Bash Arrays and String Manipulation

Arrays#

An array holds a list of values. Brackets create one, with spaces between items:

fruits=("apple" "green mango" "kiwi")
echo "${fruits[0]}"
echo "${fruits[1]}"
echo "Count: ${#fruits[@]}"

apple
green mango
Count: 3

Numbering starts at 0. Add to an array with +=:

fruits=("apple" "kiwi")
fruits+=("guava")
echo "${fruits[@]}"
echo "${fruits[@]:1:2}"

apple kiwi guava
kiwi guava

${fruits[@]:1:2} takes two items, starting at index 1.

Loop over an array#

Quote it as "${array[@]}", so that items with spaces stay together:

servers=("web 1" "web 2")
for s in "${servers[@]}"; do
  echo "Checking $s"
done

Checking web 1
Checking web 2

Fill an array from a command#

mapfile reads lines into an array, safely:

printf 'one\ntwo\nthree\n' > items.txt
mapfile -t items < items.txt
echo "${#items[@]} items, last is ${items[-1]}"

3 items, last is three

Associative arrays#

These use text keys, like a dictionary. They must be declared with declare -A:

declare -A port
port[web]=80
port[ssh]=22
echo "ssh uses ${port[ssh]}"
for name in "${!port[@]}"; do
  echo "$name ${port[$name]}"
done | sort

ssh uses 22
ssh 22
web 80

"${!port[@]}" gives the keys. They come in no fixed order, so we sort them.

This is useful for counting:

declare -A seen
for word in red blue red red; do
  ((seen[$word]++))
done
echo "red=${seen[red]} blue=${seen[blue]}"

red=3 blue=1

String length and substrings#

text="Laviyx Learn"
echo "${#text}"
echo "${text:0:6}"
echo "${text: -5}"

12
Laviyx
Learn

${text:start:length} takes part of a string. The space before -5 is needed.

Replace text#

path="a-b-c"
echo "${path/-/_}"
echo "${path//-/_}"

a_b-c
a_b_c

One / replaces the first match, and two replace them all.

Trim from either end#

# removes from the start, and % from the end. Doubling them removes as much as possible:

These are the standard way to handle paths:

file="/var/log/app.tar.gz"
echo "${file##*/}"
echo "${file%/*}"
echo "${file%.gz}"
name="${file##*/}"
echo "${name%%.*}"
echo "${name#*.}"

app.tar.gz
/var/log
/var/log/app.tar
app
tar.gz

The first two give the same results as basename and dirname, without starting a new program.

Upper and lower case#

word="Hello"
echo "${word^^}"
echo "${word,,}"

HELLO
hello

Split a string#

Use IFS with read:

line="asha:admin:hyderabad"
IFS=: read -r user role city <<< "$line"
echo "$user lives in $city"
IFS=, read -ra parts <<< "a,b,c"
echo "${#parts[@]} parts"

asha lives in hyderabad
3 parts

<<< is a here string, which feeds one string to a command.

Common mistakes#

Practice#

files=("photo.JPG" "notes.txt" "song.mp3")
for f in "${files[@]}"; do
  ext="${f##*.}"
  echo "${ext,,}"
done

jpg
txt
mp3

Key takeaway#

Next: Writing Safe Scripts.