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## General syntax ## General syntax
This command allows you to do various tests and sets its exit code to 0 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.
(*TRUE*) or 1 (*FALSE*) whenever such a test succeeds or not. Using this 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:
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 ```bash
# test if /etc/passwd exists #!/bin/bash
if test -e /etc/passwd; then # test if /etc/passwd exists
echo "Alright man..." >&2 if test -e /etc/passwd; then
else echo "Alright man..." >&2
echo "Yuck! Where is it??" >&2 else
exit 1 echo "Yuck! Where is it??" >&2
fi exit 1
fi
```
The syntax of the test command is relatively easy. Usually it\'s the The syntax of the test command is relatively easy.
command name \"`test`\" followed by a test type (here \"`-e`\" for 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`).
\"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 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> ]`.
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: Let's rewrite the above example to use it:
#!/bin/bash ```bash
# test if /etc/passwd exists #!/bin/bash
if [ -e /etc/passwd ]; then # test if /etc/passwd exists
echo "Alright man..." >&2 if [ -e /etc/passwd ]; then
else echo "Alright man..." >&2
echo "Yuck! Where is it??" >&2 else
exit 1 echo "Yuck! Where is it??" >&2
fi exit 1
fi
```
One might **think** now that these \"\[\" and \"\]\" belong to the 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!**
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 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.
parameter for a test, you have to give it one parameter. Let\'s check Let's check for some of your music files:
for some of your music files:
#!/bin/bash ```bash
#!/bin/bash
mymusic="/data/music/Van Halen/Van Halen - Right Now.mp3" mymusic="/data/music/Van Halen/Van Halen - Right Now.mp3"
if [ -e "$mymusic" ]; then if [ -e "$mymusic" ]; then
echo "Let's rock" >&2 echo "Let's rock" >&2
else else
echo "No music today, sorry..." >&2 echo "No music today, sorry..." >&2
exit 1 exit 1
fi fi
```
As you definitely noted, the filename contains spaces. Since we call a As you definitely noted, the filename contains spaces.
normal ordinary command (\"test\" or \"\[\") the shell will word-split Since we call a normal ordinary command (`test` or `[`) the shell will word-split the expansion of the variable `mymusic`:
the expansion of the variable `mymusic`: You need to quote it when you You need to quote it when you don't want the `test`-command to complain about too many arguments for this test-type!
don\'t want the `test`-command to complain about too many arguments for If you didn't understand it, please read the [article about words\...](/syntax/words).
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. 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 Don't give a glob (filename-wildcards) as it can expand to many filenames => **too many arguments!**
filenames =\> **too many arguments!**
[**Another common mistake**]{.underline} is to provide too **few** **Another common mistake** is to provide too **few** arguments:
arguments:
[ "$mystring"!="test" ] ```bash
[ "$mystring"!="test" ]
```
This provides exactly **one** test-argument to the command. With one This provides exactly **one** test-argument to the command.
parameter, it defaults to the `-n` test: It tests if a provided string With one parameter, it defaults to the `-n` test:
is empty (`FALSE`) or not (`TRUE`) - due to the lack of **spaces to 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`!
separate the arguments** the shown command always ends `TRUE`!
Well, I addressed several basic rules, now let\'s see what the Well, I addressed several basic rules, now let's see what the test-command can do for you.
test-command can do for you. The Bash test-types can be split into The Bash test-types can be split into several sections: **file tests**, **string tests**, **arithmetic tests**, **misc tests**.
several sections: **file tests**, **string tests**, **arithmetic Below, the tests marked with :warning: are non-standard tests (i.e. not in SUS/POSIX/etc..).
tests**, **misc tests**. Below, the tests marked with :!: are
non-standard tests (i.e. not in SUS/POSIX/etc..).
## File tests ## File tests
This section probably holds the most tests, I\'ll list them in some This section probably holds the most tests, I'll list them in some logical order.
logical order. Since Bash 4.1, all tests related to permissions respect Since Bash 4.1, all tests related to permissions respect ACLs, if the underlying filesystem/OS supports them.
ACLs, if the underlying filesystem/OS supports them.
