Bash Scripting

Shebang

#!/bin/bash   # bash shell binary execution location

Variables

public_var="value" 
local local_var="local_value"  # local variable inside function
echo $public_var        # access variable value
echo $0               # access script name
echo $1               # access first script argument
echo $@               # access all script arguments
echo $#               # access number of script arguments
echo $$               # access current script PID
echo $?               # access last command exit code

Arrays

Indexed

array=("value1" "value2" "value3")
echo ${array[0]}        # access first element
echo ${array[@]}        # access all elements

Associative

declare -A assoc_array=(
    ["key1"]="value1"
    ["key2"]="value2"
)
echo ${assoc_array["key1"]}         # access value by key
echo ${!assoc_array[@]}             # access all keys
echo ${assoc_array[@]}              # access all values

Loops

Numeric (c-style) - fastest loop

for ((i=1; i<=10; i++)); do
    echo "$i"
done

Array

item_list=("item1" "item2" "item3")

for items in "${item_list[@]}"
do
    echo $items
done

Infinite

while :; do # same as while true but faster
    echo "Looping..."
    sleep 1
done

If Statements

[ ] - POSIX test (portable, works in sh)

if [ "$var" -eq 5 ]; then
    echo "Equal to 5"
fi

if [ "$str" = "hello" ]; then    # use = not == for POSIX compliance
    echo "Matched"
fi

[[ ]] - Bash extended test (recommended in bash scripts)

if [[ "$str" == "hello" ]]; then    # no word splitting/globbing, supports pattern matching
    echo "Matched"
fi

if [[ "$str" =~ ^[0-9]+$ ]]; then   # regex matching
    echo "Numeric"
fi

if [[ -f "$file" && -r "$file" ]]; then   # && / || work directly inside [[ ]]
    echo "File exists and is readable"
fi

(( )) - Arithmetic evaluation

if (( var == 5 )); then        # C-style comparison, no $ needed on variables
    echo "Equal to 5"
fi

if (( var > 0 && var < 10 )); then
    echo "In range"
fi

test - Command form (identical to [ ])

if test "$var" -eq 5; then
    echo "Equal to 5"
fi

Common test flags

-e file   # exists
-f file   # regular file
-d file   # directory
-r file   # readable
-w file   # writable
-x file   # executable
-z string # string is empty
-n string # string is non-empty

Short-circuit form (no if block)

[ -f file.txt ] && echo "exists"     # runs on success
[ -f file.txt ] || echo "missing"    # runs on failure

Exit Codes

0 = Match Found / Success

exit 0

echo $?: 0

1 = No Match Found

exit 1

echo $?: 1

2 = Syntax or File Error

echo "Something went wrong" >&2
exit 2

echo $?: 2

Avoid Error Stopping Script Execution

option 1 (fastest):

git clone hello.git || :

option 2:

set +e
git clone hello.git
set -e

option 3:

if ! git clone hello.git; then
    echo "Clone failed, continuing anyway..."
fi

option 4:

git clone hello.git || /bin/true

Debug Mode

set -x  # enable debug mode
set +x  # disable debug mode