伯恩命令解释器常识。Bash 101

command expansions

Simple expansion

• Tilde expansion: echo ~ yields /home/dev
• Parameter expansion: echo $winIp yields 172.18.112.1

Arithmetic expansion

Bash shell does not natively support floating-point arithmetic. However, you can use the bc command to perform floating-point arithmetic in Bash. bc is an arbitrary precision calculator language that can handle floating-point numbers.

$ echo $(( 2 + 2 ))
4
$ echo $(( 2 * 3 ))
6
$ echo $(( 10 / 2 ))
5
$ echo $(( 10 % 3 ))
1
$ echo $(( 2 ** 3 ))
8
$ echo "scale=4; 22/7" | bc
3.1428
$ echo "scale=7; 333/106" | bc
3.1415094

In the above example, we used the $(( )) syntax to perform integer arithmetic expansion. The scale variable is used to specify the number of decimal places to display in the result. In this case, we set it to 2 to display two decimal places. The | symbol (vertical line i.e. pipe symbol) is used to pipe the output of the echo command to bc.

Pathname expansion

Pathname expansion is used to generate a list of filenames that match a pattern. Here are some examples of pathname expansion:

• echo * yields all files in the current directory
• echo *.txt yields all files in the current directory that end with .txt
• echo /usr/*/share yields all directories in /usr that have a subdirectory called share
• echo ./test/input-*.txt yields all input-1.txt, input-2.txt, input-999.txt.
• echo "*.dat* yields all file and directory names that contain .dat.

Grouped brace expansion is also supported.For example, if you have a large collection of images you want to organize into years and months, you can create a series of directories named in numeric “Year-Month” format. This way, the directory names will sort in chronological order. Instead of typing out a complete list of directories, you can use pathname expansion to create them quickly and easily. Here’s an example:

Brace expansion

• echo chapter{1..3} yields chapter1 chapter2 chapter3.
• echo {A..Z..2} yields A C E G I K M O Q S U W Y.
• echo {a..c}" "{1..3} yields a 1 a 2 a 3 b 1 b 2 b 3 c 1 c 2 c 3. Note that blank space is quoted.
• echo {a..c}{x,y}{1..3} yields ax1 ax2 ax3 ay1 ay2 ay3 bx1 bx2 bx3 by1 by2 by3 cx1 cx2 cx3 cy1 cy2 cy3.

Pathname expansion is commonly used to create lists of files or directories to be created. For example, if you have a large collection of images you want to organize into years and months, you can create a series of directories named in numeric “Year-Month” format. This way, the directory names will sort in chronological order. Instead of typing out a complete list of directories, you can use pathname expansion to create them quickly and easily. Here’s an example:

$ mkdir Photos
$ cd Photos
$ mkdir {2017..2019}-{01..12}
$ echo {2017..2019}-{01..12}
2017-01 2017-02 2017-03 2017-04 2017-05 2017-06 2017-07 2017-08 2017-09 2017-10 2017-11 2017-12
2018-01 2018-02 2018-03 2018-04 2018-05 2018-06 2018-07 2018-08 2018-09 2018-10 2018-11 2018-12
2019-01 2019-02 2019-03 2019-04 2019-05 2019-06 2019-07 2019-08 2019-09 2019-10 2019-11 2019-12

This will create directories named 2017-01, 2017-02, 2017-03, and so on, up to 2019-12.

Command substitution

Command substitution allows you to use the output of a command as an argument to another command. For example, echo $(date) will print the current date and time.

$ echo "The current date is `date +%Y-%m-%d`"
The current date is 2023-03-19