Tlcl-view the world echo in shell eyes

Tlcl-view the world echo in shell eyes

View the world in shell eyes

Echo-display a line of text

When you enter a command, bash expands the input characters before executing the command.

Expand path

Echo *

Shell expands * to the name of the file in the current directory, and prints the name of the file in the current directory,

Echo d * print all files starting with d in the current directory

Echo * d: print all files ending with d in the current directory.

~ Expand

Echo ~ Print the home directory information of the current user

Echo ~ Test prints the home directory information of the test user.

Arithmetic expression Expansion

Echo $ (expression ))

Echo $ (2*3) + 5 ))

Echo $(2 ** 3) * 5 ))

Note the expression format. arithmetic expressions only support integer, addition, subtraction, multiplication, division, and power operations.

Curly braces

Create multiple text strings from a curly brackets Pattern

Echo hello {1, 2, 3} Will output hello1 hello2 hello3

Echo hello {1, 2, 3} Will output hello 1 2 3

Echo hello {a, B, c} Will output helloa hellob helloc

Note: The elements in {} must be separated by commas (,). They can be characters or strings.

The most common application is to create a series of files or directories.

Mkdir hello {1, 2, 4, 5}

Expand Parameters

Echo $ USER print the USER name

View Variable list

Printenv | less

When the expanded variable does not exist, the system expands it into an empty string.

Echo $ HELLO

Command replacement

Use the output of a command as an extension

Echo $ (ls)

Ls-l $ (which cp) uses the whic cp output information as the ls Parameter

Or

Ls-l 'which cp' old shell support, inverted quotation marks

Reference

Echo hello world

The hello world will be printed on the screen.

Echo the total is $100.00

The total is 00.00 will be printed on the screen

In the first medium, shell will delete unnecessary spaces. In the second medium, $ will expand the variable 1, and 1 is not a variable, so it will be expanded into a null string.

Shell provides a reference mechanism that effectively controls the expansion of characters.

Double quotation marks

The first type of reference controls the expansion of characters.

If you add "" at both ends of the text, all characters except $ \ '(inverted quotation marks) in the text will not have special expanded meanings.

In double quotation marks, parameter expansion, expression expansion, and command expansion are still valid.

Echo "$ USER $(2 ** 3) $ (ls )"

Echo "hello world" will print the hello world

Word Segmentation Mechanism

Echo hello world

The output is hello world.

By default, the word segmentation mechanism searches for spaces, tabs, and line breaks in words and regards them as delimiters between words. Each word is a parameter, in the preceding example, there are two parameters. Each parameter is printed and defined by space.

Similarly, if you enter echo $ (cal)

In this case, a line of data with spaces is printed, without line breaks.

If echo "hello world" is input"

Word Segmentation is not allowed. In fact, "internal content as a parameter exists, but" not added "is two parameters.

Let's see the difference between echo $ (cal) and echo "$ (cal )".

Single quotes

The second type of reference restricts the expansion of characters. single quotation marks limit the expansion of all characters. All characters only have their own meaning and do not have any special meaning.

Escape characters

Use \ to disable the expansion of special characters. Note that \ will be ineffective in single quotes.

Echo \ "hello world \"

Use of backslash escape sequences

\ A sends an alert;

\ B Delete the previous character;

\ C does not end with a line break;

\ F line feed, but the cursor remains at the original position;

\ N wrap and move the cursor to the beginning of the line;

\ R move the cursor to the beginning of the line without line breaks;

\ T Insert tab;

\ V is the same as \ f;

For example, \ a bell

Echo-e '\ a'-e Option to explain escape characters. Although \ does not work in single quotes,-e Option explains it

Echo $ '\'

Cannot directly echo "\"

Finally, I learned a-n option.

-N: output without line breaks

Echo "hello"; echo "world" at this time, the hello world will be output in two rows

Echo-n "hello"; echo "world" will output a line of helloworld