CIS241

System-Level Programming and Utilities

bash Loops and Arrays

Erik Fredericks, frederer@gvsu.edu
Fall 2025

Based on material provided by Erin Carrier, Austin Ferguson, and Katherine Bowers

For loops - similar to Python

for var in list; do
  # loopy things
done

Samples:

for var in 1 2 3 4 5
for var in "alice" "bob" "erik"
for val in {0..10..2}
for var in $@

Syntax note

Remember that $(cmd) will run cmd

var=$(ls)

for idx in $(seq 0 10); do
  echo $idx
done

While loops

while CONDITION; do
  # while things
done

CONDITION? Same as if!

Fun use: parsing arguments

getopts

Flags/switches

• E.g., -f -r -t
  Option argument
• E.g., -d /path/to/dir
  Other arguments
• E.g., "this is a string being passed in as a single argument"
  Example:
• sh script.sh –f –r –t –d /path/to/dir "this is that string I mentioned"

Parsing arguments

OPTSTRING

• Define legal argument options

VARNAME

• Which variable to store argument data within

getopts OPTSTRING VARNAME
getopts abc:d
Allows:

• -a -b -c <SOME DATA> -d

getopts :abc:d

• Suppress error output (first colon)

Parsing arguments with getopts

Parses arguments in order

• Looping over getopts (calling multiple times) common

while getopts :ci opt; do
  case $opt in
    c)
      echo “-c found” ;;
    i)
      echo “-i found” ;;
    \?)
      echo “invalid option -$OPTARG” >&2 ;;
  esac
done

Add colon (:) after flag to indicate an expected value

• E.g., t:

while getopts :csi: opt; do
  case $opt in
    c)
      echo “-c found” ;;
    s)
      echo “-s found” ;;
    i)
      echo “-i found with argument $OPTARG” ;;
    \?)
      echo “invalid option -$OPTARG” >&2 ;;
  esac
done

Reading user input

User input : read from command line

echo –n "Enter your name: "  # -n skips the newline
read your_name
echo "Hello $your_name"

read by default dumps everything into single variable ($your_name in this case)

read –p "Enter your name: " first_name last_name
echo “Your name is $first_name $last_name

Timeout (or, not waiting for those pesky users)

May wish to provide a way to give a default response if the user isn’t watching

read –t 5 –p "Should we auto-continue [Y/n]? " answer
case $answer in
  N | n)
    echo "Exiting..."
    exit
    ;;
  Y | y | *)
    echo
    echo "Continuing on..."
    ;;
esac

Arrays

Creating arrays

arr=()           # Empty array
declare -a arr_2 # Also empty
arr_3=(10 8 6)   # Create with values

Setting/Adding values

arr[0]=test  # Set value
arr_2+=(87)  # Append value
arr_3+=(4 2) # Append values

Accessing arrays

Accessing:

${arr[0]} # Access nth element

Special accessing:

${arr[@]}           # All elements
${arr[*]}           # All elements as string
${#arr[@]}          # Count of elements
${!arr[@]}          # Indices of elements
${arr[@]:start:num} # Slice of array

Iterating over arrays (hopefully a reminder)

for x in ${arr[@]}; do
  echo "$x"
done

Associative arrays (maps/dictionaries)

declare -A dnd_stats
dnd_stats=(
  [str]=16
  [dex]=10
  [con]=18
  [int]=11
  [wis]=10
  [cha]=14
)
echo "wisdom = ${dnd_stats[wis]}"
dnd_stats[wis]=$((dnd_stats[wis] + 2))
echo "wisdom = ${dnd_stats[wis]}"