35.26. test: Testing Files and Strings
Unix has a command called test that does a lot of useful tests. For instance, test can check whether a file is writable before your script tries to write to it. It can treat the string in a shell variable as a number and do comparisons ("Is that number less than 1000?"). You can combine tests, too ("If the file exists and it's readable and the message number is more than 500..."). Some versions of test have more tests than others. For a complete list, read your shell's manual page (if your shell has test built in (Section 1.9)) or the online test(1) manual page.
The test command returns a zero status (Section 35.12) if the test was true and a nonzero status otherwise, so people usually use test with if , while, or until. Here's a way your program could check to see if the user has a readable file named .signature in the home directory:
if test -r $HOME/.signature then ...Do whatever... else
echo "$myname: Can't read your '.signature'. Quitting." 1>&2 exit 1 fi
The test command also lets you test for something that isn't true. Add an exclamation point (!) before the condition you're testing. For example, the following test is true if the .signature file is not readable:
if test ! -r $HOME/.signature then echo "$myname: Can't read your '.signature'. Quitting." 1>&2 exit 1 fi
Unix also has a version of test (a link to the same program, actually) named [. Yes, that's a left bracket. You can use it interchangeably with the test command with one exception: there has to be a matching right bracket (]) at the end of the test. The second example above could be rewritten this way:
if [ ! -r $HOME/.signature ] then echo "$myname: Can't read your '.signature'. Quitting." 1>&2 exit 1 fi
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