Improve bash skills through step by step examples.

bash-by-examples.md

Bash by examples

Table of Contents

• variable
• input
• print
• conditional
• commandline arguments
• function
• array
• file check

1. variable

syntax: KEY=VALUE

# General declaration
HELLO="HELLO"
HELLO2=$HELLO
NUMBER=1234
NUMBERS=(1 2 3 4)

# Numerical declaration
SOME_NUMBER=$(( $NUMBER + 2 + 3 ))
echo $SOME_NUMBER # prints 1239
A=$(( 3 * (2+1+(1+5)) ))
echo $A # prints 27

# function-scope local variable
function foo {
  local LOCAL_VARIABLE=1234
}

# Check variable is set or not set
if [[ -z "$SOME_VARIABLE" ]]; then
  echo "Error: variable is not set"
fi

# Unsetting variable from bash
ITEM="hello"
unset ITEM # dispose variable

# Variable as key-value store
declare -A SOME_HASHMAP # bash4 feature
SOME_HASHMAP["KEY1"]=1234
SOME_HASHMAP["KEY2"]="OH"
echo $SOME_HASHMAP # prints nothing
echo ${SOME_HASHMAP["KEY1"]} # prints 1234

2. input

bash offers builtin function read.

read A # assign input to variable A
echo -n "Enter name: " && read A # print prompt 'Enter name: ' then assign input to variable A
read -s A # disable echoing. useful to get password

3. print

GNU coreutils has echo and printf utilities. If you have to format complex string, use printf. it will work as you expected.

echo "hello" # prints "hello"
echo "$A" # prints variable A. if A is not set, it is replaced with empty string
echo -n "$A" # prints variable A without trailing newline
echo -e "\n\n$A" # enable escaping + prints variable A

4. conditional

if [[ $varA -eq 1 ]] && [[ $varB == 't1' || $varC == 't2' ]]; then 
  scale=0.05
fi

if (( $varA == 1 )) && [[ $varB == 't1' || $varC == 't2' ]]; then 
fi

5. commandline arguments

6. function

# decl
function foo {
  echo "foo!"
  echo "First ARG: $1"
}
# call
foo "some arg"

7. array

# declaration
AA=(1 2 3 4)

# getter
echo ${AA[0]}
echo ${AA[1]}

# setter
AA[4]=5

# command output to array
FILES=(`ls`) # Don't forget to wrap output with parentheses
FIRST_FILE=${FILES[0]}
# array to strings
ARR=(`ls`)
STR=${ARR[@]}
# substring
STRING="hello"
SUBSTRING=${STRING:0:2} # he
SUBSTRING=${STRING::2} # he
SUBSTRING=${STRING:1:2} # el
SUBSTRING=${STRING:2} # llo

8. file check

file test operators

• -e: exists
• -f: is_file
• -s: has_size
• -d: is_directory
• -b: is_block_device
• -c: is_char_device
• -p: is_pipe
• -h: is_symbolic
• -L: is_symbolic
• -S: is_socket
• -t: is_terminal
• -r,-w,-x: can_read,can_write,can_execute
• -g: has_sgid
• -u: has_suid
• -k: has_sticky_bit
• -O: has_ownership
• -G: has_group_ownership
• -N: is_changed