Operator syntax Description | Operator syntax | Description |
----------------------------- -------------------------------------------------------------------------------------------- -- | ------------------------- | ----------------------------------------------------------------------------------------------- |
**-a** \<FILE\> True if \<FILE\> exists. :!: (not recommended, may collide with `-a` for `AND`, see below) | **-a** <FILE\> | True if <FILE\> exists. :warning: (not recommended, may collide with `-a` for `AND`, see below) |
**-e** \<FILE\> True if \<FILE\> exists. | **-e** <FILE\> | True if <FILE\> exists. |
**-f** \<FILE\> True, if \<FILE\> exists and is a **regular** file. | **-f** <FILE\> | True, if <FILE\> exists and is a **regular** file. |
**-d** \<FILE\> True, if \<FILE\> exists and is a **directory**. | **-d** <FILE\> | True, if <FILE\> exists and is a **directory**. |
**-c** \<FILE\> True, if \<FILE\> exists and is a **character special** file. | **-c** <FILE\> | True, if <FILE\> exists and is a **character special** file. |
**-b** \<FILE\> True, if \<FILE\> exists and is a **block 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). | **-p** <FILE\> | True, if <FILE\> exists and is a **named pipe** (FIFO). |
**-S** \<FILE\> True, if \<FILE\> exists and is a **socket** file. | **-S** <FILE\> | True, if <FILE\> exists and is a **socket** file. |
**-L** \<FILE\> True, if \<FILE\> exists and is a **symbolic link**. | **-L** <FILE\> | True, if <FILE\> exists and is a **symbolic link**. |
**-h** \<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. | **-g** <FILE\> | True, if <FILE\> exists and has **sgid bit** set. |
**-u** \<FILE\> True, if \<FILE\> exists and has **suid bit** set. | **-u** <FILE\> | True, if <FILE\> exists and has **suid bit** set. |
**-r** \<FILE\> True, if \<FILE\> exists and is **readable**. | **-r** <FILE\> | True, if <FILE\> exists and is **readable**. |
**-w** \<FILE\> True, if \<FILE\> exists and is **writable**. | **-w** <FILE\> | True, if <FILE\> exists and is **writable**. |
**-x** \<FILE\> True, if \<FILE\> exists and is **executable**. | **-x** <FILE\> | True, if <FILE\> exists and is **executable**. |
**-s** \<FILE\> True, if \<FILE\> exists and has size bigger than 0 (**not empty**). | **-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. | **-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\> **-nt** <FILE2\> | True, if <FILE1\> is **newer than** <FILE2\> (mtime). :warning: |
\<FILE1\> **-ot** \<FILE2\> True, if \<FILE1\> is **older than** \<FILE2\> (mtime). :!: | <FILE1\> **-ot** <FILE2\> | True, if <FILE1\> is **older than** <FILE2\> (mtime). :warning: |
\<FILE1\> **-ef** \<FILE2\> True, if \<FILE1\> and \<FILE2\> refer to the **same device and inode numbers**. :!: | <FILE1\> **-ef** <FILE2\> | True, if <FILE1\> and <FILE2\> refer to the **same device and inode numbers**. :warning: |
## String tests ## String tests
Operator syntax Description | Operator syntax | Description |
-------------------------------- ------------------------------------------------------------------------------------------------------------------------------------ | ---------------------------- | -------------------------------------------------------------------------------------------------------------------------------- |
**-z** \<STRING\> True, if \<STRING\> is **empty**. | **-z** <STRING\> | True, if <STRING\> is **empty**. |
**-n** \<STRING\> True, if \<STRING\> is **not empty** (this is the default operation). | **-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 **equal**. |
\<STRING1\> **!=** \<STRING2\> True, if the strings are **not 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 **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 `\>` | <STRING1\> **\>** <STRING2\> | True if <STRING1\> sorts **after** <STRING2\> lexicographically (pure ASCII, not current locale!). Remember to escape! Use `\>` |
## Arithmetic tests ## Arithmetic tests
Operator syntax Description | Operator syntax | Description |
----------------------------------- --------------------------------------------------------------------- | ------------------------------- | ------------------------------------------------------------------- |
\<INTEGER1\> **-eq** \<INTEGER2\> True, if the integers are **equal**. | <INTEGER1\> **-eq** <INTEGER2\> | True, if the integers are **equal**. |
\<INTEGER1\> **-ne** \<INTEGER2\> True, if the integers are **NOT 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\> **-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\> **-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\> **-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. | <INTEGER1\> **-gt** <INTEGER2\> | True, if the first integer is **greater than** second one. |
## Misc syntax ## Misc syntax
Operator syntax Description | 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\> **-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). | <TEST1\> **-o** <TEST2\> | True, if either \<TEST1\> **or** <TEST2\> is true (OR). |
**!** \<TEST\> True, if \<TEST\> is **false** (NOT). | **!** <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 \"\\)\"! | **(** <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. | **-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. | **-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) | **-R** <VARIABLENAME\> | True if the variable <VARIABLENAME\> has been set and is a nameref variable (since 4.3-alpha) |
## Number of Arguments Rules ## Number of Arguments Rules
The `test` builtin, especially hidden under its `[` name, may seem The `test` builtin, especially hidden under its `[` name, may seem simple but is in fact **causing a lot of trouble sometimes**.
