YJL

1.2.1   Number sequence

To produce a number sequence, simply assign the starting number and end number. Here are some examples:

echo "{1..5} =" {1..5}
echo "{5..1} =" {5..1}

echo "{1..5..1} =" {1..5..1}
echo "{5..1..-1} =" {5..1..-1}

{1..5} = 1 2 3 4 5
{5..1} = 5 4 3 2 1
{1..5..1} = 1 2 3 4 5
{5..1..-1} = 5 4 3 2 1

You can see two sets, the first sets step is omitted, but not the second set. Bash knows how to walk your sequence even you do not supply the step.

step is used to skip some numbers, for example, you print every two numbers from 1 to 5 and 5 down to 1:

echo "{1..5..2} =" {1..5..2}
echo "{5..1..-2} =" {5..1..-2}
echo
echo "{5..1..2} =" {5..1..-2}

{1..5..2} = 1 3 5
{5..1..-2} = 5 3 1

{5..1..2} = 5 3 1

The first two results are what we expected. But the last one is interesting, even we set the step to negative number for a sequence in descending order, Bash still knows how to walk it correctly. In other words, you should set step always to be positive number, there is no need for you to change steps sign.

echo "{001..3} =" {001..3}
echo "{1..003} =" {1..003}

{001..3} = 001 002 003
{1..003} = 001 002 003

echo "{-1..003} =" {-1..003}
echo "{-001..3} =" {-001..3}

{-1..003} = -01 000 001 002 003
{-001..3} = -001 0000 0001 0002 0003

1.2.2   Alphabet sequence

The amazing part of Sequence is not only you can have numbers but also the letters. Its the only one language I know that you can generate a string of a-z in only 6 characters: {a..z}. When start and end is the type of alphabet, namely a-z or A-Z, the sequence would become a list of alphabets in similar fashion as number sequence. For example,

echo "{a..e} =" {a..e}
echo "{e..a} =" {e..a}

echo "{a..e..2} =" {a..e..2}
echo "{e..a..2} =" {e..a..2}

{a..e} = a b c d e
{e..a} = e d c b a
{a..e..2} = a c e
{e..a..2} = e c a

But dont use mixed cases like {X..c}, you will get unexpected results.

echo "{X..c} =" {X..c}

{X..c} = X Y Z [  ] ^ _ ` a b c

Even it prints out some non-alphabet characters, you still cant set characters other than alphabets as start and end.

1.4.1   Items

In the end of List, an nesting example is shown. If you look at it and ask how many items of final result?

{abc,{"def",'123 456'}}

The answer is 3. The following sample code using array to demonstrate:

arr=({abc,{"def",'123 456'}})
echo ${arr[0]}
echo ${arr[1]}
echo ${arr[2]}

abc
def
123 456

You can see it as a flattened list.

1.4.2   Multiple number sequence

The Sequence expression can only represent one sequence at a time, but what if you need 1 to 3 and 99 to 101? You can use separate Brace Expansions, for instance:

echo {1..3} {99..101}

1 2 3 99 100 101

But this actually have a slight disadvantage when Preamble and Postscript is needed. You will have to add them to two sequences. If you nest two sequences inside a Brace Expansion, it reads more clear:

echo foo{{1..3},{99..101}}bar

foo1bar foo2bar foo3bar foo99bar foo100bar foo101bar

2.2.1   Tilde and Pathname Expansions

When Brace Expansion is expanded and Tilde and/or Pathname Expansions is formed in the result, they will also be expanded. If there is no match filename or ~user, then the string will be kept untouched, that is * or ? will not be removed. But if matches, then the item is replaced by the match item(s).

touch /tmp/ABC{,D}
echo {/tmp/{AB*,NOSUCHFILE?},~{,nosuchuser}}

/tmp/ABC /tmp/ABCD /tmp/NOSUCHFILE? /home/livibetter ~nosuchuser

As you can see, first item is replaced by the filenames in /tmp/. The second item is left untouched, third item expands into home directory of current user, forth or last item is left untouched.