Revisions to Simultaneously check for empty output and successful exit status

replaced http://unix.stackexchange.com/ with https://unix.stackexchange.com/

Source Link

edited Apr 13, 2017 at 12:36

1

First of all, while they are functionally equivalent, $(…) is widely considered to be clearer than `…` — see this this, this this, and this this. Secondly, you don’t need to use $? to check whether a command succeeded or failed. My attention was recently drawn to Section 2.9.1, Simple Commands of The Open Group Base Specifications for Shell & Utilities (Issue 7):

A "simple command" is a sequence of optional variable assignments and redirections, in any sequence, optionally followed by words and redirections, terminated by a control operator.

When a given simple command is required to be executed …
⋮ (blah, blah, blah …)

If there is a command name, execution shall continue as described in Command Search and Execution. If there is no command name, but the command contained a command substitution, the command shall complete with the exit status of the last command substitution performed. …

For example,

the exit status of the command
```
  ls -ld "module_$(uname).c"
```

is the exit status from the ls, but

the exit status of the command
```
  myfile="module_$(uname).c"
```

is the exit status from the uname.

So ferada’s answer ferada’s answer can be streamlined a bit:

if output=$(/etc/grub.d/30_os-prober)  &&  [ -z "$output" ]
    # i.e., if 30_os-prober successfully produced no output
then
    install_linux_only
else
    install_dual_boot
fi

Note that it is good practice to use all-upper-case names only for environment variables (or variables to be visible throughout the script). Variables of limited scope are usually named in lower case (or, if you prefer, camelCase).

First of all, while they are functionally equivalent, $(…) is widely considered to be clearer than `…` — see this, this, and this. Secondly, you don’t need to use $? to check whether a command succeeded or failed. My attention was recently drawn to Section 2.9.1, Simple Commands of The Open Group Base Specifications for Shell & Utilities (Issue 7):

A "simple command" is a sequence of optional variable assignments and redirections, in any sequence, optionally followed by words and redirections, terminated by a control operator.

When a given simple command is required to be executed …
⋮ (blah, blah, blah …)

If there is a command name, execution shall continue as described in Command Search and Execution. If there is no command name, but the command contained a command substitution, the command shall complete with the exit status of the last command substitution performed. …

For example,

the exit status of the command
```
  ls -ld "module_$(uname).c"
```

is the exit status from the ls, but

the exit status of the command
```
  myfile="module_$(uname).c"
```

is the exit status from the uname.

So ferada’s answer can be streamlined a bit:

if output=$(/etc/grub.d/30_os-prober)  &&  [ -z "$output" ]
    # i.e., if 30_os-prober successfully produced no output
then
    install_linux_only
else
    install_dual_boot
fi

Note that it is good practice to use all-upper-case names only for environment variables (or variables to be visible throughout the script). Variables of limited scope are usually named in lower case (or, if you prefer, camelCase).

First of all, while they are functionally equivalent, $(…) is widely considered to be clearer than `…` — see this, this, and this. Secondly, you don’t need to use $? to check whether a command succeeded or failed. My attention was recently drawn to Section 2.9.1, Simple Commands of The Open Group Base Specifications for Shell & Utilities (Issue 7):

A "simple command" is a sequence of optional variable assignments and redirections, in any sequence, optionally followed by words and redirections, terminated by a control operator.

When a given simple command is required to be executed …
⋮ (blah, blah, blah …)

If there is a command name, execution shall continue as described in Command Search and Execution. If there is no command name, but the command contained a command substitution, the command shall complete with the exit status of the last command substitution performed. …

For example,

the exit status of the command
```
  ls -ld "module_$(uname).c"
```

is the exit status from the ls, but

the exit status of the command
```
  myfile="module_$(uname).c"
```

is the exit status from the uname.

So ferada’s answer can be streamlined a bit:

if output=$(/etc/grub.d/30_os-prober)  &&  [ -z "$output" ]
    # i.e., if 30_os-prober successfully produced no output
then
    install_linux_only
else
    install_dual_boot
fi

Note that it is good practice to use all-upper-case names only for environment variables (or variables to be visible throughout the script). Variables of limited scope are usually named in lower case (or, if you prefer, camelCase).

Added POSIX reference and best-practice guidance regarding case of identifiers.

Source Link

edited Jun 20, 2015 at 7:31

Scott - Слава Україні

10.9k
10
42
70

First of all, while they are functionally equivalent, $(…) is widely considered to be clearer than `…` — see this, this, and this. Secondly, you don’t need to use $? to check whether a command succeeded or failed. SoMy attention was recently drawn to Section 2.9.1, Simple Commands of The Open Group Base Specifications for Shell & Utilities (Issue 7):

A "simple command" is a sequence of optional variable assignments and redirections, in any sequence, optionally followed by words and redirections, terminated by a control operator.