simple but is in fact **causing a lot of trouble sometimes**. One of the One of the difficulty is that the behaviour of `test` not only depends on its arguments but also on the **number of its arguments**.
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 ([**Note:**]{.underline} This Here are the rules taken from the manual:
is for the command `test`, for `[` the number of arguments is calculated
without the final `]`, for example `[ ]` follows the \"zero arguments\"
rule):
- **0 arguments** !!! note
- The expression is false. This is for the command `test`, for `[` the number of arguments is calculated without the final `]`, for example `[ ]` follows the "zero arguments" rule.
- **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 - **0 arguments**
them might help you to explain some of the \"unexplicable\" behaviours - The expression is false.
you might encounter: - **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
var="" These rules may seem complex, but it's not so bad in practice.
if [ -n $var ]; then echo "var is not empty"; fi Knowing them might help you to explain some of the "unexplicable" behaviours you might encounter:
This code prints \"var is not empty\", even though `-n something` is ```bash
supposed to be true if `$var` is not empty - **why?** var=""
if [ -n $var ]; then echo "var is not empty"; fi
```
Here, as `$var` is **not quoted**, word splitting occurs and `$var` This code prints "var is not empty", even though `-n something` is supposed to be true if `$var` is not empty - **why?**
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 "$var" ]` so that the test has always 2
arguments, even if the second one is the null string.
These rules also explain why, for instance, -a and -o can have several Here, as `$var` is **not quoted**, word splitting occurs and `$var` results in actually nothing (Bash removes it from the command's argument list!).
meanings. So the test is in fact `[ -n ]` **and falls into the "one argument" rule**, the only argument is "-n" which is not null and so the test returns true.
The solution, as usual, is to **quote the parameter expansion**:
`[ -n "$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 ## AND and OR
### The Prefered Way ### The Prefered Way
The way often recommended to logically connect several tests with AND 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**.
and OR is to use **several single test commands** and to **combine**
them with the shell `&&` and `||` **list control operators**.
See this: See this:
if [ -n "$var"] && [ -e "$var"]; then ```bash
echo "\$var is not null and a file named $var exists!" if [ -n "$var"] && [ -e "$var"]; then
fi echo "\$var is not null and a file named $var exists!"
fi
```
The return status of AND and OR lists is the exit status of the last The return status of AND and OR lists is the exit status of the last command executed in the list
command executed in the list
- With `command1 && command2`, `command2` is executed if, and only if, - With `command1 && command2`, `command2` is executed if, and only if, `command1` returns an exit status of zero (true)
`command1` returns an exit status of zero (true) - With `command1 ││ command2`, `command2` is executed if, and only if, `command1` returns a non-zero exit status (false)
- With `command1 ││ command2`, `command2` is executed if, and only if,
`command1` returns a non-zero exit status (false)
### The other way: -a and -o ### The other way: -a and -o
The logical operators AND and OR for the test-command itself are `-a` The logical operators AND and OR for the test-command itself are `-a` and `-o`, thus:
and `-o`, thus:
if [ -n "$var" -a -e "$var" ] ; then ```bash
echo "\$var is not null and a file named $var exists" if [ -n "$var" -a -e "$var" ] ; then
fi echo "\$var is not null and a file named $var exists"
fi
```
They are **not** `&&` or `||`: They are **not** `&&` or `||`:
$ if [ -n "/tmp" && -d "/tmp"]; then echo true; fi # DOES NOT WORK ```bash
bash: [: missing `]' $ if [ -n "/tmp" && -d "/tmp"]; then echo true; fi # DOES NOT WORK
bash: [: missing `]'
```
You might find the error message confusing, `[` does not find the 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**.
