Bourne-Again SHell

Bash Cheatsheet

made-with-bash Ubuntu 22.10 Become a sponsor to JV-conseil Follow JV conseil on StackOverflow Follow JVconseil on Twitter Follow JVconseil on Mastodon Follow JV conseil on GitHub

The only way to really learn scripting is to write scripts – Mendel Cooper

Writing shell scripts leaves a lot of room to make mistakes, in ways that will cause your scripts to break on certain input, or (if some input is untrusted) open up security vulnerabilities… The simplest step is to avoid using shell at all – MIT Student Information Processing Board

Table of Content

Create Symbolic Links 🔗

ln is a command-line utility for creating links between files. By default, the ln command creates hard links. To create a symbolic link, use the -s (–symbolic) option.

The ln command syntax for creating symbolic links is as follows: ln -s [OPTIONS] FILE LINK

ln -s source_file symbolic_link
ln -s ../README.md ./README.md

src 👉 Create Symbolic Links

Extract a substring from a string

With grep -Eo and a regex pattern

grep -Eo "http[^ ']+" <<EOF
curl 'https://cloud.sdu.dk/api/ingresses/browse?itemsPerPage=100&includeOthers=true&sortDirection=ascending'
-H 'Accept-Language: en-US,en;q=0.5'
-H 'Accept-Encoding: gzip, deflate, br'
-H 'Authorization: Bearer ****'
-H 'Content-Type: application/json; charset=utf-8'
-H 'Cookie: refreshToken=cf6220bd%2D650c%2D4a48%2D9836%2D58ea9ecb7567'
-H 'Sec-Fetch-Dest: empty'
-H 'Sec-Fetch-Mode: cors'
-H 'Sec-Fetch-Site: same-origin'
-H 'Pragma: no-cache'
-H 'Cache-Control: no-cache'
EOF

will output

https://cloud.sdu.dk/api/ingresses/browse?itemsPerPage=100&includeOthers=true&sortDirection=ascending

src 👉 stackoverflow.com

Extract a value from a file

Example of a pyproject.toml file

[tool.poetry]
name = "docs"
version = "0.1.0"
description = "Collection of Cheatsheets 🗒️ PostgreSQL 🐘, Git..."
repository = "https://github.com/JV-conseil/docs"

With grep and sed

grep -Eo "^repository.+$" pyproject.toml | sed -E 's/^repository = "(.+)"$/\1/'

will output

https://github.com/JV-conseil/docs

Padding characters with printf

for i in $(seq -f "%04g" 1 470)
do
  echo $i
done

will output

0001
0002
...
0469
0470

src 👉 How to zero pad a sequence of integers in bash so that all have the same width

printf '%0.1s' "."{0..10} will output ...........

src 👉 Padding characters in printf and printf syntax

Read a JSON stream

With jq

curl 'https://cloud.sdu.dk/api/ingresses/browse?itemsPerPage=100&includeOthers=true&sortDirection=ascending' \
-H 'Accept-Language: en-US,en;q=0.5' \
-H 'Accept-Encoding: gzip, deflate, br' -H 'Authorization: Bearer *****' \
-H 'Content-Type: application/json; charset=utf-8' \
--compressed | jq '.items[].specification.domain' >~/Downloads/example.json

will output

"app-9a7f8023b8b09392140f3ff9f12c91f2.cloud.sdu.dk"
"app-githubbing.cloud.sdu.dk"
"app-health-check.cloud.sdu.dk"
"app-health-status.cloud.sdu.dk"
"app-mission-ocean.cloud.sdu.dk"
"app-research-funding.cloud.sdu.dk"
"app-santa-maria-josefina-do-coracao-de-jesus-sancho-de-guerra.cloud.sdu.dk"
"app-thalassa.cloud.sdu.dk"
"app-yerun.cloud.sdu.dk"

src 👉 jq/manual

Replace multiline text with Bash

With ed line editor replace / update a part of multiline text in a text file with automatically generated content (eg: Table of contents) 👇

_lines=""

for i in {1..10}; do
  _lines+=$'\n'"${i}. ""$(openssl rand -hex 12)"
done

ed -s "./sample.md" <<EOF
/## BEGIN GENERATED/+,/## END GENERATED/-d
/## BEGIN GENERATED/a
${_lines}

.
wq
EOF

src 👉 gist

Throbber & Spinner

A throbber, also known as a loading icon, is an animated graphical control element used to show that a computer program is performing an action in background — wikipedia

# Usage: throbber & my_long_process ; kill %-1
throbber() {
  printf "Please wait..."
  while true; do
    printf "."
    sleep .3
  done
}

Add a display to let user wait in bash

  • kill %-1 will kill the n-1'th backgrounded process
throbber &
sleep 10
kill %-1

will output

Please wait.............

Spinner alternative

alias spinner='while :; do for s in / - \\ \|; do printf "\n$s"; sleep .1; done; done'

src 👉 How to kill the (last - 1) PID and How to Write Better Bash Spinners

Wait for several subprocesses to complete

declare -A pids=()

# run processes and store pids in array
for i in {1..5}; do
  sleep "${i}" &
  pids["${i}"]=$!
done

# wait for all pids
for pid in "${pids[@]}"; do
  wait "${pid}"
done

src 👉 Wait for several subprocesses to complete

Array Slicing

Method Syntax Description Example
Basic Slice array[@]:start:length Extract slice from start index with specified length fruits[@]:1:3
From Start array[@]:0:length Get first n elements fruits[@]:0:3
From Index array[@]:start Get from start index to end fruits[@]:2
Last N array[@]: -n Get last n elements fruits[@]: -2
Negative Indexing array[@]: -n:m Get m elements from end fruits[@]: -3:2
Full Array array[@]:0:${#array[@]} Get entire array fruits[@]:0:${#fruits[@]}
Conditional Slice array[@]:start:${#array[@]}-start Slice with dynamic length fruits[@]:1:${#fruits[@]}-1

Key Points:

  • start is 0-based index
  • length is optional (defaults to end of array)
  • Negative indices count from end (e.g., -1 is last element)
  • Slice creates new array, doesn't modify original
  • Use local -n arr_ref="array_name" for indirect access
  • Can be used with mapfile for more complex operations

Example Usage:

# Initialize array
fruits=("apple" "banana" "cherry" "date" "elderberry" "fig")

# Extract first 3 elements
first_three=("${fruits[@]:0:3}")

# Extract from index 2 to end
from_two=("${fruits[@]:2}")

# Extract last 2 elements
last_two=("${fruits[@]: -2}")

# Extract 2 elements starting from index 1
slice=("${fruits[@]:1:2}")

Tools for Shell Script Development ⚙️

ShellCheck ShellCheck
Integrates ShellCheck into VS Code, a linter for Shell scripts
shell-format shell-format
A formatter for shell scripts implementing shfmt parser, formatter, and interpreter
See Google Shell Style Guide

General 📚

Features 🔎

Coding Guidelines, Style, Linter ✍️

Sponsorship

If this project helps you, you can offer me a cup of coffee ☕️ :-)

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