hyperreal/wiki.bash-hackers.org
1
0
Fork 0
mirror of https://github.com/rawiriblundell/wiki.bash-hackers.org synced 2024-12-25 14:10:42 +01:00
Code Issues Actions Packages Projects Releases Wiki Activity
4bf053250d
wiki.bash-hackers.org/commands/classictest.md

572 lines
25 KiB
Markdown
Raw Normal View History

First pandoc conversion
2023-04-16 10:04:24 +02:00
# The classic test command
`test <EXPRESSION>`
`[ <EXPRESSION> ]`
## General syntax
This command allows you to do various tests and sets its exit code to 0
(*TRUE*) or 1 (*FALSE*) whenever such a test succeeds or not. Using this
exit code, it's possible to let Bash react on the result of such a test,
here by using the command in an if-statement:
#!/bin/bash
# test if /etc/passwd exists
if test -e /etc/passwd; then
echo &quot;Alright man...&quot; >&2
else
echo &quot;Yuck! Where is it??&quot; >&2
exit 1
fi
The syntax of the test command is relatively easy. Usually it's the
command name "`test`" followed by a test type (here "`-e`" for "file
exists") followed by test-type-specific values (here the filename to
check, "`/etc/passwd`").
There's a second standardized command that does exactly the same: the
command "`[`" - the difference just is that it's called "`[`" and the
last argument to the command must be a "`]`": It forms
"`[ <EXPRESSION> ]`"
Let's rewrite the above example to use it:
#!/bin/bash
# test if /etc/passwd exists
if [ -e /etc/passwd ]; then
echo &quot;Alright man...&quot; >&2
else
echo &quot;Yuck! Where is it??&quot; >&2
exit 1
fi
One might **think** now that these "\[" and "\]" belong to the syntax of
Bash's if-clause: **No they don't! It's a simple, ordinary command,
still!**
Another thing you have to remember is that if the test command wants one
parameter for a test, you have to give it one parameter. Let's check for
some of your music files:
#!/bin/bash
mymusic=&quot;/data/music/Van Halen/Van Halen - Right Now.mp3&quot;
if [ -e &quot;$mymusic&quot; ]; then
echo &quot;Let's rock&quot; >&2
else
echo &quot;No music today, sorry...&quot; >&2
exit 1
fi
As you definitely noted, the filename contains spaces. Since we call a
normal ordinary command ("test" or "\[") the shell will word-split the
expansion of the variable `mymusic`: You need to quote it when you don't
want the `test`-command to complain about too many arguments for this
test-type! If you didn't understand it, please read the [article about
words...](/syntax/words)
Please also note that the file-tests want **one filename** to test.
Don't give a glob (filename-wildcards) as it can expand to many
filenames =\> **too many arguments!**
<u>**Another common mistake**</u> is to provide too **few** arguments:
[ &quot;$mystring&quot;!=&quot;test&quot; ]
This provides exactly **one** test-argument to the command. With one
parameter, it defaults to the `-n` test: It tests if a provided string
is empty (`FALSE`) or not (`TRUE`) - due to the lack of **spaces to
separate the arguments** the shown command always ends `TRUE`!
Well, I addressed several basic rules, now let's see what the
test-command can do for you. The Bash test-types can be split into
several sections: **file tests**, **string tests**, **arithmetic
tests**, **misc tests**. Below, the tests marked with :!: are
non-standard tests (i.e. not in SUS/POSIX/etc..).
## File tests
This section probably holds the most tests, I'll list them in some
logical order. Since Bash 4.1, all tests related to permissions respect
ACLs, if the underlying filesystem/OS supports them.
