List of 'if' switches anywhere?
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list of lists python
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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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
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 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..
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
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
ls command returned a
0 exit status (since the file exists and the file exists and can be stat'ed by the
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
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
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
10958 is the inode. Files with the same inode are two different names for the same file. Thus
test command are soft linked1. This means you can use
[ instead of
if [ -e foo.test.txt ] then echo "This file exists" else echo "I can't find it anywhere.." fi
1. In BASH, the
[ are builtin, so when you run these commands in BASH, it isn't running
/bin/[. However, they're still linked to each other.
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[is a short name for the
- @Barmar you mean
man testis the documentation for
/bin/test, and it supports different options from the
- ffs just run
-zis is an option to
test; it has nothing (directly) to do with
- 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
[is not an alias of test. synonym is the word the manpage uses. Alias has a specific meaning (in the bash manpage, read the section
- Okay. I'll change that. I was simply looking for a word that would explain how the two are linked.