executing if-statement from command prompt

Question

In bash I can do the following:

if [ -f /tmp/test.txt ]; then echo "true"; fi

However, if I add sudo in front, it doesn't work anymore:

sudo if [ -f /tmp/test.txt ]; then echo "true"; fi
-bash: syntax error near unexpected token `then'

How can I make it work?

Answer 1

sudo executes its argument using exec, not via a shell interpreter. Therefore, it is limited to actual binary programs and cannot use shell functions, aliases, or builtins (if is a builtin). Note that the -i and -s options can be used to execute the given commands in a login or non-login shell, respectively (or just the shell, interactively; note that you'll have to escape the semicolons or quote the command).

$ sudo if [ -n x ]; then echo y; fi
-bash: syntax error near unexpected token `then'
$ sudo if [ -n x ]\; then echo y\; fi
sudo: if: command not found
$ sudo -i if [ -n x ]\; then echo y\; fi
y
$ sudo -s 'if [ -n x ]; then echo y; fi'
y

Answer 2

Try calling the line as a string argument through the shell.

sudo /bin/sh -c 'if [ -f /tmp/test.txt ]; then echo "true"; fi'