required final `]`, because as seen above `&&` is used to write a **list The `if` statement actually **sees two commands**:
of commands**. The `if` statement actually **sees two commands**:
- `[ -n "/tmp"` - `[ -n "/tmp"`
- `-d "/tmp" ]` - `-d "/tmp" ]`
\...which **must** fail. ...which **must** fail.
### Why you should avoid using -a and -o ### Why you should avoid using -a and -o
#### If portability is a concern #### If portability is a concern
POSIX(r)/SUSv3 does **not** specify the behaviour of `test` in cases POSIX(r)/SUSv3 does **not** specify the behaviour of `test` in cases where there are more than 4 arguments.
where there are more than 4 arguments. If you write a script that might If you write a script that might not be executed by Bash, the behaviour might be different! [^1]
not be executed by Bash, the behaviour might be different! [^1]
#### If you want the cut behaviour #### If you want the cut behaviour
Let\'s say, we want to check the following two things (AND): Let's say, we want to check the following two things (AND):
1. if a string is null (empty) 1. if a string is null (empty)
2. if a command produced an output 2. if a command produced an output
Let\'s see: Let\'s see:
if [ -z "false" -a -z "$(echo I am executed >&2)" ] ; then ... ```bash
if [ -z "false" -a -z "$(echo I am executed >&2)" ] ; then ...
```
=\> The arguments are all expanded **before** `test` runs, thus the => The arguments are all expanded **before** `test` runs, thus the echo-command **is executed**.
echo-command **is executed**.
if [ -z "false" ] && [ -z "$(echo I am not executed >&2)" ]; then... ```bash
if [ -z "false" ] && [ -z "$(echo I am not executed >&2)" ]; then...
```
=\> Due to the nature of the `&&` list operator, the second test-command => 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**.
runs only if the first test-command returns true, our echo-command **is
not executed**.
[**Note:**]{.underline} In my opinion, `-a` and `-o` are also less !!! note
readable `[pgas]` In my opinion, `-a` and `-o` are also less readable `[pgas]`
### Precedence and Parenthesis ### Precedence and Parenthesis
Take care if you convert your scripts from using `-a` and `-o` to use Take care if you convert your scripts from using `-a` and `-o` to use the list way (`&&` and `||`):
the list way (`&&` and `||`):
- in the test-command rules, `-a` has **precedence over** `-o` - in the test-command rules, `-a` has **precedence over** `-o`
- in the shell grammar rules, `&&` and `||` have **equal precedence** - in the shell grammar rules, `&&` and `||` have **equal precedence**
That means, **you can get different results**, depending on the manner That means, **you can get different results**, depending on the manner of use:
of use:
$ if [ "true" ] || [ -e /does/not/exist ] && [ -e /does/not/exist ]; then echo true; else echo false; fi ```bash
false $ if [ "true" ] || [ -e /does/not/exist ] && [ -e /does/not/exist ]; then echo true; else echo false; fi
false
$ if [ "true" -o -e /does/not/exist -a -e /does/not/exist ]; then echo true; else echo false;fi $ if [ "true" -o -e /does/not/exist -a -e /does/not/exist ]; then echo true; else echo false;fi
true true
```
As a result you have to think about it a little or add precedence As a result you have to think about it a little or add precedence control (parenthesis).
control (parenthesis).
