mirror of
https://gitea.com/mcereda/oam.git
synced 2026-02-09 05:44:23 +00:00
284 lines
8.3 KiB
Markdown
284 lines
8.3 KiB
Markdown
# Bourne Again SHell
|
|
|
|
1. [TL;DR](#tldr)
|
|
2. [Startup files loading order](#startup-files-loading-order)
|
|
3. [Functions](#functions)
|
|
4. [Substitutions](#substitutions)
|
|
1. [!! (command substitution)](#-command-substitution)
|
|
2. [^^ (caret substitution)](#-caret-substitution)
|
|
5. [Here documents](#here-documents)
|
|
6. [Keys combinations](#keys-combinations)
|
|
7. [Check if a script is sourced by another](#check-if-a-script-is-sourced-by-another)
|
|
8. [Gotchas](#gotchas)
|
|
1. [Exist statuses of killed commands](#exist-statuses-of-killed-commands)
|
|
2. [Go incognito](#go-incognito)
|
|
9. [Further readings](#further-readings)
|
|
10. [Sources](#sources)
|
|
|
|
## TL;DR
|
|
|
|
```sh
|
|
# Declare arrays.
|
|
ARRAY=(
|
|
"first_element"
|
|
"second_element" "nth_element"
|
|
)
|
|
|
|
# Get the length of arrays.
|
|
# A.K.A. number of elements.
|
|
ARRAY_LEN=${#ARRAY[@]}
|
|
|
|
# Access all elements in arrays with referencing.
|
|
echo ${ARRAY[@]}
|
|
echo ${ARRAY[*]}
|
|
|
|
# Access the last value of arrays.
|
|
echo ${ARRAY[-1]}
|
|
echo ${ARRAY: -1}
|
|
|
|
# Get a slice of 4 elements from an array.
|
|
# Start from the element with index number 2.
|
|
echo ${ARRAY:2:4}
|
|
|
|
# Declare functions.
|
|
functionName () { … }
|
|
function functionName { … }
|
|
|
|
# Declare functions on a single line.
|
|
functionName () { command1 ; … ; command N ; }
|
|
|
|
# Get all the arguments in input.
|
|
echo $@
|
|
|
|
# Of all the arguments in input, return only those which are existing directories.
|
|
DIRECTORIES=()
|
|
for (( I = $# ; I >= 0 ; I-- )); do
|
|
if [[ -d ${@[$I]} ]]; then
|
|
DIRECTORIES+=${@[$I]}
|
|
else
|
|
local COMMAND="${@:1: 1-$I}"
|
|
break
|
|
fi
|
|
done
|
|
|
|
# Print all shell and environment variables.
|
|
( set -o posix ; set )
|
|
|
|
# Print exported variables only.
|
|
export -p
|
|
|
|
# Run a command or function on exit, kill or error.
|
|
trap "rm -f $tempfile" EXIT SIGTERM ERR
|
|
trap function-name EXIT SIGTERM ERR
|
|
|
|
# Disable CTRL-C.
|
|
trap "" SIGINT
|
|
|
|
# Re-enable CTRL-C.
|
|
trap - SIGINT
|
|
|
|
# Bash 3 and `sh` have no built-in means to convert case of a string, but the
|
|
# `awk`, `sed` or `tr` tools can be used instead.
|
|
echo $(echo "$name" | tr '[:upper:]' '[:lower:]' )
|
|
echo $(tr '[:upper:]' '[:lower:]' <<< "$name")
|
|
|
|
# Bash 5 has a special parameter expansion for upper- and lowercasing strings.
|
|
echo ${name,,}
|
|
echo ${name^^}
|
|
|
|
# Add a clock to the top-right part of the terminal.
|
|
while sleep 1
|
|
do
|
|
tput sc;
|
|
tput cup 0 $(($(tput cols)-29))
|
|
date
|
|
tput rc
|
|
done &
|
|
|
|
# Show a binary clock.
|
|
watch -n 1 'echo "obase=2; `date +%s`" | bc'
