Understanding Bash if statement that invokes a command

Question

Does anyone know what this is doing:

if ! /fgallery/fgallery -v -j3 /images /usr/share/nginx/html/ "${GALLERY_TITLE:-Gallery}"; then
  mkdir -p /usr/share/nginx/html

I understand the first part is saying if /fgallery/fgallery directory doesn't exist but after this it it not clear for me.

It is NOT a directory check. `if` checks the return of the execution of `/fgallery/fgallery ...` If the command returns an `error` condition (e.g. NOT `0`), then `mkdir` is called. — David C. Rankin, May 17 '17 at 19:32
It means if `/fgallery/fgallery` command doesn't exit with status 0 (it fails), run the `mkdir` command. — codeforester, May 17 '17 at 19:33
Frankly, the code looks backwards. Why not simply run `mkdir` first to *ensure* the directory exists, then run `fgallery`? — chepner, May 17 '17 at 19:39
The condition in an `if` is *always* a command (possibly preceded by `!` to negate it). A directory existence test, for example, might look like `if [ -d /foo/bar ]`. In that condition, `[` is a command, nearly equivalent to the `test` command; it's not as you might reasonably assume, a built-in part of the shell's syntax. `[` is the most common command to use as a condition, but you can use any command you like. — Keith Thompson, May 18 '17 at 01:20

score 3 · Answer 1 · edited May 23 '17 at 12:26

In Bash, we can build an if based on the exit status of a command this way:

if command; then
  echo "Command succeeded"
else
  echo "Command failed"
fi

then part is executed when the command exits with 0 and else part otherwise.

Your code is doing exactly that.

It can be rewritten as:

/fgallery/fgallery -v -j3 /images /usr/share/nginx/html/ "${GALLERY_TITLE:-Gallery}"; fgallery_status=$?
if [ "$fgallery_status" -ne 0 ]; then
  mkdir -p /usr/share/nginx/html
fi

But the former construct is more elegant and less error prone.

See these posts:

I see Thank you very much guys! – Donislav Belev May 17 '17 at 19:45 — Donislav Belev, May 17 '17 at 19:45

Understanding Bash if statement that invokes a command

1 Answers1