| Operator syntax | Description | |
|:----------------------------|:-------------------------------------------------------------------------------------------|:---:|
| **-a** \<FILE\> | True if \<FILE\> exists. :!: (not recommended, may collide with `-a` for `AND`, see below) | |
| **-e** \<FILE\> | True if \<FILE\> exists. | |
| **-f** \<FILE\> | True, if \<FILE\> exists and is a **regular** file. | |
| **-d** \<FILE\> | True, if \<FILE\> exists and is a **directory**. | |
| **-c** \<FILE\> | True, if \<FILE\> exists and is a **character special** file. | |
| **-b** \<FILE\> | True, if \<FILE\> exists and is a **block special** file. | |
| **-p** \<FILE\> | True, if \<FILE\> exists and is a **named pipe** (FIFO). | |
| **-S** \<FILE\> | True, if \<FILE\> exists and is a **socket** file. | |
| **-L** \<FILE\> | True, if \<FILE\> exists and is a **symbolic link**. | |
| **-h** \<FILE\> | True, if \<FILE\> exists and is a **symbolic link**. | |
| **-g** \<FILE\> | True, if \<FILE\> exists and has **sgid bit** set. | |
| **-u** \<FILE\> | True, if \<FILE\> exists and has **suid bit** set. | |
| **-r** \<FILE\> | True, if \<FILE\> exists and is **readable**. | |
| **-w** \<FILE\> | True, if \<FILE\> exists and is **writable**. | |
| **-x** \<FILE\> | True, if \<FILE\> exists and is **executable**. | |
| **-s** \<FILE\> | True, if \<FILE\> exists and has size bigger than 0 (**not empty**). | |
| **-t** \<fd\> | True, if file descriptor \<fd\> is open and refers to a terminal. | |
| \<FILE1\> **-nt** \<FILE2\> | True, if \<FILE1\> is **newer than** \<FILE2\> (mtime). :!: | |
| \<FILE1\> **-ot** \<FILE2\> | True, if \<FILE1\> is **older than** \<FILE2\> (mtime). :!: | |
| \<FILE1\> **-ef** \<FILE2\> | True, if \<FILE1\> and \<FILE2\> refer to the **same device and inode numbers**. :!: | |
## String tests
| Operator syntax | Description |
|--------------------------------|------------------------------------------------------------------------------------------------------------------------------------|
| **-z** \<STRING\> | True, if \<STRING\> is **empty**. |
| **-n** \<STRING\> | True, if \<STRING\> is **not empty** (this is the default operation). |
| \<STRING1\> **=** \<STRING2\> | True, if the strings are **equal**. |
| \<STRING1\> **!=** \<STRING2\> | True, if the strings are **not equal**. |
| \<STRING1\> **\<** \<STRING2\> | True if \<STRING1\> sorts **before** \<STRING2\> lexicographically (pure ASCII, not current locale!). Remember to escape! Use `\<` |
| \<STRING1\> **\>** \<STRING2\> | True if \<STRING1\> sorts **after** \<STRING2\> lexicographically (pure ASCII, not current locale!). Remember to escape! Use `\>` |
## Arithmetic tests
| Operator syntax | Description |
|-----------------------------------|---------------------------------------------------------------------|
| \<INTEGER1\> **-eq** \<INTEGER2\> | True, if the integers are **equal**. |
| \<INTEGER1\> **-ne** \<INTEGER2\> | True, if the integers are **NOT equal**. |
| \<INTEGER1\> **-le** \<INTEGER2\> | True, if the first integer is **less than or equal** second one. |
| \<INTEGER1\> **-ge** \<INTEGER2\> | True, if the first integer is **greater than or equal** second one. |
| \<INTEGER1\> **-lt** \<INTEGER2\> | True, if the first integer is **less than** second one. |
| \<INTEGER1\> **-gt** \<INTEGER2\> | True, if the first integer is **greater than** second one. |
## Misc syntax
| Operator syntax | Description |
|:---------------------------|:-------------------------------------------------------------------------------------------------------------------------|
| \<TEST1\> **-a** \<TEST2\> | True, if \<TEST1\> **and** \<TEST2\> are true (AND). Note that `-a` also may be used as a file test (see above) |
| \<TEST1\> **-o** \<TEST2\> | True, if either \<TEST1\> **or** \<TEST2\> is true (OR). |
| **!** \<TEST\> | True, if \<TEST\> is **false** (NOT). |
| **(** \<TEST\> **)** | Group a test (for precedence). **Attention:** In normal shell-usage, the "(" and ")" must be escaped; use "\\" and "\\"! |
| **-o** \<OPTION_NAME\> | True, if the [shell option](/internals/shell_options) \<OPTION_NAME\> is set. |
| **-v** \<VARIABLENAME\> | True if the variable \<VARIABLENAME\> has been set. Use `var[n]` for array elements. |
| **-R** \<VARIABLENAME\> | True if the variable \<VARIABLENAME\> has been set and is a nameref variable (since 4.3-alpha) |
## Number of Arguments Rules
The `test` builtin, especially hidden under its `[` name, may seem
simple but is in fact **causing a lot of trouble sometimes**. One of the
difficulty is that the behaviour of `test` not only depends on its
arguments but also on the **number of its arguments**.
Here are the rules taken from the manual (<u>**Note:**</u> This is for
the command `test`, for `[` the number of arguments is calculated
without the final `]`, for example `[ ]` follows the "zero arguments"
rule):
- **0 arguments**
- The expression is false.