|
|
|
|
# Fork bomb.
|
|
:(){ :|: & };:
|
|
```
|
|
|
|
## Startup files loading order
|
|
|
|
On startup:
|
|
|
|
1. (if login shell) `/etc/profile`
|
|
1. (if interactive and non login shell) `/etc/bashrc`
|
|
1. (if login shell) `~/.bash_profile`
|
|
1. (if login shell and `~/.bash_profile` not found) `~/.bash_login`
|
|
1. (if login shell and no `~/.bash_profile` nor `~/.bash_login` found) `~/.profile`
|
|
1. (if interactive and non login shell) `~/.bashrc`
|
|
|
|
Upon exit:
|
|
|
|
1. (if login shell) `~/.bash_logout`
|
|
1. (if login shell) `/etc/bash_logout`
|
|
|
|
## Functions
|
|
|
|
A function automatically returns the exit code of the last command in it.
|
|
|
|
## Substitutions
|
|
|
|
### !! (command substitution)
|
|
|
|
Substitutes `!!` with the last command in your history
|
|
|
|
```sh
|
|
$ echo 'hallo!'
|
|
hallo!
|
|
|
|
$ !!
|
|
echo 'hallo!'
|
|
hallo!
|
|
|
|
$ sudo !!
|
|
sudo echo 'hallo!'
|
|
[sudo] password for user:
|
|
hallo!
|
|
```
|
|
|
|
### ^^ (caret substitution)
|
|
|
|
Re-runs a command replacing a string.
|
|
|
|
```sh
|
|
$ sudo apt search tmux
|
|
…
|
|
|
|
$ ^search^show
|
|
sudo apt show tmux
|
|
…
|
|
```
|
|
|
|
## Here documents
|
|
|
|
A _Here document_ (_heredoc_) is a type of redirection that allows you to pass multiple lines of input to a command.
|
|
|
|
```sh
|
|
[COMMAND] <<[-] 'DELIMITER'
|
|
HERE-DOCUMENT
|
|
DELIMITER
|
|
```
|
|
|
|
- the first line must start with an **optional command** followed by the special redirection operator `<<` and the **delimiting identifier**
|
|
- one can use **any string** as a delimiting identifier, the most commonly used being `EOF` or `END`
|
|
- if the delimiting identifier is **unquoted**, the shell will substitute all variables, commands and special characters before passing the here-document lines to the command
|
|
- appending a **minus sign** to the redirection operator (`<<-`), will cause all leading tab characters to be **ignored**
|
|
this allows one to use indentation when writing here-documents in shell scripts
|
|
leading whitespace characters are not allowed, only tabs are
|
|
- the here-document block can contain strings, variables, commands and any other type of input
|
|
- the last line must end with the delimiting identifier
|
|
white space in front of the delimiter is not allowed
|
|
|
|
```sh
|
|
$ cat << EOF
|
|
The current working directory is: $PWD
|
|
You are logged in as: $(whoami)
|
|
EOF
|
|
The current working directory is: /home/user
|
|
You are logged in as: user
|
|
```
|
|
|
|
```sh
|
|
$ cat <<-'EOF' | sed 's/l/e/g' > file.txt
|
|
Hello
|
|
World
|
|
EOF
|
|
$ cat file.txt
|
|
Heeeo
|
|
Wored
|
|
```
|
|
|
|
## Keys combinations
|
|
|
|
- `Ctrl+L`: clear the screen (instead of typing `clear`)
|
|
- `Ctrl+R`: reverse search your Bash history for a command that you have already run and wish to run again
|
|
|
|
## Check if a script is sourced by another
|
|
|
|
```sh
|
|
(return 0 2>/dev/null) \
|
|
&& echo "this script is not sourced" \
|
|
|| echo "this script is sourced"
|
|
```
|
|
|
|
## Gotchas
|
|
|
|
### Exist statuses of killed commands
|
|
|
|
The exit status of a killed command is **128 + _n_** if the command was killed by signal _n_:
|
|
|
|
```sh
|
|
$ pgrep tryme.sh
|
|
880
|
|
$ kill -9 880
|
|
$ echo $?
