What does it mean to expand a parameter in Bash?

What does it mean to expand a parameter in Bash?

Bash uses the value formed by expanding the rest of parameter as the new parameter; this is then expanded and that value is used in the rest of the expansion, rather than the expansion of the original parameter. This is known as indirect expansion.

What happens when substring expansion is applied to an array?

Substring expansion applied to an associative array produces undefined results. Substring indexing is zero-based unless the positional parameters are used, in which case the indexing starts at 1 by default. If offset is 0, and the positional parameters are used, $0 is prefixed to the list.

Is the exclamation point subject to expansion in Bash?

The exceptions to this are the expansions of $ {! prefix *} and $ {! name [@]} described below. The exclamation point must immediately follow the left brace in order to introduce indirection. In each of the cases below, word is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion.

Which is an example of substring expansion in Bash?

Here are some examples illustrating substring expansion on parameters and subscripted arrays: If parameter is ‘ @ ’, the result is length positional parameters beginning at offset . A negative offset is taken relative to one greater than the greatest positional parameter, so an offset of -1 evaluates to the last positional parameter.

What happens if the expansion of a parameter is unset?

Otherwise, the value of parameter is substituted. $ {parameter:=word}. If parameter is unset or null, the expansion of word is assigned to parameter . The value of parameter is then substituted. Positional parameters and special parameters may not be assigned to in this way.

How are variables and command substitution used in Bash?

Bash variables and command substitution Using variables to refer to data, including the results of a command. An essential feature of programming is the ability to use a name or a label to refer to some other quantity: such as a value, or a command. This is commonly referred to as variables.

Which is the final form of parameter expansion?

The final form of parameter expansion I want to mention is one which simply expands to the length of the variable’s value, its form is $ {#VAR}. So for example: outputs “6”. Using these forms of parameter expansion in your shell scripts can simplify and shorten your scripts.

What does indirect expansion do to a parameter?

This is known as indirect expansion . The value is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion. If parameter is a nameref, this expands to the name of the variable referenced by parameter instead of performing the complete indirect expansion.

What happens when a parameter is unset in Bash?

If parameter is null or unset, the expansion of word (or a message to that effect if word is not present) is written to the standard error and the shell, if it is not interactive, exits. Otherwise, the value of parameter is substituted.

How are expansions performed in a shell script?

An expansion in Shell is performed on the script after it has been split into tokens. A token is a sequence of characters considered a single unit by the shell. It can either be a word or an operator.

When to use case modification in parameter expansion?

Case modification can be used to create the proper capitalization for names or titles. Just assign it to an array: For array expansion, the case modification applies to every expanded element, no matter if you expand an individual index or mass-expand the whole array using @ or * subscripts.

How can I use shell syntax in makefile targets?

So put SHELL := /bin/bash at the top of your makefile, and you should be good to go. BTW: You can also do this for one target, at least for GNU Make. Each target can have its own variable assignments, like this: See “Target-specific Variable Values” in the documentation for more details.

Where do I put Shell in my makefile?

If this variable is not set in your makefile, the program `/bin/sh’ is used as the shell. So put SHELL := /bin/bash at the top of your makefile, and you should be good to go. BTW: You can also do this for one target, at least for GNU Make. Each target can have its own variable assignments, like this:

Where do operators Go in a parameter expansion?

Inside the braces of a parameter expansion, certain operators, along with their arguments, may be placed after the name, before the closing brace. These operators may invoke conditional, subset, substring, substitution, indirection, prefix listing, element counting, and case modification expansion methods, modifying the result of the expansion.

Bash uses the value formed by expanding the rest of parameter as the new parameter; this is then expanded and that value is used in the rest of the expansion, rather than the expansion of the original parameter . This is known as indirect expansion .

How to get filename and extension in Bash?

Using Bash, there’s also $ {file%.*} to get the filename without the extension and $ {file##*.} to get the extension alone. That is, file=”thisfile.txt” echo “filename: $ {file%.*}” echo “extension: $ {file##*.}”

How is the name of a variable used in Bash?

Bash uses the value of the variable formed from the rest of parameter as the name of the variable; this variable is then expanded and that value is used in the rest of the substitution, rather than the value of parameter itself. This is known as indirect expansion.

What’s the best way to manipulate strings in Bash?

There is many ways to do string manipulation with bash, like finding a filename extension using expr, separating the directory part from a filename using dirname and basename…. or even more sophisticated ones based on regex, sed…. Why using a sledgehammer to crack a nut when you could use bash builtin functionalities!

Can you use braces for parameter expansion in Bash?

Without using braces for parameter expansion, Bash will interpret the sequence of all valid characters from the introducing ” $ ” up to the last valid character as name of the parameter. When using braces you just force Bash to only interpret the name inside your braces .

When to use an exclamation point in a parameter in Bash?

If the first character of parameter is an exclamation point (!), and parameter is not a nameref, it introduces a level of indirection. Bash uses the value formed by expanding the rest of parameter as the new parameter; this is then expanded and that value is used in the rest of the expansion, rather than the expansion of the original parameter.

How to extract the first two characters of a string in Bash?

Probably the most efficient method, if you’re using the bash shell (and you appear to be, based on your comments), is to use the sub-string variant of parameter expansion: This will set short to be the first two characters of long.

What are the variables and expansions in Bash?

Bash parameters and variables; environment variables, special parameters and array parameters; expanding parameters, expansion operators, command substitution and process substitution; pathname expansion, tilde expansion and brace expansion. ✅ Thanks for your contribution! Back to the front page.

How to remove all whitespace from a variable in Bash?

However, multiple spaces will be condensed to single spaces, so “foo bar” will become “foo bar”. In addition it doesn’t remove end of lines characters. Let’s define a variable containing leading, trailing, and intermediate whitespace: How to remove all whitespace (denoted by [:space:] in tr ):

Why are there spaces in a variable in Bash?

Execute it like this: “$VAR”. This is one of the most significant gotchas in shell scripting because strings are always substituted literally and any contained spaces are treated as token delimiters rather than as characters of the string. Think of substituting a variable as a kind of code pasting at runtime.

How to use parameter extension in bash script?

Bash also provides other forms of parameter extension, which can do some processing for variable values and play the effect of string operation. For example: $ {parameter#word} from parameter Remove match after beginning of variable value word The rest of the rest is reserved.