For `&&` and `||` parenthesis means (shell-ly) grouping the commands, For `&&` and `||` parenthesis means (shell-ly) grouping the commands, and since `( ... )` introduces a subshell we will use `{ ... }` instead:
and since `( ... )` introduces a subshell we will use `{ ... }` instead:
$ if [ "true" ] || { [ -e /does/not/exist ] && [ -e /does/not/exist ] ;} ; then echo true; else echo false; fi ```bash
true $ if [ "true" ] || { [ -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, `( )`, 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:
but you need to escape or quote them, so that the shell doesn\'t try to
interpret them:
$ if [ \( "true" -o -e /does/not/exist \) -a -e /does/not/exist ]; then echo true; else echo false; fi ```bash
false $ if [ \( "true" -o -e /does/not/exist \) -a -e /does/not/exist ]; then echo true; else echo false; fi
false
# equivalent, but less readable IMHO: # equivalent, but less readable IMHO:
$ if [ '(' "true" -o -e /does/not/exist ')' -a -e /does/not/exist ]; then echo true; else echo false; fi $ if [ '(' "true" -o -e /does/not/exist ')' -a -e /does/not/exist ]; then echo true; else echo false; fi
false false
```
## NOT ## NOT
As for AND and OR, there are 2 ways to negate a test with the shell As for AND and OR, there are 2 ways to negate a test with the shell keyword `!` or passing `!` as an argument to `test`.
keyword `!` or passing `!` as an argument to `test`.
Here `!` negates the exit status of the command `test` which is 0 Here `!` negates the exit status of the command `test` which is 0 (true), and the else part is executed:
(true), and the else part is executed:
if ! [ -d '/tmp' ]; then echo "/tmp doesn't exists"; else echo "/tmp exists"; fi ```bash
if ! [ -d '/tmp' ]; then echo "/tmp doesn't exists"; else echo "/tmp exists"; fi
```
Here the `test` command itself exits with status 1 (false) and the else Here the `test` command itself exits with status 1 (false) and the else is also executed:
is also executed:
if [ ! -d '/tmp' ]; then echo "/tmp doesn't exists"; else echo "/tmp exists"; fi ```plain
if [ ! -d '/tmp' ]; then echo "/tmp doesn't exists"; else echo "/tmp exists"; fi
```
Unlike for AND and OR, both methods for NOT have an identical behaviour, Unlike for AND and OR, both methods for NOT have an identical behaviour, at least for doing one single test.
at least for doing one single test.
## Pitfalls summarized ## Pitfalls summarized
In this section you will get all the mentioned (and maybe more) possible In this section you will get all the mentioned (and maybe more) possible pitfalls and problems in a summary.
pitfalls and problems in a summary.
### General ### General
Here\'s the copy of a mail on bug-bash list. A user asking a question Here's the copy of a mail on bug-bash list.
about using the test command in Bash, **he\'s talking about a problem, 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**:
which you may have already had yourself**:
From: (PROTECTED) ```plain
Subject: -d option not working. . .? From: (PROTECTED)
Date: Tue, 11 Sep 2007 21:51:59 -0400 Subject: -d option not working. . .?
To: bug-bash@gnu.org Date: Tue, 11 Sep 2007 21:51:59 -0400
To: bug-bash@gnu.org
Hi All, Hi All,
I've got a script that I'm trying to set up, but it keeps telling me I've got a script that I'm trying to set up, but it keeps telling me
that "[-d command not found". Can someone please explain what is that "[-d command not found". Can someone please explain what is
wrong with this?: wrong with this?:
#!/bin/sh #!/bin/sh
for i in $* for i in $*
do do
{ {
if [-d $i] if [-d $i]
then then
echo "$i is a directory! Yay!" echo "$i is a directory! Yay!"
else else
echo "$i is not a directory!" echo "$i is not a directory!"
fi fi
} }
done done
Regards
```
See the problem regarding the used test-command (the other potential problems are not of interest here)?
Regards ```bash
[-d $i]
```
See the problem regarding the used test-command (the other potential He simply didn't know that `test` or `[` is a normal, simple command.
problems are not of interest here)? 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:
[-d $i] ```plain
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
He simply didn\'t know that `test` or `[` is a normal, simple command. > (QUOTED TEXT WAS REMOVED)
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) The shell is first and foremost a way to launch other commands. The
Subject: Re: -d option not working. . .? syntax is simply "if" followed by a command-list, (e.g. if /some/foo;
Date: Wed, 12 Sep 2007 10:32:35 -0600 or even if cmd1; cmd2; cmd3; then). Plus the '( ... )' syntax is
To: bug-bash@gnu.org already taken by the use of starting a subshell.
