Bash command expansion

1. What is expansion in bash?
2. How do you expand a variable in bash?
3. What is shell command line expansion?
4. What does #! Mean in bash?
5. What is path name expansion?
6. What is filename expansion?
7. How do you check if a variable is defined in bash?
8. How do you do if in bash?
9. What is the use of tee command?

What is expansion in bash?

The expansion is a string that is the value of parameter with backslash escape sequences expanded as with the $'…' quoting mechanism. P. The expansion is a string that is the result of expanding the value of parameter as if it were a prompt string (see Controlling the Prompt).

How do you expand a variable in bash?

Parameter expansion comes in many forms in bash, the simplest is just a dollar sign followed by a name, eg $a. This form merely substitutes the value of the variable in place of the parameter expansion expression. The variable name can also optionally be surround by braces, eg $a.

What is shell command line expansion?

History expansion is the process that bash uses to turn a history command into an executable command line. For example, when you give the command !!, history expansion changes that command line so it is the same as the previous one. ... History expansion does not apply to noninteractive shells (shell scripts).

What does #! Mean in bash?

6. @ShivanRaptor #!/bin/bash Means run this script in bash. #!/bin/sh means run this script in sh which is the default unix shell, which might be bash or any other variant like ksh, dash, zsh, etc – Karthik T Dec 14 '12 at 3:10.

What is path name expansion?

When you specify an abbreviated filename that contains special characters, also called metacharacters, the shell can generate filenames that match the names of existing files. ... The process the shell performs on these filenames is called pathname expansion or globbing.

What is filename expansion?

Another way to save time in your commands is to use special characters to abbreviate filenames. You can specify many files at once by using these characters. This feature of the shell is sometimes called "globbing."

How do you check if a variable is defined in bash?

To find out if a bash variable is defined:

Return true if a bash variable is unset or set to the empty string: if [ -z $my_variable+x ]; Also try: [ -z $my_bash_var+y ] && echo "\$my_bash_var not defined" Determine if a bash variable is set or not : [[ ! -z $PURGEIMAGE+z ]] && echo "Set" || echo "Not defined"

How do you do if in bash?

A Shell script usually needs to test if a command succeeds or a condition is met. In Bash, this test can be done with a Bash if statement.
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What are the Bash Conditional Expressions?

What is the use of tee command?

The tee command is normally used to split the output of a program so that it can be both displayed and saved in a file. The command can be used to capture intermediate output before the data is altered by another command or program. The tee command reads standard input, then writes its content to standard output.