bash_goodies

bash_goodies.txt

# -------------------------
# Bash stuff 
# -------------------------

Never understood arrays in bash (even though they were added in bash2.0 - 2009) 
This explains them:

(can you compare bash to perl5? 
 No, when declaring a data structure in bash, then you cannot nest a map inside an array or map.
 Also runs slower - bash interpreted line by line, instead of bytecode translation/execution of perl5
)

# you don't need to declare arrays with integer keys before use (but you do for hash maps/associative arrays)
# fun part: keys can have gaps and there is 'index out of range'
$ b[0]=1
$ b[2]=2

# shows all values (but it doesn't show, which keys are set...)
$ echo "${b[@]}"
1 2

# this sets the set key values!
$ echo "${!b[@]}"
0 2

# show number of set entries in array - sort of like len(arr) in python
$ echo "${#b[@]}"
2

# access a an entry by index
$ echo "${b[0]}"
1

# this guy is not set, the array can have gaps!
$ echo "${b[1]}"

$ echo "${b[2]}"
2

# assign an arrays with a bunch of values
$ c=(1 2 3 4 5 6)

# iterate over the assigned array
$ for x in ${c[@]}; do echo $x; done
1
2
3
4
5
6

# show first element
$ echo ${c[0]}
1

# show the whole array
$ echo ${c[@]}
1,2,3,4,5,6

# and you can have slices too!

$ echo ${c[@]:2}
3 4 5 6

$ echo ${c[@]:1:2}
2 3

$ echo ${c[@]:1:3}
2 3 4

# -----

Since bash 4.0 (released in 2009) we have hash maps in bash! almost a complete perl5...

--
# Initialize the empty map
#declare -A my_map

# Initialize a map with some values (note: you need to have my_map=( without spaces!)
declare -A my_map=(
 ['Star']='cat'
 ['Chucha']='cat'
)

# Add or update key-value pairs
my_map['Michael']='Moser'
echo "Michael maps to ${my_map['Michael']}"

echo "Iterate over key-value pairs"

for user in "${!my_map[@]}"; do
    role="${my_map[${user}]}"
    echo "User: $user | Role: $role"
done

echo "It is not an error to access nonexisting key - returns empty string"
echo "${my_map['nonexist']}"

# check if key exists
if [[ -v my_map['Star'] ]]; then 
    echo "key Star exists in map my_map"
fi

# Delete a key-value pair
unset my_map[role]
--

# -------------------------

# didn't know that:
# in bash: 
# |& 
# is the same as 
# 2>&1 |

$ cat no-such-file.txt | grep 'No such file' | wc -l
cat: no-such-file.txt: No such file or directory
0

$ cat no-such-file.txt 2>&1 | grep 'No such file' | wc -l
1

$ cat no-such-file.txt |& grep 'No such file' | wc -l
1

# - useless stuff, it is easy to mix up |& with &|
$ - the second one runs the command in the background.
# - better do 2>&1 |