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 } |
3. Shell comparison operator & reference
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