3. Shell comparison operator & reference

—

by

Command	Description	Example
&	Run the previous command in the background	`ls &`
&&	Logical AND	`if [ "$X" -ge "0" ] && [ "$X" -le "9"]`
\|\|	Logical OR	`if [ "$X" -lt "0" ] \|\| [ "$X" -gt "9" ]`
^	Start of line	`grep "^X"`
$	End of line	`grep "X$"`
=	String equality (cf. -eq)	`if [ "$X" = "bar" ]`
!	Logical NOT	`if [ "$X" != "bar" ]`
$$	PID (Process ID)of current shell	`echo "my PID = $$"`
$!	PID of last background command	`ls & echo "PID of ls = $!"`
$?	exit status of last command	`ls ; echo "ls returned code $?"`
$0	Name of current command (as called)	`echo "I am $0"`
$1	Name of current command’s first parameter	`echo "The first argument is $1"`
$9	Name of current command’s ninth parameter	`echo "The ninth argument is $9"`
$@	All of current command’s parameters (preserving whitespace and quoting)	`echo "The arguments are $@"`
$*	All of current command’s parameters (not preserving whitespace and quoting)	`echo "The arguments are $*"`
-eq	Numeric Equality	`if [ "$X" -eq "9" ]`
-ne	Numeric Inquality	`if [ "$X" -ne "9" ]`
-lt	Less Than	`if [ "$X" -lt "9" ]`
-le	Less Than or Equal	`if [ "$X" -le "9" ]`
-gt	Greater Than	`if [ "$X" -gt "9" ]`
-ge	Greater Than or Equal	`if [ "$X" -ge "9" ]`
-z	String is zero length	`if [ -z "$X" ]`
-n	String is not zero length	`if [ -n "$X" ]`
-nt	Newer Than	`if [ "$file1" -nt "$file2" ]`
`-ot`	Older Than	if [ “$file1” -ot “$file2” ]
-d	Is a Directory	`if [ -d /bin ]`
-f	Is a File	`if [ -f /bin/ls ]`
-r	Is a readable file	`if [ -r /bin/ls ]`
-w	Is a writable file	`if [ -w /bin/ls ]`
-x	Is an executable file	`if [ -x /bin/ls ]`
`-a`, `-e`	file exists
-ef	Paths refer to the same file
-O	File is owned by the user running the test
-S	Denotes Socket File
( … )	Function definition	`function thefunc() { echo hello world }`

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