List of 'if' switches anywhere?
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Is there a list of all the if switches for use in bash scripting? Sometimes I see someone using it and I wonder what the switch they're using actually does.
Example is the -z in this one. I know how to use it, but I don't know where it was derived from.
if [ -z "$BASH_VERSION" ]; then
echo -e "Error: this script requires the BASH shell!"
exit 1
fi
Any references, guides, posts, answers would be appreciated.
Look at the bash manpage (man bash). The options are specified in the CONDITIONAL EXPRESSIONS section:
CONDITIONAL EXPRESSIONS
Conditional expressions are used by the [[ compound command and the
test and [ builtin commands to test file attributes and perform string
and arithmetic comparisons. Expressions are formed from the following
unary or binary primaries. If any file argument to one of the pri-
maries is of the form /dev/fd/n, then file descriptor n is checked. If
the file argument to one of the primaries is one of /dev/stdin,
/dev/stdout, or /dev/stderr, file descriptor 0, 1, or 2, respectively,
is checked.
Unless otherwise specified, primaries that operate on files follow sym-
bolic links and operate on the target of the link, rather than the link
itself.
-a file
True if file exists.
... more options ...
It is also explained in the help:
$ help [ [: [ arg... ]
This is a synonym for the "test" builtin, but the last
argument must be a literal `]', to match the opening `['.
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Yes. These are called conditional expressions and these are used by the [[ compound command and the test and [ builtin commands ([ is simply a synonym for test).
Read section 6.4 Bash Conditional Expressions of the Bash Reference Manual, which contains a list of all these switches and their usage.
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They are not switches for the if statement, but for the test command ([ is a synonym for the test builtin). See help test in bash for a complete list.
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It's actually not if that's providing those — it's [, better known by the name of test. help test should give you a list of all options it can take. You could also look at the standard, if you care.
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The single square brackets ([ ... ]) is an synonym of the test command. If you look at the manpage for test, you will see almost all (BASH might have a few extra not mentioned here) of the various if switches as you called them. All in one easy to find place.
If you use double square brackets ([[ ... ]]), you are using an extended BASH set of tests. These mainly have to do with regular expression matching, and glob matching (and extended glob matching if you have that set too). For that, you'll have to read that BASH manpage.
You called them if switches, but that's not really correct. These are tests and really have nothing to do with the if command.
The if command merely executes the command you give it, and then if that command returns an exit code of 0, will run the if portion of the if statement. Otherwise, it will run the else portion (if that's present).
Let's look at this:
$ rm foo.test.txt # Hope this wasn't an important file $ if ls foo.test.txt > then > echo "This file exists" > else > echo "I can't find it anywhere.." > fi ls: foo.test.txt: No such file or directory I can't find it anywhere..
The if statement runs the ls foo.test.txt command and the ls command returns a non-zero because the file does not exist. This causes the if statement to execute the else clause.
Let's try that again...
$ touch foo.test.txt # Now this file exists. $ if ls foo.test.txt # Same "if/else" statement as above > then > echo "This file exists" > else > echo "I can't find it anywhere.." > fi foo.test.txt This file exists
Here, the ls command returned a 0 exit status (since the file exists and the file exists and can be stat'ed by the ls command.
Normally, you shouldn't use the ls command to test for a file. I merely used it here to show that the if statement executes the command, then executed the if or else clause depending upon the exit status of that command. If you want to test whether or not a file exists, you should use the test -e command instead of ls command:
if test -e foo.test.txt # Same as above, but using "test" instead of "ls"
then
echo "This file exists"
else
echo "I can't find it anywhere.."
fi
If the file exists, test -e will return an exit status of 0. Otherwise, it will return a non-zero exit status.
If you do this:
$ ls -i /bin/test /bin/[ 10958 /bin/[ 10958 /bin/test
That 10958 is the inode. Files with the same inode are two different names for the same file. Thus [ and test command are soft linked1. This means you can use [ instead of test:
if [ -e foo.test.txt ]
then
echo "This file exists"
else
echo "I can't find it anywhere.."
fi
Looks familiar?
1. In BASH, the test and [ are builtin, so when you run these commands in BASH, it isn't running /bin/test or /bin/[. However, they're still linked to each other.
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Comments
- gnu.org/software/bash/manual/bashref.html
[is a short name for thetestcommand. Tryman test.- @Barmar you mean
help test.man testis the documentation for/bin/test, and it supports different options from thetestbuiltin - ffs just run
man [! -zis is an option to[ortest; it has nothing (directly) to do withif.- Online manual: Bash Conditional Expressions
- Single page: 6.4 Bash Conditional Expressions
- -1: test is both a command
/bin/testand a bash builtin.man testgives you the doc for the command, but[is a synonym for the builtin. As an example of divergent behavior, on OSX the bash builtin supports the-Ntest but the/bin/testdoes not [is not an alias of test. synonym is the word the manpage uses. Alias has a specific meaning (in the bash manpage, read the sectionALIASES)- Okay. I'll change that. I was simply looking for a word that would explain how the two are linked.