Bash Pathname Expansion in Linux
When you type a command and press enter, bash performs several processes upon the text before carrying out our command as a command. The process that makes this happen is called expansion. For example, suppose you use the echo command for standard output.
echo hi Output hi
If you use echo with asterisk wildcard echo will print your directory. Suppose you have two files in the current working directory. The first file name is file1.txt and file2.txt then you will use echo with asterisk wildcard then It will print your directory name.
Example:
Types of Expansion
- Pathname Expansion
- Tilde Expansion
- Arithmetic Expansion
- Brace Expansion
- Parameter Expansion
- Command Substitution
Pathname Expansion
The mechanism by which wildcards work is called pathname expansion. you can use wildcard after the filename or before the filename.
echo filestarting_name*
Example:
Tilde Expansion
“~” Tilde Represents the home directory in the Linux system. Use tilde as an expansion
This Command change working directory to home directory cd ~ This Command change working directory to foo cd ~/foo
Arithmetic Expansion
The Bash shell allows arithmetic expansion. This allows us to perform a Linux shell as a calculator.
Syntax:
echo $((expression))
Example
echo $((3+3))
Brace Expansion
With this Expansion, you can create multiple text strings from a pattern containing braces
Example:
Parameter Expansion
If you know any programming language basics then parameter expansion is easy for you because it’s similar to a data variable. In Linux, You can define a variable like this
# This is $variable_name parameter expansion name="GFG" echo $name GFG
Example:
Mostly Parameter Expansion is used in Bash Script
Command Substitution
Command substitution uses the output of a command as an expansion
Example:
echo $(ls)
file $(ls -d /usr/bin/* | grep zip)
You can also use quotes for command substitution
Example:
ls -l ' which cp'
Quoting
Quoting helps to control Linux command expansion
In this example, You will see your extra space remove by the Linux shell.
$ echo Hi My name is GFG output : Hi My name is GFG
Output
Double Quotes
Double quotes stop the expansion. If you place characters inside of double quotes, all special characters used by the shell have their special meaning and are treated as ordinary characters. This means some expansions do not work in double quotes example: pathname expansion, tilde expansion, and brace expansion are suppressed.
Example:
Now you can see here that the shell does not remove any space
echo "Hi GFG" Expected Output: Hi GFG
Output
But Some expansion still works example: arithmetic expansion, and command substitution.
Single Quotes
If you want to suppress all expansions. You can use single quotes.
echo ‘text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER’
Expected Output: text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER
Escaping Characters
Sometimes we want to quote a single character. To do this, we can precede characters with a backslash called escape characters.
Example
echo "HI \n GFG"
Expected Output: Hi
GFG
Backslash Tricks
Let’s take an example, On our desktop, we have a “Backslash Folder” in this folder we also have 1 to 50 folders and the first Folder also has 1 to 50 fifty folders. Sometimes It’s very tedious work to type every folder name so we can solve this problem by backslash and asterisk. See the Syntax
Syntax
cd SourceFolder/*/Destination Folder
Example
cd 1/*/50
Output
You can see directory read-write permission without changing the current working directory using backslash
ls -l /directory_name/*
