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The Theory

There are two concepts that are in play here :

• IFS is the Input Field Separator, which means the string read will be split based on the characters in IFS. On a command line, IFS is normally any whitespace characters, that's why the command line splits at spaces.
• Doing something like VAR=value command means "modify the environment of command so that VAR will have the value value". Basically, the command command will see VAR as having the value value, but any command executed after that will still see VAR as having its previous value. In other words, that variable will be modified only for that statement.

In this case

So when doing IFS= read -r line, what you are doing is setting IFS to an empty string (no character will be used to split, therefore no splitting will occur) so that read will read the entire line and see it as one word that will be assigned to the line variable. The changes to IFS only affect that statement, so that any following commands won't be affected by the change.

As a side note

While the command is correct and will work as intended, setting `IFS` in this case is not *might¹ not be* necessary. As written in the `bash` man page in the `read` builtin section :

One line is read from the standard input [...] and the first word is assigned to the first name, the second word to the second name, and so on, with leftover words and their intervening separators assigned to the last name. If there are fewer words read from the input stream than names, the remaining names are assigned empty values. The characters in IFS are used to split the line into words. [...]

Since you only have the line variable, every words will be assigned to it anyway, so if you don't need any of the preceding and trailing whitespace characters¹ you could just write read -r line and be done with it.

[1] As rightly noted by mikeserv, the preceding and trailing characters won't survive if IFS is not unset. Furthermore, some strange things could happen if IFS was to be modified somewhere earlier in the script.

The Theory

There are two concepts that are in play here :

• IFS is the Input Field Separator, which means the string read will be split based on the characters in IFS. On a command line, IFS is normally any whitespace characters, that's why the command line splits at spaces.
• Doing something like VAR=value command means "modify the environment of command so that VAR will have the value value". Basically, the command command will see VAR as having the value value, but any command executed after that will still see VAR as having its previous value. In other words, that variable will be modified only for that statement.

In this case

So when doing IFS= read -r line, what you are doing is setting IFS to an empty string (no character will be used to split, therefore no splitting will occur) so that read will read the entire line and see it as one word that will be assigned to the line variable. The changes to IFS only affect that statement, so that any following commands won't be affected by the change.

As a side note

While the command is correct and will work as intended, setting `IFS` in this case is not *might¹ not be* necessary. As written in the `bash` man page in the `read` builtin section :

One line is read from the standard input [...] and the first word is assigned to the first name, the second word to the second name, and so on, with leftover words and their intervening separators assigned to the last name. If there are fewer words read from the input stream than names, the remaining names are assigned empty values. The characters in IFS are used to split the line into words. [...]

Since you only have the line variable, every words will be assigned to it anyway, so if you don't need any of the preceding and trailing whitespace characters¹ you could just write read -r line and be done with it.

[1] Just as an example of how an unset or default $IFS value will cause read to regard leading/trailing IFS whitespace, you might try:

echo ' where are my spaces? ' | { 
    unset IFS
    read -r line
    printf %s\\n "$line"
} | sed -n l

Run it and you will see that the preceding and trailing characters won't survive if IFS is not unset. Furthermore, some strange things could happen if $IFS was to be modified somewhere earlier in the script.

The Theory

There are two concepts that are in play here :

• IFS is the Input Field Separator, which means the string read will be split based on the characters in IFS. On a command line, IFS is normally any whitespace characters, that's why the command line splits at spaces.
• Doing something like VAR=value command means "modify the environment of command so that VAR will have the value value". Basically, the command command will see VAR as having the value value, but any command executed after that will still see VAR as having its previous value. In other words, that variable will be modified only for that statement.

In this case

So when doing IFS= read -r line, what you are doing is setting IFS to an empty string (no character will be used to split, therefore no splitting will occur) so that read will read the entire line and see it as one word that will be assigned to the line variable. The changes to IFS only affect that statement, so that any following commands won't be affected by the change.

Going further

While the command is correct and will work as intended, setting `IFS` in this case is not necessary. As written in the `bash` man page in the `read` builtin section :

One line is read from the standard input [...] and the first word is assigned to the first name, the second word to the second name, and so on, with leftover words and their intervening separators assigned to the last name. If there are fewer words read from the input stream than names, the remaining names are assigned empty values. The characters in IFS are used to split the line into words. [...]

