bash & structured programming
InstallFest 2018

Lukáš Bařinka

Barinka does not simply be in time with his talk.

Introduction
Blocks of Commands
Functions
Functions Vs ...
Scope of Variables
Settings and Debugging

Length of spaghetti code is too damn high.

Clarity, Quality, Development Time? - Structured Programming.

Brace Yourself, Structured Programming is Comming.

(list) # in sub-shell

{ list; } # in current shell

{ read first; read second; } <file

tar cf - dir | (cd /tmp; tar xf -)

Different syntaxes for function creation.

error() { echo "$0[error]: $@"; exit 1; } >&2
warn() { echo "$0[warn]: $@"; } >&2
info() { ((VERBOSE)) && echo "$0[info]: $@"; } >&2

debug() {
    local var quoted
    for var do
        quoted=${!var//\'/\'\\\'\'}
        info "$var='$quoted'"
    done }

return [n]
exit [n]

f() {
    trap date RETURN
    sleep 5
}

What if I told you, for almost every purpose aliases are superseded by shell functions.

Functions are more powerful, but aliases are simplier to use.

alias ls='ls --color=auto'
alias ll='ls -l'


ls() { command ls --color=auto "$@"; }
ll() { ls -l "$@"; }

If you can redefine built-in command, that would be great.

cd() {
    echo "OLD: $OLDPWD"
    builtin cd "$@"
    E=$?
    echo "NEW: $PWD"
    return $E
}

You don't have to reset you variables if you run separate script.

I'll create my own context, with variables and parameters.

f() {
    local i
    for i
    do
        echo "$0:$i
    done
}
f "$@"

get_id() (
    set -o pipefail
    getent passwd "$1" \
    | cut -d: -f3
)
uid=$(get_id user)
found=$?

eval for call by reference, well that escalated quickly.

list() { eval '
    for i in "${!'$1'[@]}"
    do
        printf -- "[%s] -> %s\n" \
            "$i" "${'$1'["$i"]}"
    done'
}
declare -A a=([1st]=foo [2nd]=bar)
list a

array_cp() {
    declare -gA "$2=()"
    declare -n from=$1 to=$2
    local i
    for i in "${!from[@]}"
    do
        to[$i]=${from[$i]}
    done
}
declare -A a=([1st]=foo [2nd]=bar)
array_cp a b

inc() {
    declare -n var=$1 R=BASH_REMATCH
    [[ $var =~ (.*)_([0-9]+)\.(.*) ]] \
    && printf -- '%s_%03d.%s' \
        "${R[1]}" \
        $((10#${R[2]}+1)) \
        "${R[3]}"
}
file=/var/tmp/backup_data_021.tgz
inc file

# DEBUG / RETURN
set -o functrace
declare -tf f_name

# ERR
set -o errtrace

FUNCNEST=0
export -f f_name

So, you're telling me you understand everything and have no questions?

Not upvoting this talk, that's paddlin'.

bash & structured programmingInstallFest 2018

bash & structured programming
InstallFest 2018