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

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^{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 [[ ... ]].}