> (QUOTED TEXT WAS REMOVED) 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:
The shell is first and foremost a way to launch other commands. The if /bin/test -d somedir
syntax is simply "if" 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 Although the full path /bin/test was never used. I showed it that way
not built-in. It was provided by the standalone '/bin/test' command. here for emphasis that following the 'if' statement is a command list.
The result was effectively this: Normally it would simply have been:
if /bin/test -d somedir if test -d somedir
Although the full path /bin/test was never used. I showed it that way Of course that is fine and for the best portability that style is
here for emphasis that following the 'if' statement is a command list. still the recommended way today to use the test command. But many
Normally it would simply have been: 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.
if test -d somedir ...modify /bin/test to ignore ']' as last argument...
cp /bin/test /bin/[
Of course that is fine and for the best portability that style is This allows:
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... if [ -d somedir ]
cp /bin/test /bin/[
This allows: 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.
if [ -d somedir ] it test -f *.txt
test: too many arguments
Doesn't that look more normal? People liked it and it caught on. It Oops. I have twenty .txt files and so test got one -f followed by the
was so popular that both 'test' and '[' are now shell built-ins. They first file followed by the remaining files. (e.g. test -f 1.txt 2.txt
don't launch an external '/bin/test' program anymore. But they *used* 3.txt 4.txt)
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 if test -d $file
test: too many arguments test: argument expected
Oops. I have twenty .txt files and so test got one -f followed by the Oops. I meant to set file.
first file followed by the remaining files. (e.g. test -f 1.txt 2.txt
3.txt 4.txt)
if test -d $file file=/path/some/file
test: argument expected if test -d $file
Oops. I meant to set file. If variables such as that are not set then they wlll be expanded by
the shell before passing them to the (possibly external) command and
disappear entirely. This is why test arguments should always be quoted.
file=/path/some/file if test -d "$file"
if test -d $file if [ -d "$file" ]
If variables such as that are not set then they wlll be expanded by Actually today test is defined that if only one argument is given as
the shell before passing them to the (possibly external) command and in this case "test FOO" then then test returns true if the argument is
disappear entirely. This is why test arguments should always be quoted. non-zero in text length. Because "-d" is non-zero length "test -d" 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.
if test -d "$file" DATA="something"
if [ -d "$file" ] if test "$DATA" # true, $DATA is non-zero length
Actually today test is defined that if only one argument is given as DATA=""
in this case "test FOO" then then test returns true if the argument is if test "$DATA" # false, $DATA is zero length
non-zero in text length. Because "-d" is non-zero length "test -d" is
true. The number of arguments affects how test parses the args. This
avoids a case where depending upon the data may look like a test
operator.
DATA="something" But the problem case is how should test handle an argument that looks
if test "$DATA" # true, $DATA is non-zero length 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 $DATA.
DATA="" DATA="-d"
if test "$DATA" # false, $DATA is zero length if test "$DATA" # true, $DATA is non-zero length
if test -d # true, same as previous case.
But the problem case is how should test handle an argument that looks Because test and [ are possibly external commands all of the parts of
like an operator? This used to generate errors but now because it is them are chosen to avoid shell metacharacters. The Fortran operator
only one argument is defined to be the same as test -n $DATA. naming was well known at the time (e.g. .gt., .eq., etc.) and was
pressed into service for the shell test operator too. Comming from
Fortran using -gt, -eq, etc. looked very normal.
DATA="-d" Incorrect use generating unlikely to be intended results:
if test "$DATA" # 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 if test 5 > 2 # true, "5" is non-zero length, creates file named "2"
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: Intended use:
if test 5 > 2 # true, "5" is non-zero length, creates file named "2" if test 5 -gt 2 # true (and no shell meta characters needing quoting)
Intended use: 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 test 5 -gt 2 # true (and no shell meta characters needing quoting) if [[ -d $file ]] # okay
if [[ 5 > 2 ]] # okay
Then much later, sometime in the mid 1980's, the Korn sh decided to I am sure that I am remembering a detail wrong but hopefully this is
improve upon this situation. A new test operator was introduced. useful as a gentle introduction and interesting anyway.