Since you only have the line variable, every words will be assigned to it anyway, so you could just write read -r line and be done with it.

The Theory

There are two concepts that are in play here :

• IFS is the Input Field Separator, which means the string read will be split based on the characters in IFS. On a command line, IFS is normally any whitespace characters, that's why the command line splits at spaces.
• Doing something like VAR=value command means "modify the environment of command so that VAR will have the value value". Basically, the command command will see VAR as having the value value, but any command executed after that will still see VAR as having its previous value. In other words, that variable will be modified only for that statement.

In this case

So when doing IFS= read -r line, what you are doing is setting IFS to an empty string (no character will be used to split, therefore no splitting will occur) so that read will read the entire line and see it as one word that will be assigned to the line variable. The changes to IFS only affect that statement, so that any following commands won't be affected by the change.

As a side note

While the command is correct and will work as intended, setting `IFS` in this case is not *might¹ not be* necessary. As written in the `bash` man page in the `read` builtin section :

One line is read from the standard input [...] and the first word is assigned to the first name, the second word to the second name, and so on, with leftover words and their intervening separators assigned to the last name. If there are fewer words read from the input stream than names, the remaining names are assigned empty values. The characters in IFS are used to split the line into words. [...]

Since you only have the line variable, every words will be assigned to it anyway, so if you don't need any of the preceding and trailing whitespace characters¹ you could just write read -r line and be done with it.

[1] As rightly noted by mikeserv, the preceding and trailing characters won't survive if IFS is not unset. Furthermore, some strange things could happen if IFS was to be modified somewhere earlier in the script.

There are two concepts that are in play here :

• IFS is the Input Field Separator, which means the string read will be split based on the characters in IFS. On a command line, IFS is normally any whitespace characters, that's why the command line splits at spaces.
• Doing something like VAR=value command means "modify the environment of command so that VAR will have the value value". Basically, the command command will see VAR as having the value value, but any command executed after that will still see VAR as having its previous value. In other words, that variable will be modified only for that statement.

So when doing IFS= read -r line, what you are doing is setting IFS to an empty string (no character will be used to split, therefore no splitting will occur) so that read will read the entire line and see it as one word that will be assigned to the line variable. The changes to IFS only affect that statement, so that any following commands won't be affected by the change.

In this case, this is not very useful. As written in the bash man page in the read builtin section :

One line is read from the standard input [...] and the first word is assigned to the first name, the second word to the second name, and so on, with leftover words and their intervening separators assigned to the last name. If there are fewer words read from the input stream than names, the remaining names are assigned empty values. The characters in IFS are used to split the line into words. [...]

Since you only have the line variable, every words will be assigned to it anyway, so you could just write read -r line and be done with it.

The Theory

There are two concepts that are in play here :

• IFS is the Input Field Separator, which means the string read will be split based on the characters in IFS. On a command line, IFS is normally any whitespace characters, that's why the command line splits at spaces.
• Doing something like VAR=value command means "modify the environment of command so that VAR will have the value value". Basically, the command command will see VAR as having the value value, but any command executed after that will still see VAR as having its previous value. In other words, that variable will be modified only for that statement.

In this case

So when doing IFS= read -r line, what you are doing is setting IFS to an empty string (no character will be used to split, therefore no splitting will occur) so that read will read the entire line and see it as one word that will be assigned to the line variable. The changes to IFS only affect that statement, so that any following commands won't be affected by the change.

Going further

While the command is correct and will work as intended, setting `IFS` in this case is not necessary. As written in the `bash` man page in the `read` builtin section :

One line is read from the standard input [...] and the first word is assigned to the first name, the second word to the second name, and so on, with leftover words and their intervening separators assigned to the last name. If there are fewer words read from the input stream than names, the remaining names are assigned empty values. The characters in IFS are used to split the line into words. [...]

Since you only have the line variable, every words will be assigned to it anyway, so you could just write read -r line and be done with it.