Builtins
Declare
The declare builtin is used to explictly declare variables
declare -r varis to declare a read-only variable
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declare -r var="123"
var="456" # see "var: readonly variable"
declare -a indexed_arraydeclare -A associative_array-
declare -i integer_value -
declare -x exported_var="exported var"can also define and export a variable - It can also print all variable (custom & environment variables) values.
declare -p
Though a most variables (except for arrays) can be defined directly VAR=123, I found declare more “type-safe”.
Data Structures
Array
Bash has 1D indexed array (the common array we normally see), and associative array. Any variable can be used as an array. To declare an array, use declare.
Bash Array
An indexed array is “common”. However, it doesn’t guarantee that items are stored contiguously.
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#!/bin/bash
declare -a array1 array2 # one can declare multiple variables
array1=("apple" "peach")
echo ${array1[1]} # see apple,
echo ${array1[@]} # see apple,peach
array2=("pear")
# $array2 is the first element in array2 only, similar to c.
# array1+=$array2 #applepear peach
# This is also applepear peach
# array1+="${array2[@]}"
array1+=("${array2[@]}") #see apple, peach, pear ?
echo ${array1[@]} # see apple,peach
array1+=("pecan")
echo ${array1[@]} #see apple, peach, pear, pecan
Associative Array
A bash associative array is similar to a python dictionary. E.g., below is an excerpt of a script that downloads ROS bags
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declare -A bags_link_lookup #?
# This is very dictionary-like
bags_link_lookup["data/rgbd_dataset_freiburg1_xyz.bag"]="https://cvg.cit.tum.de/rgbd/dataset/freiburg1/rgbd_dataset_freiburg1_xyz.bag"
bags_link_lookup["data/rgbd_dataset_freiburg1_rpy.bag"]="https://cvg.cit.tum.de/rgbd/dataset/freiburg1/rgbd_dataset_freiburg1_rpy.bag"
echo "Download bag files if they are missing ..."
for key in "${!bags_link_lookup[@]}"; do
if [[ -f "${key}" ]]; then
echo "No need to download dataset ${key}..."
else
echo "Downloading dataset ${key}"
wget -P data "${bags_link_lookup[$key]}"
fi
done
echo "Done Downloading"