When a given simple command is required to be executed …
⋮ (blah, blah, blah …)

If there is a command name, execution shall continue as described in Command Search and Execution. If there is no command name, but the command contained a command substitution, the command shall complete with the exit status of the last command substitution performed. …

For example,

the exit status of the command
```
  ls -ld "module_$(uname).c"
```

is the exit status from the ls, but

the exit status of the command
```
  myfile="module_$(uname).c"
```

is the exit status from the uname.

So ferada’s answer can be streamlined a bit:

if output=$(/etc/grub.d/30_os-prober)  &&  [ -z "$output" ]
       # i.e., if 30_os-prober successfully produced no output
then
    install_linux_only
else
    install_dual_boot
fi

Note that it is good practice to use all-upper-case names only for environment variables (or variables to be visible throughout the script). Variables of limited scope are usually named in lower case (or, if you prefer, camelCase).

First of all, while they are functionally equivalent, $(…) is widely considered to be clearer than `…` — see this, this, and this. Secondly, you don’t need to use $? to check whether a command succeeded or failed. So ferada’s answer can be streamlined a bit:

if output=$(/etc/grub.d/30_os-prober)  &&  [ -z "$output" ]
       # i.e., if 30_os-prober successfully produced no output
then
    install_linux_only
else
    install_dual_boot
fi

First of all, while they are functionally equivalent, $(…) is widely considered to be clearer than `…` — see this, this, and this. Secondly, you don’t need to use $? to check whether a command succeeded or failed. My attention was recently drawn to Section 2.9.1, Simple Commands of The Open Group Base Specifications for Shell & Utilities (Issue 7):

A "simple command" is a sequence of optional variable assignments and redirections, in any sequence, optionally followed by words and redirections, terminated by a control operator.

When a given simple command is required to be executed …
⋮ (blah, blah, blah …)

If there is a command name, execution shall continue as described in Command Search and Execution. If there is no command name, but the command contained a command substitution, the command shall complete with the exit status of the last command substitution performed. …

For example,

the exit status of the command
```
  ls -ld "module_$(uname).c"
```

is the exit status from the ls, but

the exit status of the command
```
  myfile="module_$(uname).c"
```

is the exit status from the uname.

So ferada’s answer can be streamlined a bit:

if output=$(/etc/grub.d/30_os-prober)  &&  [ -z "$output" ]
    # i.e., if 30_os-prober successfully produced no output
then
    install_linux_only
else
    install_dual_boot
fi

Note that it is good practice to use all-upper-case names only for environment variables (or variables to be visible throughout the script). Variables of limited scope are usually named in lower case (or, if you prefer, camelCase).

While we're at pointing out best-practice, all upper-case should be reserved for environment variables (or variables to be visible throughout the script)

Source Link

edited Jun 9, 2015 at 10:25

Stéphane Chazelas

584.9k
96
1.1k
1.7k

First of all, while they are functionally equivalent, $(…) is widely considered to be clearer than `…` — see this, this, and this. Secondly, you don’t need to use $? to check whether a command succeeded or failed. So ferada’s answer can be streamlined a bit:

if PROBE_VALUE=$output=$(/etc/grub.d/30_os-prober)  &&  [ -z "$PROBE_VALUE""$output" ]
       # i.e., if 30_os-prober successfully produced no output
then
    install_linux_only
else
    install_dual_boot
fi

First of all, while they are functionally equivalent, $(…) is widely considered to be clearer than `…` — see this, this, and this. Secondly, you don’t need to use $? to check whether a command succeeded or failed. So ferada’s answer can be streamlined a bit:

if PROBE_VALUE=$(/etc/grub.d/30_os-prober)  &&  [ -z "$PROBE_VALUE" ]
       # i.e., if 30_os-prober successfully produced no output
then
    install_linux_only
else
    install_dual_boot
fi

First of all, while they are functionally equivalent, $(…) is widely considered to be clearer than `…` — see this, this, and this. Secondly, you don’t need to use $? to check whether a command succeeded or failed. So ferada’s answer can be streamlined a bit:

if output=$(/etc/grub.d/30_os-prober)  &&  [ -z "$output" ]
       # i.e., if 30_os-prober successfully produced no output
then
    install_linux_only
else
    install_dual_boot
fi

Source Link

answered Jun 9, 2015 at 7:02

Scott - Слава Україні

10.9k
10
42
70

Loading

Stack Exchange Network

Return to Answer