Differences between Ubuntu Dash and bash

Ubuntudash Shell

 

Https://wiki.ubuntu.com/DashAsBinSh

Http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_title.html

From Ubuntu 6.10, dash (theDebian almshells) is used by default instead of bash (the GNUBourne-Again Shell ). but Login Shell is still bash. the reason is that dash is faster and more efficient, and it complies with POSIX specifications. Ubuntu runs a lot of shell scripts during startup. Using dash can speed up startup.

 

• If you solve the problem caused by bash and dash compatibility

  • Write "#! /Bin/bash"
  • In Makefile, you can set

SHELL =/bin/bash

• To change the default value to bash, run the following command and select no. Note that this will affect all system scripts. If a script requires a specific dash function, it will cause a problem (this case is rare ).

Sudodpkg-reconfigure dash

• Avoid using the bash extension feature (bashism) in newly written shell scripts ).

  • Use the checkbashisms command of the devscripts package to check whether bashism exists in the shell script.
  • Run the info autoconf command to install the autoconf-doc package. Read the documentation in "Portable Shell" to learn more about POSIX Shell.
  • Avoid using-a,-o in the "[" command (test). Use multiple "[]" commands and connect with "&", "|.
    For example, the following shell statement

[\ ("$ Foo" = "$ bar"-a-f/bin/baz \)-o! -X/bin/quux]

Should be replaced:

(["$ Foo" = "$ bar"] & [-f/bin/baz]) | [! -X/bin/quux])

• Instead of using the "[[" command, you should use the "[" command
• Use $ ((...)) Instead ((...)) Perform arithmetic calculations.
• You cannot use $ (n ++), $ (-- n), instead of $ (n = n + 1 )) and $ (n = N-1 ))
• Do not use {} For character extension, such as/usr/lib/libfoo. {a, so };
• Avoid using $ '... 'Extended escape characters. For example, replace $ '\ t' with "$ (printf' \ t'
• Do not use $ "... "For string translation. Use the gettext. sh script.
• Most of the $ {...} Variable extension is portable. But the following are not.

  • $ {!...} Run the eval command for non-direct variable extension.
  • $ {Parameter/pattern/string} to replace the pattern
  • $ {Parameter: offset: length} intercepts a substring
• Do not use $ {parm /? /Pat [/str]} should replace characters, and echo, sed, grep, awk and other commands should be used. For example:

OPENGL_VERSION = $ (glxinfo | grep "OpenGL version string :")

OPENGL_VERSION =$ {OPENGL_VERSION /*:/}

 

Use:

OPENGL_VERSION = $ (glxinfo | grep "OpenGL version string:" | awk 'in in {FS = ": [[: space:] +" };{ print $2 }')

• Do not use $ {foo: 3 [: 1]} to cut substrings. Use echo, sed, grep, awk, and other commands.

For example:

String_after = "somestring"

String =$ {string_after: 0: 3}

 

Use:

String = $ (echo $ string_after | awk '{string = substr ($, 3); print string ;}')

• Use [!] In case statements Instead of [^]. For example:

Case "foo" in

[^ F] *)

Echo 'not f *'

;;

Esac

 

Replace:

Case "foo" in

[! F] *)

Echo 'not f *'

;;

Esac

• Dash does not support $ LINENO, although it is a POSIX standard.
• Do not use $ PIPESTATUS
• Avoid using $ RANDOM, but read/dev/urandom or/dev/random. For example:

Random = 'hexdump-n 2-E'/2 "% u" '/dev/urandom'

• Some echo options are not portable, and may not be supported by other shells. For example,-e,-n
• Do not add the function keyword before the function name.
• Do not use the let Command. Use = to assign values directly. For example

Let time = 10 is the same as time = 10

Let time -- the same as time =$ (time-1)

• Bash and dash have different interpretations of the local keyword.

Local a = 5 B = 6; // dash: a and B are global variables, While bash considers a and B as local variables.

• Printf % q | % B is not supported
• Do not use the select keyword, which is only supported by bash.
• The source command is also supported by bash. The '.' command should be used.
• During path search, 'dash 'does not support '~ 'Extended, use $ HOME
• Declare or typeset is not supported
• Bash and dash have different options for ulimit and type.
• Time is a built-in bash command, but the time program must be used in dash.
• Kill-[0-9] or-[A-Z] is the bash built-in command
• In bash, if read is not connected to a variable, it is saved in the REPLY variable. In dash, read REPLY should be used instead.
• Do not use <, but use <instead. For example:

$ Cat <"$ HOME is where the heart is ."

/Home/ralphis where the heart is.

 

Replace:

$ Cat <E

> $ HOME is where the heart is.

> E

/Home/ralphis where the heart is.

$