check if a file is jpeg format using shell script

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

Try (assumes Bash v3.0+, using =~, the regex-matching operator):

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

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

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

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

Alternatively, if you’d rather use a globbing-style pattern:

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

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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 ...

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

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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 [[ ... ]].}