Could someone explain me or point me out some document where the differences between "if () ..." and "if [] ..." can be clarified please?
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4() is for specifying function and [ ] for test condition/evaluation .. – Nitin4873 Jun 05 '13 at 18:08
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3Here is another question where this has already been answered: http://stackoverflow.com/questions/12765340/difference-in-bash-between-if-statements-with-parenthesis-and-square-brackets – betseyb Jun 05 '13 at 18:11
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http://stackoverflow.com/questions/2188199/bash-double-or-single-bracket-parentheses-curly-braces – Cloud Jun 05 '13 at 18:11
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2@NSD, `()` in this context would be for running a command pipeline in a subshell and `if` would act on the exit status. – glenn jackman Jun 05 '13 at 19:46
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if simply takes an ordinary shell command as an argument, and evaluates the exit code of the command. For example, you can do
if grep pattern file; then ...; fi
() in bash executes the contents in a subshell, so you can specify basically any command in the subshell.
[] in bash is a shell command (technically the [ command), which evaluates an expression according to a specific syntax.
So, if (...) is used to test the exit code of a command (in most cases the () are redundant), while if [...] is used to test an expression using the syntax of test.
nneonneo
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Ok, based in your last sentence, can you explain why it is not working? I mean, after "crontab -l > /dev/null 2>&1" $? is 1. if (crontab -l > /dev/null 2>&1); then echo "There is something scheduled" else echo "There is nothing scheduled" fi – VeryNiceArgumentException Jun 05 '13 at 18:34
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The [ symbol is actually a command. It is equivalent to the test command.
For instance
if test "$foo" = 'bar'; then ...
is the same as
if [ "$foo" = 'bar' ]; then ...
Whereas, if (command) executes command in a subshell.
Colonel Panic
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