bash check if a string contains a substring ([[ $str == *sub* ]])

In bash, [[ $string == *"$substring"* ]] is true when $substring occurs anywhere in $string. The * on both sides are glob wildcards and must stay outside the quotes; the quotes around the variable make its content literal. For POSIX sh, use a case statement with the same pattern, and for regular expressions use [[ $string =~ regex ]].

Ways to test

str="Error: connection refused (port 5432)"
sub="refused"

[[ $str == *"$sub"* ]] && echo "1. [[ == *sub* ]]: found"
[[ $str == "*$sub*" ]] || echo "2. quoted pattern: not found (compares the literal *refused*)"

case "$str" in
  *"$sub"*) echo "3. case: found" ;;
  *) echo "3. case: not found" ;;
esac

[[ $str =~ port\ ([0-9]+) ]] && echo "4. regex: port is ${BASH_REMATCH[1]}"

[[ ${str,,} == *"error"* ]] && echo "5. case-insensitive with \${str,,}: found"

if grep -qF -- "$sub" <<< "$str"; then echo "6. grep -qF: found"; fi

[[ $str == *"timeout"* ]] || echo "7. timeout: not present"

1. [[ == *sub* ]]: found
2. quoted pattern: not found (compares the literal *refused*)
3. case: found
4. regex: port is 5432
5. case-insensitive with ${str,,}: found
6. grep -qF: found
7. timeout: not present

All seven tests behave as their labels say. The second line is the most common mistake: with the whole pattern in quotes, bash looks for the literal characters *refused*.

Options

Pitfalls

• [ ] instead of [[ ]]. In single brackets == does no pattern matching, and [ $str == *sub* ] lets the shell expand *sub* against file names, which can fail with too many arguments.
• sh scripts. A script run with sh script.sh (dash on Debian and Ubuntu) fails on [[ with [[: not found. Run it with bash or use case.
• macOS bash 3.2. ${str,,} is Bash 4 syntax; on bash 3.2 it fails with bad substitution. Use tr '[:upper:]' '[:lower:]' or shopt -s nocasematch.
• Quoted regex. [[ $s =~ "port [0-9]+" ]] matches the literal text; put the regex in a variable (re='port ([0-9]+)'; [[ $s =~ $re ]]) to keep it readable.
• Special characters in $sub. With the variable quoted, *, ? and [ in it are matched literally, which is usually what you want.

Confirm it worked

Print a message in both branches (&& echo found || echo missing) and test with a value that contains the substring, one that does not, and one that differs only in case.

FAQ