the TLS handshake.; openssl for TLS
sort key: The part of each line that sort compares, chosen with -k. Without -k, sort compares whole lines.; Sorting and counting
standard error: Descriptor 2, where ./build.sh writes its error: line. > and | leave it on the terminal unless you move it.; Pipes and redirects
starting point: The directory you give find first, such as . or logs. It searches that directory and everything below it.; Finding files
stash entry: A saved set of uncommitted changes, with a message. git stash push adds one and resets the working tree and the index to HEAD.; Stash
Status code: The three-digit number the server puts on the first line of its answer: 200 for OK, 301 for moved, 404 for not found.; curl
symbolic mode: The mode written as who, an operator and letters, such as u+x or go-rwx. It changes only the bits it names.; Permissions
table alias: A short name for a table in one query, such as c in customers c. A self join needs two aliases for the same table.; Joins
test: A condition such as -name or -size that is true or false for each file. find prints a file only if every test is true.; Finding files
transaction block: The statements between BEGIN and COMMIT. Some commands, such as CREATE INDEX CONCURRENTLY, refuse to run inside one.; Tables and indexes
umask: A mask kept by the shell. Each bit set in it is turned off on files and directories that the shell’s commands create.; Permissions
Unit: One object systemd manages: a service, a timer, a socket, a mount. Its name ends with its type, as in orders.service.; systemctl
Unit file: The text file that defines a unit, such as /etc/systemd/system/orders.service, with sections like [Service] and [Install].; systemctl
untracked file: A file in the working tree that was never added with git add. git status --short marks it ??.; Stash
Update-assignment: |= in a filter. It runs the old value of a field through a filter and stores the result in the same place.; jq transforms
upstream: The remote-tracking branch a local branch is linked to. On that branch, git status compares with it and a plain git pull pulls from it.; Branches
upstream branch: The remote branch a local branch is linked to. A plain git push or git pull on that branch uses it.; Remotes
word character: A letter, a digit or _. -w uses them to decide where a word starts and ends.; Searching text
working tree: The files on your disk, as you edit them.; Seeing changes