Bash

Table of Contents

1. Shell movements

 Shell movements
 ===============

Cursor position represented by []
---------------------------------

 ctrl-a <-----------------> ctrl-e  | moving
  alt-b    <-------->        alt-f  |
 ctrl-b       <-->          ctrl-f  |

      $ mv file[].txt dir/

 ctrl-w    <-------->       alt-d   | erasing
 ctrl-u <-----------------> ctrl-k  |

2. Interactive Shells

3. Bash Startup Files

3.1. Prevent autostart for programs when inside an Emacs shell

To avoid running programs that may not render well within an Emacs shell when loading ~/.bashrc etc 

if [[ -z $INSIDE_EMACS ]]
then
    # Things to do when not using a shell inside Emacs
fi

4. Shell Parameter Expansion (/#, ##, %% etc)

4.1. Return the same value from 2 different strings

youtube_link_channel="https://yewtu.be/feed/channel/foo"
# OR
youtube_link_channel="https://www.youtube.com/feeds/videos.xml?channel_id=foo"

# both youtube_link_channel return "foo"
youtube_channel_id="${youtube_link_channel##*[=/]}"

5. Redirections (>, >>, <, 2>&1 etc)

6. List function names and bindings

bind -p

7. Bind a function named water to M-s f

bind -x '"\esf":water'

8. Display all locations containing an executable

type -a foo

9. Print the commands as they are executed

++ in the output mean forking/subshell 

set -x
<commands>

bash -x <file>

10. Check for unset variables

bash -u <file>

11. Pipe status exit codes

false | true | false | false
echo "${PIPESTATUS[*]}"

12. Date using printf builtin

local date
printf -v date '%(%Y-%m-%d)T'
echo -n "$date"

13. Array of files basename

fn() {
    local files=(/path/to/files/file*)
    files=("${files[@]##*/}")

    declare -p files
}

14. Unset values in array

# Iterate over the green blocklists
for green_blocklist in "${green_blocklists[@]}"
do
    # Remove green blocklists from the blue blocklists array
    for index in "${!blue_blocklists[@]}"
    do
        if [[ ${blue_blocklists[index]} == "$green_blocklist" ]]
        then
            unset 'blue_blocklists[index]'
            blue_blocklists=("${blue_blocklists[@]}")
        fi
    done
done

15. Example with trap builtin

cleanup() {
    echo "bar"
}

main() {
    trap cleanup EXIT
    echo "foo"
    some-command
}

main

16. Change prompt displayed (PS3) in a select menu

some_menu() {
    local PS3="Choose value: "
    local values=("foo" "bar")
    local v
    select v in "${values[@]}"
    do
        echo "Value: ${v}"
        break
    done
}

17. Debug functions with set -x and PS4

Single quotes for PS4 prevent early expansion of its value. 

foo() {
    echo "hello"
}

bar() {
    echo "world"
}

main() {
    local PS4='+<${BASH_SOURCE[0]}><${BASH_LINENO[0]}><${FUNCNAME[0]}> '
    set -x
    foo
    bar
}

main

+<./foo><19><main> foo
+<./foo><15><foo> echo hello
hello
+<./foo><19><main> bar
+<./foo><16><bar> echo world
world

18. Functions

18.1. Wrap command (alternate buffer)

Send journalctl output to an alternate buffer, see notes:ansi for the escape sequences. 

journalctl() {
    printf '\e[?1049h'
    printf '\e[?25l'
    printf '\e[H'
    command journalctl "${@}"
    printf '\e[?1049l'
    printf '\e[?25h'
}

18.2. Print 256 colors

lscolors() {
    local i
    tput bold
    for ((i=0; i<256; i++))
    do
        # tput setaf "${i}"
        # printf 'color %s\n' "${i}"
        printf '\x1b[38;5;%dmcolor %d\n' "${i}" "${i}"
    done | column
    tput sgr0
}

18.3. Draw a colorful line

draw_line() {
    local color symbol nc n
    color="${1:-34}" # default to `blue'
    symbol="${2:--}" # default to `-'
    nc="$(tput cols)"
    for ((n=0; n<nc; n++))
    do
        printf '\x1b[1;%dm%s\x1b[0m' "${color}" "${symbol}"
    done
}

19. One liners

19.1. Find dotfiles packages not managed by Ansible

while read -r pkg; do if grep -rq "$pkg" ~/projects/ansible-dotfiles; then echo "pkg found: $pkg"; else echo "pkg not found: $pkg"; fi done < ~/.dotfiles/.pkg*

19.2. Flatpak remove all document ids for exported files

while read -r id; do flatpak document-unexport --doc-id "$id" && echo "done $id"; done < <(flatpak documents)

20. References

dim. 04 oct. 2026

Generated with Emacs 30.1 (Org mode 9.7.11)