Description

This is a reference for bash scripting. For more general instructions take a look here.

Operators

Usage

Bash operators are mainly used in if statements, to check against a variable or a hard coded object. Example:

   1 #!/bin/bash 
   2 name=Foo  
   3 
   4 if [ -e $name ] 
   5 then 
   6     echo File Exist 
   7 else
   8     echo File doesnot exist 
   9 fi

File Test Operators

Operator	Checks for	Returns
(`-a`)	File exits DEPRECATED	True if it exits, otherwise false
`-b`	File is a block device	True if the file is a block device, otherwise false
`-c`	File is a character device	True if it is a character device, otherwise false
`-d`	File is a directory	True if it is a directory, otherwise false
`-e`	File exits	True if it exits, otherwise false
`-ef`	Files are hard links to the same file	True if files before and after operator are hard links to the same file, otherwise false
`-f`	File is regular file	True if regular, false if directory or device file
`-g`	Set-group-id (sgid) flag set on directory¹	True if flag is set, otherwise false
`-G`	Group-id of file is equal to the current user's	True group-id is equal to the user's, otherwise false
`-h`	File is a symbolic link	True if file is a symbolic link, otherwise false
`-k`	Sticky bit set²	True if bit is set, otherwise false
`-L`	File is a symbolic link	True if file is a symbolic link, otherwise false
`-N`	File is modified since last read	True file is modified since last read, otherwise false
`-nt`	File is newer than other file	True if file preceding operator is newer than file behind the operator, otherwise false
`-ot`	File is older than other file	True if file preceding operator is older than file behind the operator, otherwise false
`-O`	Current user is owner of file	True if user is owner, otherwise false
`-p`	File is a pipe	True if file is a pipe, otherwise false
`-r`	File has read permission (for the user running the test)	True if it has read access, otherwise false
`-s`	File Size	True if file > 0, otherwise false
`-S`	File is a socket	True if file is a socket, otherwise false
`-t`	File (descriptor) is associated with a terminal device³	True if file is associated with a terminal device, otherwise false
`-u`	Set-user-id (suid) flag set on file⁴	True if flag is set, otherwise false
`-w`	File has write permission (for the user running the test)	True if it has write access, otherwise false
`-x`	File has execute permission (for the user running the test)	True if it has execute access, otherwise false
`!`	NOT, reverses the tests above	True if condition absent, otherwise reverse of condition

Integer Comparison Operators

Operator	C-style variant	Meaning	Usage	Usage C-Style
`-eq`	none	Is equal to	`if [ "$a" -eq "$b" ]`	-
`-ne`	none	Is not equal to	`if [ "$a" -ne "$b" ]`	-
`-gt`	`>`	Is greater than	`if [ "$a" -gt "$b" ]`	`(("$a" > "$b"))`
`-ge`	`>=`	Is greater or equal to	`if [ "$a" -ge "$b" ]`	`(("$a" >= "$b"))`
`-lt`	`<`	Is less than	`if [ "$a" -lt "$b" ]`	`(("$a" < "$b"))`
`-le`	`<=`	Is less than or equal to	`if [ "$a" -le "$b" ]`	`(("$a" <= "$b"))`

String Comparison Operators

Operator	Meaning	Usage
`=`	Is equal to	`if [ "$a" = "$b" ]`
`==`	Is equal to⁵	`if [ "$a" == "$b" ]`
`!=`	Is not equal to	`if [ "$a" != "$b" ]`

TODO: Continue here https://tldp.org/LDP/abs/html/comparison-ops.html

=

Is equal to

if [ "$a" = "$b" ]

Arithmetic Operators

Operator	Name	Funtion
`+`	Plus	3 + 6 = 6
`-`	Minus	6 - 3 = 3
`*`	Multiply	3 * 3 = 9
`/`	Divide	9 / 3 = 3
`**`	Exponent	3 ** 2 = 9
`%`	Modulo	24 % 4 = 4

Each arithmetic operator can be followed with an equals sign(=), which writes the output of the operation into the first variable. An example:

   1 #!/bin/bash
   2 a=2
   3 $((a += 1)) 
   4 echo $a
   5 
   6 # returns: 3
   7

Logical Operators

Operator	Name	Funtion
`&&`	AND	"true" && "false" = false
`\|\|`	OR	"true" `\|\|` "false" = true
`!`	NOT	! "true" = false

Usage:

   1 #!/bin/bash 
   2 if [ $condition1 ] && [ $condition2 ]; ...
   3 if [[ $condition1 && $condition2 ]]; ...
   4 if [ ! -f $FILENAME ]; ...

