bash / string-comparison

bash string comparison — equality, contains, starts with

Quick Answer

# Equal
[[ "$a" == "$b" ]] && echo "equal"

# Not equal
[[ "$a" != "$b" ]] && echo "different"

# Empty / not empty
[[ -z "$var" ]] && echo "empty"
[[ -n "$var" ]] && echo "not empty"

# Contains (glob pattern)
[[ "$str" == *"substr"* ]] && echo "contains"

Usage

String comparison in bash has many pitfalls. Use [[ ]] (not [ ]) for most cases.

Other causes & fixes

Regex match with =~

# Check if string matches a regex
if [[ "$email" =~ ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$ ]]; then
  echo "valid email"
fi

# Access capture groups via BASH_REMATCH
if [[ "2024-01-15" =~ ^([0-9]{4})-([0-9]{2})-([0-9]{2})$ ]]; then
  echo "year=${BASH_REMATCH[1]}"
fi

Starts with / ends with

# Starts with "prefix"
[[ "$str" == prefix* ]] && echo "starts with prefix"

# Ends with ".log"
[[ "$str" == *.log ]] && echo "ends with .log"

Case-insensitive comparison

shopt -s nocasematch
[[ "Hello" == "hello" ]] && echo "match"
shopt -u nocasematch  # restore

String length

str="hello"
echo ${#str}   # 5

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