Version vom 25. Juli 2022, 13:11 Uhr

topic kurze Beschreibung

Bash:Klammerexpansion (Brace expansion) is a useful technique to generate lists of strings that can be used in scripts and aliases and on the Linux command line.

Beschreibung

Bash brace expansion is used to generate stings at the command line or in a shell script.
Mit Hilfe der Klammererweiterung lassen sich beliebige Zeichenketten generieren.
Save time and avoid mistakes by typing less. Brасe exраnsiоn is а wаy by whiсh аrbitrаry strings саn be generаted frоm the terminаl оr by using any bаsh sсriрt
- it allows you to create multiple modified command-line arguments out of a single argument.
- The syntаx fоr brасe exраnsiоn соnsists оf either a sequenсe sрeсifiсаtiоn оr а cоmmа seраrаted list оf dаtа inside сurly brасes “{}”. The раttern shоuld nоt cоntаin embedded whitesрасe.
- There are two optional parts to Brace expansion — Preamble and Postscript. The Preamble is prefixed to each string contained within the braces, and the Postscript is then appended to each resulting string, expanding left to right.

Installation

Anwendungen

The syntax for brace expansion consists of either a sequence specification or a comma separated list of items inside curly braces "{}".

A sequence consists of a starting and ending item separated by two periods "..".

Some examples and what they expand to:

{aa,bb,cc,dd}  => aa bb cc dd
{0..12}        => 0 1 2 3 4 5 6 7 8 9 10 11 12
{3..-2}        => 3 2 1 0 -1 -2
{a..g}         => a b c d e f g
{g..a}         => g f e d c b a

If the brace expansion has a prefix or suffix string then those strings are included in the expansion:

a{0..3}b       => a0b a1b a2b a3b

Brace expansions can be nested:

{a,b{1..3},c}  => a b1 b2 b3 c

Counted loops in bash can be implemented a number of ways without brace expansion:

# Three expression for loop:
for (( i = 0; i < 20; i++ ))
do
    echo $i
done
# While loop:
i=0
while $i -lt 20 
do
    echo $i
    let i++
done
# For loop using seq:
for i in $(seq 0 19)
do
    echo $i
done

A counted for loop using bash sequences requires the least amount of typing:

for i in {0..19}
do
    echo $i
done

But beyond counted for loops, brace expansion is the only way to create a loop with non-numeric "indexes":

for i in {a..z}
do
    echo $i
done

Brace expansion can also be useful when passing multiple long pathnames to a command.

Instead of

# rm /a/long/path/foo /a/long/path/bar

You can simply do

# rm /a/long/path/{foo,bar}

Brace expansion is enabled via the "set -B" command and the "-B" command line option to the shell and disabled via "set +B" and "+B" on the command line.

Q: https://www.linuxjournal.com/content/bash-brace-expansion

Syntax

{string1,string2,...,stringN}
{<START>..<END>}
{<START>..<END>..<INCR>} (Bash 4)
<PREFIX>{........}
{........}<SUFFIX>
<PREFIX>{........}<SUFFIX>

Optionen

Parameter

Umgebungsvariablen

Exit-Status

Konfiguration

Dateien

Sicherheit

Dokumentation

RFC

Man-Pages

Info-Pages

Siehe auch

Links

Projekt-Homepage

Weblinks

Introduction to expansion and substitution

Einzelnachweise

Testfragen

Testfrage 1

Antwort1

Testfrage 2

Antwort2

Testfrage 3

Antwort3

Testfrage 4

Antwort4

Testfrage 5

Antwort5

TMP

Brace expansion

Using Comma-separated lists

echo {geeks,for,geeks}
echo {"hello","world"}

Using Ranges

Different types of ranges can be used, like numeric, alphabetic, or both.
A sequence consists of a starting and ending term separated by 2 periods “..” inside curly braces “{}”.

echo {A..D}
echo {3..8}
echo {A..C}{1..2}

Using Preamble

Patterns to be brace expanded may contain a leading term called a preamble.
The brace expression may contain either a comma-separated list or a range

echo gfg{1..4}
echo gfg{1,5,9}

Using Postscript

Like Preamble, it can also have an ending portion known as Postscript

echo {1..3}gfg
echo {1,5,9}gfg

Bash Expansion can be nested.

echo {a,b{1..3},c}

Bash Expansion can be used with other commands also, like, we can make multiple folders with one command using Brace expansion with mkdir command.

Q: https://www.geeksforgeeks.org/bash-brace-expansion-in-linux-with-examples/

Brace expansion

Bash uses brace expansion to generate a sequence of strings from the terminal or by using any bash script.

A sequence of expressions or comma-separated list of data with curly brackets is used to define brace expansion.
Two optional parts of brace expansion are preamble and postscript.
The preamble is used to add text at the front of each generated string and postscript is used to append text at the end of the generated string by using brace expansion.
How the user can create different types of random strings using bash brace expansion is explained in this tutorial by using various examples.

String List

{String1, String2,... ,StringN }# Range List
{<start> . . <end>}# Preamble and postscript
<preamble>{ string or range }{ string or range }<postscript><preamble{ string or range }<postscript>

The above syntax shows that you can use brace expansion without preamble and postscript or with preamble or with postscript or with both.

Different uses of brace expansions are shown in the next part of this tutorial by using various examples.

Using comma-separated lists

The following command shows the use of brace expansion with comma-separated list only.

You have to remember one thing while defining the list.
That is, list items must be separated by comma only and don’t add any space between the items, otherwise brace expansion will not work.
Here, the first command will display the list items with space.
Two lists are used in the second command and the output will generate by combining each items of each list.

$ echo {PHP,Javascript,JQuery}$ echo {"I like ","Learn "}{"PHP","Programming"}

Output:

The following output will appear after running the script.

In the second command, there are two items in each list and there are two lists.
So, the second command will print (2X2=4), four text as output.

Using Ranges

Different types of ranges can be used in brace expansion to generate the list of data.

The uses of four types of ranges are shown in this example.
The first range will create a list of numeric data, starting from 50 to 60.
The second range will generate a list of alphabetic characters, starting from A to F.
The third range will generate a list of number, starting from 1 to 5 with leading zero.
The forth range will generate a list of alpha-numeric data by combining A to C and 1 to 3.

$ echo {50..60}$ echo {A..E}$ echo {01..05}$ echo {A..C}{1..3}

Output:

The following output will appear after running the above commands.

In the last command, the first brace expansion contains three items and the second brace expansion contains three items.
So, the total items in the output will be, 3X3 = 9.

Using preamble

This example shows the use of preamble in brace expansion.

The first command will add the string, “Hi “ with each item of the list and generate the output.
The second command will add ‘b’ with each item of the list.
This type of task is beneficial if you want to add a common text or character at the front of each list item.

$ echo "Hi "{John, Mohammed, Lisa}$ echo b{all, ell, oat, eef}

Output