Linux bash script - variables, command outputs, and determine statements

Variables

Variables should be named as the combination of numbers, letters, and underscore.

Special variables

Here is the list of special variables and their meanings, which should not be occupied when programming.

VARIABLES	MEANINGS
$0	Name + path of the current script
$n	n is an integer > 0. External variables, entered after the script name before executing
$#	The number of external variables
$*	All the external variables, "$1", "$2", …
"$*"	All the external variables as a single variable, "$1 $2 ..."
$@, "$@"	Same as $*
$?	Exit status of the last command. Successful, =0; Failed, >0
$$	PID of the current process

Environmental variables

The specific definition of environmental variables depend on the specific environment. Here are some common environmental variables.

VARIABLES	MEANINGS
$HOME	Home directory
$USER	User name
$PWD	Current directory
$PATH	Path to executables
$LD_LIBRARY_PATH	Path to libraries to be linked with -D -l flag
$INCLUDE	Path to header files to be included with -I flag

Variable operations

List variables are defined as (elem1 elem2 ...). Common operations for a list variable $LIST:

OPERATIONS	MEANINGS
${LIST[n]}	n >= 0, the nth element of $LIST
$LIST	The first element of $LIST, ${LIST[0]}
${LIST[@]}, ${LIST[*]}	All the elements of $LIST
${#LIST[@]}	The length of $LIST

String variables can be truncated from a specified character or a specified index. Taking link='spica-vir.github.io' as an example:

OPERATIONS	MEANINGS
${link%.*}	Cut the right most ‘.’ and the content on its right, spica-vir.github
${link%%.*}	Cut the left most ‘.’ and the content on its right, spica-vir
${link#*.}	Cut the left most ‘.’ and the content on its left, github.io
${link##*.}	Cut the right most ‘.’ and the content on its left, io
${link: 3:6}	Return to the string with indices 3~6, including 2 ends, ca-v
${link: 6}	Return to the string starting from the index 6, vir.github.io
${link: -6}	Return to the string of the last 6 characters, hub.io

Command outputs

The output of commands can be redirected using the symbols listed below:

SYMBOLS	MEANINGS
0	Standard input
1	Standard output
2	Standard error
command > file	Redirect the standard output of ‘command’ to a newly created ‘file’
command >> file	Redirect the standard output of ‘command’ to the end of ‘file’. If ‘file’ does not exist, create it
command >& file, command &> file	Redirect the standard output and standard error of ‘command’ to a newly created ‘file’
2>&1	Copy the behavior of standard output and use it for standard error
: > file	Create an empty file

Analysis:

# Command 1
$ command > file.out 2>&1
# Command 2
$ command 2>&1 > file.out

• Command 1: Standard output redirected to ‘file.out’; Copied to standard error; Standard error redirected to ‘file.out’. —> Output and error printed in ‘file.out’.
• Command 2: Standard output by default printed on screen; Copied to standard error; Standard output redirected to ‘file.out’. —> Output printed in ‘file.out’, error printed on screen.

Determine statements

Determine statements are important for both ‘if’ sentences and ‘while’ loops. The differences between statements acting on variables / files, or numbers / strings should be noticed.

For files and directories

OPERATORS	MEANINGS
-e	File / directory exists, True
-s	File / directory not empty, True
-d	Is a directory, True
-f	Is a regular file, True
-h, -L	Is a soft link, True
-r	Readable, True
-w	Writable, True
-x	Executable, True

For variables

OPERATORS	MEANINGS
$variable	Variable exists, True
-n	Variable is not empty, True
-z	Variable is empty, True

For strings

For string comparison, in case of spaces, adding “” to protect variables is recommended. Besides, == is preferred to -eq due to the same reason.

OPERATOR	MEANING
"$string_1" == *"$string_2"*	If $string_2 is contained within $string_1, True

For numbers

To compare integer values in if statement, use:

COMMAND	MEANING
==, -eq	Equal to
-gt	Greater than
-lt	Less than
-ge	Greater than or equal to
-le	Less than or equal to

To calculate integers, use:

COMMAND	MEANING
c=$(( a + b ))	Save the sum of $a and $b to $c. Note there is no dollar sign in parenthesis

To compare float numbers, bc command should be used. bc command returns an integer variable, and it cannot be used as determine statements, otherwise the statement can only tell whether the specific variable is empty.

COMMAND	MEANING
$(echo "$a==$b" | bc) == 1	Float equal
$(echo "$a>$b" | bc) == 1	Float greater than

Use bc to run float calculations:

COMMAND	MEANING
echo "scale=2;$a+$b" | bc	Float sum, rounded to 2 decimal places