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- when referencing a positional argument variable with more than one digit (e.g. ${10})
- to separate the variable evaluation from text immediately following (e.g. ${prefix}_file.txt)
- since underscore characters ( _ ) are allowed in variable names, the braces are needed so that the shell does not think the variable name is prefix_file.
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When defining or evaluating environment variables there's also a difference between enclosing the value in double quotes ( "$foo$foo" ) or single quotes ( '$foo$foo' ) – see Intro Unix: Quoting in the shell.
Example:
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myvar="some text" echo "$myvar" echo '$myvar' |
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Importantly, if an argument to be passed itself contains spaces, the argument must be enclosed in single or double quotes (or, more ugly, the spaces can be backspace-quotedescaped, e.g. "\ ")
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# Call a custom script passing 2 arguments my_script.sh arg1 "arg2 has embedded spaces" # Call a function passing 2 arguments my_function arg1 'arg2 has embedded spaces' |
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function myfn() { echo "arg 1: $1"; echo "arg 2: $2"; echo "all args: $@"; }
myfn foo bar baz wonk
myfn foo "bar baz" wonk
myfn "foo bar" baz wonk |
As described in Intro Unix: Quoting in the shell, this is the main function of both single and double quotes – to group text containing whitespace into one item.
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Script arguments will initially be:
- $1 - helloWorld
- $2 - My
- $3 - name
- $4 - is
- $5 - Anna
- $@ - helloWorld My name is Anna
Command line processing captures the 1st script argument then pops it off with shift.
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After shift is called, script arguments are:
- $1 - My My
- $2 - name name
- $3 - is is
- $4 - Anna Anna
- $@ - My name is Anna
The CMD variable matches the helloWorld) clause of the case/esac statement, so arguments are passed to the helloWorld function in the "$@" built-in variable.
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The helloWorld function captures the first two arguments into local variables txt1 and txt2, then shifts them off, storing the remaining function arguments ($@) in the rest variable. It then echos the variable values, surrounded by single quotes:
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- $1 - My → stored in local variable txt1
- $2 - name → stored in local variable txt2
- $3 - is
- $4 - Anna
- $@ - My name is Anna
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- My → now stored in local variable txt1
- name → now stored in local variable txt2
- $1 - is
- $2 - Anna
- $@ - is Anna → stored in local variable rest
excercise 1
How would you call the helloWorld command to produce this output?
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Here single quotes or backslash must be used so that the shell does not try to evaluate "$foo$foo" and "\" before passing the arguments into the script.
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