|
|
137
|
|
```
|
|
|
|
### Go incognito
|
|
|
|
See [How do I open an incognito bash session] on [unix.stackexchange.com]
|
|
|
|
```sh
|
|
HISTFILE=
|
|
```
|
|
|
|
You can also avoid recording a single command simply preceding it with space
|
|
|
|
```shell
|
|
echo $RECORDED
|
|
echo $NOT_RECORDED
|
|
```
|
|
|
|
## Further readings
|
|
|
|
- [Trap]
|
|
- [Upper- or lower-casing strings]
|
|
|
|
## Sources
|
|
|
|
- [The Bash trap command]
|
|
- [Bash startup files loading order]
|
|
- [How to detect if a script is being sourced]
|
|
|
|
<!-- internal references -->
|
|
[trap]: trap.md
|
|
|
|
<!-- external references -->
|
|
[bash startup files loading order]: https://youngstone89.medium.com/unix-introduction-bash-startup-files-loading-order-562543ac12e9
|
|
[how to detect if a script is being sourced]: https://stackoverflow.com/questions/2683279/how-to-detect-if-a-script-is-being-sourced#28776166
|
|
[the bash trap command]: https://www.linuxjournal.com/content/bash-trap-command
|
|
[upper- or lower-casing strings]: https://scriptingosx.com/2019/12/upper-or-lower-casing-strings-in-bash-and-zsh/
|
|
|
|
<!-- FIXME -->
|
|
|
|
[add directory to $path if it's not already there]: https://superuser.com/questions/39751/add-directory-to-path-if-its-not-already-there
|
|
[append elements to an array]: https://linuxhint.com/bash_append_array/
|
|
[bash-heredoc]: https://linuxize.com/post/bash-heredoc
|
|
[bashrc ps1 generator]: http://bashrcgenerator.com/
|
|
[check if a string represents a valid path]: https://unix.stackexchange.com/questions/214886/check-if-string-represents-valid-path-in-bash
|
|
[command line arguments in a shell script]: https://tecadmin.net/tutorial/bash-scripting/bash-command-arguments/
|
|
[find out the exit codes of all piped commands]: https://www.cyberciti.biz/faq/unix-linux-bash-find-out-the-exit-codes-of-all-piped-commands
|
|
[histsize vs histfilesize]: https://stackoverflow.com/questions/19454837/bash-histsize-vs-histfilesize#19454838
|
|
[how do i open an incognito bash session]: https://unix.stackexchange.com/questions/158933/how-do-i-open-an-incognito-bash-session/158937#158937
|
|
[how to find a bash shell array length]: https://www.cyberciti.biz/faq/finding-bash-shell-array-length-elements/
|
|
[how to slice an array]: https://stackoverflow.com/questions/1335815/how-to-slice-an-array-in-bash#1336245
|
|
[is there a way of reading the last element of an array?]: https://unix.stackexchange.com/questions/198787/is-there-a-way-of-reading-the-last-element-of-an-array-with-bash#198789
|
|
[linux-terminal-trick]: https://opensource.com/article/20/1/linux-terminal-trick
|
|
[printing array elements in reverse]: https://www.unix.com/shell-programming-and-scripting/267967-printing-array-elements-reverse.html
|
|
[regular expressions in a case statement]: https://stackoverflow.com/questions/9631335/regular-expressions-in-a-bash-case-statement#9631449
|
|
[reverse an array]: https://unix.stackexchange.com/questions/412868/bash-reverse-an-array
|
|
[set]: https://ss64.com/bash/set.html
|
|
[slice of positional parameters]: https://unix.stackexchange.com/questions/82060/bash-slice-of-positional-parameters#82061
|
|
[how to list all variables names and their current values?]: https://askubuntu.com/questions/275965/how-to-list-all-variables-names-and-their-current-values#275972
|
|
|
|
[linuxize.com]: https://linuxize.com
|
|
[opensource.com]: https://opensource.com
|
|
[unix.stackexchange.com]: https://unix.stackexchange.com
|