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 $file ]] # okay Bob
if [[ 5 > 2 ]] # okay ```
I am sure that I am remembering a detail wrong but hopefully this is I hope this text protects you a bit from stepping from one pitfall into the next.
useful as a gentle introduction and interesting anyway.
Bob I find it very interesting and informative, that's why I quoted it here.
Many thanks, Bob, also for the permission to copy the text here!
I hope this text protects you a bit from stepping from one pitfall into
the next.
I find it very interesting and informative, that\'s why I quoted it
here. Many thanks, Bob, also for the permission to copy the text here!
## Code examples ## Code examples
@ -535,40 +500,34 @@ here. Many thanks, Bob, also for the permission to copy the text here!
Some code snipplets follow, different ways of shell reaction is used. Some code snipplets follow, different ways of shell reaction is used.
- **check if a variable is defined/non-NULL** - **check if a variable is defined/non-NULL**
- `test "$MYVAR"` - `test "$MYVAR"`
- `[ "$MYVAR" ]` - `[ "$MYVAR" ]`
- **Note:** There are possibilities to make a difference if a - **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)
variable is *undefined* or *NULL* - see [Parameter Expansion - - **check if a directory exists, if not, create it**
Using an alternate value](/syntax/pe#use_an_alternate_value) - `test ! -d /home/user/foo && mkdir /home/user/foo`
- **check if a directory exists, if not, create it** - `[ ! -d /home/user/foo ] && mkdir /home/user/foo`
- `test ! -d /home/user/foo && mkdir /home/user/foo` - `if [ ! -d /home/user/foo ]; then mkdir /home/user/foo; fi`
- `[ ! -d /home/user/foo ] && mkdir /home/user/foo` - **check if minimum one parameter was given, and that one is "Hello"**
- `if [ ! -d /home/user/foo ]; then mkdir /home/user/foo; fi` - `test $# -ge 1 -a "$1" = "Hello" || exit 1`
- **check if minimum one parameter was given, and that one is - `[ $# -ge 1 ] && [ "$1" = "Hello" ] || exit 1` (see [lists description](/syntax/basicgrammar#lists))
\"Hello\"**
- `test $# -ge 1 -a "$1" = "Hello" || exit 1`
- `[ $# -ge 1 ] && [ "$1" = "Hello" ] || exit 1` (see [lists
description](/syntax/basicgrammar#lists))
### Listing directories ### Listing directories
Using a [for-loop](/syntax/ccmd/classic_for) to iterate through all Using a [for-loop](/syntax/ccmd/classic_for) to iterate through all entries of a directory, if an entry is a directory (`[ -d "$fn" ]`), print its name:
entries of a directory, if an entry is a directory (`[ -d "$fn" ]`),
print its name:
for fn in *; do ```bash
[ -d "$fn" ] && echo "$fn" for fn in *; do
done [ -d "$fn" ] && echo "$fn"
done
```
## See also ## See also
- Internal: [conditional - Internal: [conditional expression](/syntax/ccmd/conditional_expression) (aka "the new test command")
expression](/syntax/ccmd/conditional_expression) (aka \"the new test - Internal: [the if-clause](/syntax/ccmd/if_clause)
command\")
- Internal: [the if-clause](/syntax/ccmd/if_clause)
[^1]: \<rant\>Of course, one can wonder what is the use of including the [^1]: <rant\>Of course, one can wonder what is the use of including the
parenthesis in the specification without defining the behaviour with parenthesis in the specification without defining the behaviour with
more than 4 arguments or how usefull are the examples with 7 or 9 more than 4 arguments or how usefull are the examples with 7 or 9
arguments attached to the specification.\</rant\> arguments attached to the specification.</rant\>

11
mkdocs.yml
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@ -38,6 +38,17 @@ plugins:
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- Start: index.md - Start: index.md