- **1 argument**
- The expression is true if, and only if, the argument is not null
- **2 arguments**
- If the first argument is `!` (exclamation mark), the expression is
true if, and only if, the second argument is null
- If the first argument is one of the unary conditional operators
listed above under the syntax rules, the expression is true if the
unary test is true
- If the first argument is not a valid unary conditional operator, the
expression is false
- **3 arguments**
- If the second argument is one of the binary conditional operators
listed above under the syntax rules, the result of the expression is
the result of the binary test using the first and third arguments as
operands
- If the first argument is `!`, the value is the negation of the
two-argument test using the second and third arguments
- If the first argument is exactly `(` and the third argument is
exactly `)`, the result is the one-argument test of the second
argument. Otherwise, the expression is false. The `-a` and `-o`
operators are considered binary operators in this case
(**Attention:** This means the operator `-a` is not a file operator
in this case!)
- **4 arguments**
- If the first argument is `!`, the result is the negation of the
three-argument expression composed of the remaining arguments.
Otherwise, the expression is parsed and evaluated according to
precedence using the rules listed above
- **5 or more arguments**
- The expression is parsed and evaluated according to precedence using
the rules listed above
These rules may seem complex, but it's not so bad in practice. Knowing
them might help you to explain some of the "unexplicable" behaviours you
might encounter:
var=&quot;&quot;
if [ -n $var ]; then echo &quot;var is not empty&quot;; fi
This code prints "var is not empty", even though `-n something` is
supposed to be true if `$var` is not empty - **why?**
Here, as `$var` is **not quoted**, word splitting occurs and `$var`
results in actually nothing (Bash removes it from the command's argument
list!). So the test is in fact `[ -n ]` **and falls into the "one
argument" rule**, the only argument is "-n" which is not null and so the
test returns true. The solution, as usual, is to **quote the parameter
expansion**: `[ -n "$var" ]` so that the test has always 2 arguments,
even if the second one is the null string.
These rules also explain why, for instance, -a and -o can have several
meanings.
## AND and OR
### The Prefered Way
The way often recommended to logically connect several tests with AND
and OR is to use **several single test commands** and to **combine**
them with the shell `&&` and `||` **list control operators**.
See this:
if [ -n &quot;$var&quot;] && [ -e &quot;$var&quot;]; then
echo &quot;\$var is not null and a file named $var exists!&quot;
fi
The return status of AND and OR lists is the exit status of the last
command executed in the list
- With `command1 && command2`, `command2` is executed if, and only if,
`command1` returns an exit status of zero (true)
- With `command1 ││ command2`, `command2` is executed if, and only if,
`command1` returns a non-zero exit status (false)
### The other way: -a and -o
The logical operators AND and OR for the test-command itself are `-a`
and `-o`, thus:
if [ -n &quot;$var&quot; -a -e &quot;$var&quot; ] ; then
echo &quot;\$var is not null and a file named $var exists&quot;
fi
They are **not** `&&` or `||`:
$ if [ -n &quot;/tmp&quot; && -d &quot;/tmp&quot;]; then echo true; fi # DOES NOT WORK
bash: [: missing `]'
You might find the error message confusing, `[` does not find the
required final `]`, because as seen above `&&` is used to write a **list
of commands**. The `if` statement actually **sees two commands**:
- `[ -n "/tmp"`
- `-d "/tmp" ]`
...which **must** fail.
### Why you should avoid using -a and -o
#### If portability is a concern
POSIX(r)/SUSv3 does **not** specify the behaviour of `test` in cases
where there are more than 4 arguments. If you write a script that might
not be executed by Bash, the behaviour might be different! [^1]
#### If you want the cut behaviour
Let's say, we want to check the following two things (AND):
1. if a string is null (empty)
2. if a command produced an output
Let's see:
if [ -z &quot;false&quot; -a -z &quot;$(echo I am executed >&2)&quot; ] ; then ...
=\> The arguments are all expanded **before** `test` runs, thus the
echo-command **is executed**.
if [ -z &quot;false&quot; ] && [ -z &quot;$(echo I am not executed >&2)&quot; ]; then...
=\> Due to the nature of the `&&` list operator, the second test-command
runs only if the first test-command returns true, our echo-command **is
not executed**.
