check if a file is jpeg format using shell script

Question

I know I can use file pic.jpg to get the file type, but how do I write an if statement to check it in a shell script?

E.g. (pseudo code):

if pic.jpg == jpeg file then

Answer 1

Try (assumes bash (v3.0+), zsh, or ksh) , using =~, the regex-matching operator):

if [[ $(file -b 'pic.jpg') =~ JPEG ]]; then ...; fi

If you want to match file's output more closely:

if [[ $(file -b 'pic.jpg') =~ ^'JPEG ' ]]; then ...; fi

This will only match if the output starts with 'JPEG', followed by a space.

Alternatively, if you'd rather use a globbing-style (wildcard) pattern:

if [[ $(file -b 'pic.jpg') == 'JPEG '* ]]; then ...; fi

---

POSIX-compliant conditionals ([ ... ]) do not offer regex or pattern matching, so a different approach is needed:

if expr "$(file -b 'pic.jpg')" : 'JPEG ' >/dev/null; then ...; fi

Note: expr only supports basic regular expressions and is implicitly anchored at the start of the string (no need for ^).

Alternatively, use a case statement, which does support pattern matching per POSIX, as also shown in triplee's helpful answer:

case "$(file -b 'pic.jpg')" in
  'JPEG '*)
      # ...
      ;;
esac

---

^{As for why [[ ... ]] rather than [ ... ] is needed in the Bash snippets:
Advanced features such as the regex operator (=~) or pattern matching (e.g., use of unquoted * to represent any sequence of chars.) are nonstandard (not part of the POSIX shell specification).
Since these features are incompatible with how standard conditionals ([ ... ]) work, a new syntax was required; Bash, Ksh, and Zsh use [[ ... ]].}

Answer 2

Good old case is worth a mention, too.

case $(file -b pic.jpg) in
  'JPEG '*)
    echo is
    ;;
  *)
    echo is not
    ;;
esac

The lone right parentheses might seem uncanny at first, but other than that, this is reasonably simple, readable, versatile, and portable way back to the original Bourne shell. (POSIX allows for a matching left parenthesis before the expression, too.)