Bitwise Operators

Operator	Name	Funtion
`&`	AND	1010 & 1100 = 1000
`\|`	OR	1010 \| 1100 = 1110
`^`	XOR	1010 ^ 1100 = 0110
`~`	NOT/Complement	~1010 = 0101
`<<`	Left Shift	1100 << 1 = 1000
`>>`	Right Shift	1100 >> 1 = 0110

Bitwise operators are usually not used in if statements, but for number manipulation. An example:

   1 #!/bin/bash 
   2 
   3 a=$((2#1010))
   4 b=12
   5   
   6 bitwiseAND=$(( a&b )) 
   7 echo Bitwise AND of a and b is $bitwiseAND 
   8 
   9 # returns: Bitwise AND of a and b is 8
  10

Each bitwise operator can be followed with an equals sign(=), which writes the output of the operation into the first variable. An example:

   1 #!/bin/bash
   2 a=2
   3 $((a <<= 1)) 
   4 echo $a
   5 
   6 # returns: 4
   7

If a directory has the sgid flag set, then a file created within that directory belongs to the group that owns the directory, not necessarily to the group of the user who created the file. This may be useful for a directory shared by a workgroup. (1)
Commonly known as the sticky bit, the save-text-mode flag is a special type of file permission. If a file has this flag set, that file will be kept in cache memory, for quicker access. [3] If set on a directory, it restricts write permission. Setting the sticky bit adds a t to the permissions on the file or directory listing. This restricts altering or deleting specific files in that directory to the owner of those files. (2)
This test option may be used to check whether the stdin [ -t 0 ] or stdout [ -t 1 ] in a given script is a terminal. (3)
A binary owned by root with set-user-id flag set runs with root privileges, even when an ordinary user invokes it. (4)
The == comparison operator behaves differently within a double-brackets test than within single brackets. See this documentation (5)

-  ⇤ ← Revision 2 as of 2020-05-27 21:32:57 → 
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+   ← Revision 3 as of 2020-05-27 21:58:05 → ⇥
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+== Integer Comparison Operators ==
||'''Operator'''||'''C-style variant'''||'''Meaning'''||'''Usage'''||'''Usage C-Style'''||
||<:>`-eq` ||<:> none || Is equal to || `if [ "$a" -eq "$b" ]` || - ||
||<:>`-ne` ||<:> none || Is not equal to || `if [ "$a" -ne "$b" ]` || - ||
||<:>`-gt` ||<:> `>` || Is greater than || `if [ "$a" -gt "$b" ]` || `(("$a" > "$b"))` ||
||<:>`-ge` ||<:> `>=` || Is greater or equal to || `if [ "$a" -ge "$b" ]` || `(("$a" >= "$b"))` ||
||<:>`-lt` ||<:> `<` || Is less than || `if [ "$a" -lt "$b" ]` || `(("$a" < "$b"))` ||
||<:>`-le` ||<:> `<=` || Is less than or equal to || `if [ "$a" -le "$b" ]` || `(("$a" <= "$b"))` ||

== String Comparison Operators ==
||'''Operator'''||'''Meaning'''||'''Usage'''||
||<:>`=` || Is equal to || `if [ "$a" = "$b" ]` ||
||<:>`==` || Is equal to<<FootNote(The == comparison operator behaves differently within a double-brackets test than within single brackets. See [[https://tldp.org/LDP/abs/html/comparison-ops.html | this documentation ]])>> || `if [ "$a" == "$b" ]` ||
||<:>`!=` || Is not equal to || `if [ "$a" != "$b" ]` ||
TODO: Continue here https://tldp.org/LDP/abs/html/comparison-ops.html
||<:>`=` || Is equal to || `if [ "$a" = "$b" ]` ||

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