<u>**Note:**</u> In my opinion, `-a` and `-o` are also less readable
`[pgas]`
### Precedence and Parenthesis
Take care if you convert your scripts from using `-a` and `-o` to use
the list way (`&&` and `||`):
- in the test-command rules, `-a` has **precedence over** `-o`
- in the shell grammar rules, `&&` and `||` have **equal precedence**
That means, **you can get different results**, depending on the manner
of use:
$ if [ &quot;true&quot; ] || [ -e /does/not/exist ] && [ -e /does/not/exist ]; then echo true; else echo false; fi
false
$ if [ &quot;true&quot; -o -e /does/not/exist -a -e /does/not/exist ]; then echo true; else echo false;fi
true
As a result you have to think about it a little or add precedence
control (parenthesis).
For `&&` and `||` parenthesis means (shell-ly) grouping the commands,
and since `( ... )` introduces a subshell we will use `{ ... }` instead:
$ if [ &quot;true&quot; ] || { [ -e /does/not/exist ] && [ -e /does/not/exist ] ;} ; then echo true; else echo false; fi
true
For the test command, the precedence parenthesis are, as well, `( )`,
but you need to escape or quote them, so that the shell doesn't try to
interpret them:
$ if [ \( &quot;true&quot; -o -e /does/not/exist \) -a -e /does/not/exist ]; then echo true; else echo false; fi
false
# equivalent, but less readable IMHO:
$ if [ '(' &quot;true&quot; -o -e /does/not/exist ')' -a -e /does/not/exist ]; then echo true; else echo false; fi
false
## NOT
As for AND and OR, there are 2 ways to negate a test with the shell
keyword `!` or passing `!` as an argument to `test`.
Here `!` negates the exit status of the command `test` which is 0
(true), and the else part is executed:
if ! [ -d '/tmp' ]; then echo &quot;/tmp doesn't exists&quot;; else echo &quot;/tmp exists&quot;; fi
Here the `test` command itself exits with status 1 (false) and the else
is also executed:
if [ ! -d '/tmp' ]; then echo &quot;/tmp doesn't exists&quot;; else echo &quot;/tmp exists&quot;; fi
Unlike for AND and OR, both methods for NOT have an identical behaviour,
at least for doing one single test.
## Pitfalls summarized
In this section you will get all the mentioned (and maybe more) possible
pitfalls and problems in a summary.
### General
Here's the copy of a mail on bug-bash list. A user asking a question
about using the test command in Bash, **he's talking about a problem,
which you may have already had yourself**:
From: (PROTECTED)
Subject: -d option not working. . .?
Date: Tue, 11 Sep 2007 21:51:59 -0400
To: bug-bash@gnu.org
Hi All,
I've got a script that I'm trying to set up, but it keeps telling me
that &quot;[-d command not found&quot;. Can someone please explain what is
wrong with this?:
#!/bin/sh
for i in $*
do
{
if [-d $i]
then
echo &quot;$i is a directory! Yay!&quot;
else
echo &quot;$i is not a directory!&quot;
fi
}
done
Regards
See the problem regarding the used test-command (the other potential
problems are not of interest here)?
[-d $i]
He simply didn't know that `test` or `[` is a normal, simple command.
Well, here's the answer he got. I quote it here, because it's a well
written text that addresses most of the common issues with the "classic"
test command:
From: Bob Proulx (EMAIL PROTECTED)
Subject: Re: -d option not working. . .?
Date: Wed, 12 Sep 2007 10:32:35 -0600
To: bug-bash@gnu.org
> (QUOTED TEXT WAS REMOVED)
The shell is first and foremost a way to launch other commands. The
syntax is simply &quot;if&quot; followed by a command-list, (e.g. if /some/foo;
or even if cmd1; cmd2; cmd3; then). Plus the '( ... )' syntax is
already taken by the use of starting a subshell.
As I recall in the original shell language the file test operator was
not built-in. It was provided by the standalone '/bin/test' command.
The result was effectively this:
if /bin/test -d somedir
Although the full path /bin/test was never used. I showed it that way
here for emphasis that following the 'if' statement is a command list.
Normally it would simply have been:
if test -d somedir
Of course that is fine and for the best portability that style is
still the recommended way today to use the test command. But many
people find that it looks different from other programming languages.
To make the test operator (note I mention the test operator and not
the shell language, this is a localized change not affecting the
language as a whole) look more like other programming languages the
'test' program was coded to ignore the last argument if it was a ']'.
Then a copy of the test program could be used as the '[' program.
...modify /bin/test to ignore ']' as last argument...
cp /bin/test /bin/[
This allows:
if [ -d somedir ]
Doesn't that look more normal? People liked it and it caught on. It
was so popular that both 'test' and '[' are now shell built-ins. They
don't launch an external '/bin/test' program anymore. But they *used*
to launch external programs. Therefore argument parsing is the same
as if they still did launch an external program. This affects
argument parsing.
it test -f *.txt
test: too many arguments
Oops. I have twenty .txt files and so test got one -f followed by the
first file followed by the remaining files. (e.g. test -f 1.txt 2.txt
3.txt 4.txt)
if test -d $file
test: argument expected
Oops. I meant to set file.
file=/path/some/file
if test -d $file
If variables such as that are not set then they wlll be expanded by
the shell before passing them to the (possibly external) command and
disappear entirely. This is why test arguments should always be quoted.
if test -d &quot;$file&quot;
if [ -d &quot;$file&quot; ]
Actually today test is defined that if only one argument is given as
in this case &quot;test FOO&quot; then then test returns true if the argument is
non-zero in text length. Because &quot;-d&quot; is non-zero length &quot;test -d&quot; is
true. The number of arguments affects how test parses the args. This
avoids a case where depending upon the data may look like a test
operator.
DATA=&quot;something&quot;
if test &quot;$DATA&quot; # true, $DATA is non-zero length
DATA=&quot;&quot;
if test &quot;$DATA&quot; # false, $DATA is zero length
But the problem case is how should test handle an argument that looks
like an operator? This used to generate errors but now because it is
only one argument is defined to be the same as test -n $DATA.
DATA=&quot;-d&quot;
if test &quot;$DATA&quot; # true, $DATA is non-zero length
if test -d # true, same as previous case.
Because test and [ are possibly external commands all of the parts of
them are chosen to avoid shell metacharacters. The Fortran operator
naming was well known at the time (e.g. .gt., .eq., etc.) and was
pressed into service for the shell test operator too. Comming from
Fortran using -gt, -eq, etc. looked very normal.
Incorrect use generating unlikely to be intended results:
if test 5 > 2 # true, &quot;5&quot; is non-zero length, creates file named &quot;2&quot;
Intended use:
if test 5 -gt 2 # true (and no shell meta characters needing quoting)
Then much later, sometime in the mid 1980's, the Korn sh decided to
improve upon this situation. A new test operator was introduced.
This one was always a shell built-in and therefore could act upon the
shell arguments directly. This is '[[' which is a shell keyword.
(Keyword, metacharacters, builtins, all are different.) Because the
shell processes [[ internally all arguments are known and do not need
to be quoted.
if [[ -d $file ]] # okay
if [[ 5 > 2 ]] # okay
I am sure that I am remembering a detail wrong but hopefully this is
useful as a gentle introduction and interesting anyway.
Bob
I hope this text protects you a bit from stepping from one pitfall into
the next.
I find it very interesting and informative, that's why I quoted it here.
Many thanks, Bob, also for the permission to copy the text here!
## Code examples
### Snipplets
Some code snipplets follow, different ways of shell reaction is used.
- **check if a variable is defined/non-NULL**
- `test "$MYVAR"`
- `[ "$MYVAR" ]`
- **Note:** There are possibilities to make a difference if a variable
is *undefined* or *NULL* - see [Parameter Expansion - Using an
alternate value](/syntax/pe#use_an_alternate_value)
- **check if a directory exists, if not, create it**
- `test ! -d /home/user/foo && mkdir /home/user/foo`
- `[ ! -d /home/user/foo ] && mkdir /home/user/foo`
- `if [ ! -d /home/user/foo ]; then mkdir /home/user/foo; fi`
- **check if minimum one parameter was given, and that one is "Hello"**
- `test $# -ge 1 -a "$1" = "Hello" || exit 1`
- `[ $# -ge 1 ] && [ "$1" = "Hello" ] || exit 1` (see [lists
description](/syntax/basicgrammar#lists))
### Listing directories
Using a [for-loop](/syntax/ccmd/classic_for) to iterate through all
entries of a directory, if an entry is a directory (`[ -d "$fn" ]`),
print its name:
for fn in *; do
[ -d &quot;$fn&quot; ] && echo &quot;$fn&quot;
done
## See also
- Internal: [conditional
expression](/syntax/ccmd/conditional_expression) (aka "the new test
command")
[^1]: \<rant\>Of course, one can wonder what is the use of including the
parenthesis in the specification without defining the behaviour with
more than 4 arguments or how usefull are the examples with 7 or 9
arguments attached to the specification.\</rant\>
Reference in New Issue Copy Permalink