Bash Arrays#

Serialize and Store Bash Array to a File#

Let’s save the array nums below as NUL-separated values in nums.dat

$ nums=(1 2 3 4)
$ printf '%s\0' > nums.dat

If you less nums-arr.dat or open it in vim or emacs, it’ll show like this:

1^@2^@3^@4^@

The numbers are the numbers we stored 🤣, and the “^@” is not a caret followed by an at sign, but a visual representation of the NUL. Just that when some tools wants to display them, they have to use something visible and the convention is to use “^@”, which many tools do.

We can also inspect the file with od:

$ od -cax nums.txt
0000000   1  \0   2  \0   3  \0   4  \0
          1 nul   2 nul   3 nul   4 nul
           0031    0032    0033    0034

Anyway, we serialized our array and stored it in a file.

Deserialize Bash Array From a File#

How to deserialize the contents of nums.dat and turn it back into code‽ By using mapfile.

Using mapfile#

To deserialize the data and turn it back into code – an array – we use mapfile:

$ IFS= readarray -d '' them < nums-arr.dat
$ printf '%d\n' "${them[@]}"
1
2
3
4

Why can’t we use read‽#

If we try read instead of mapfile, we see it doesn’t work:

$ IFS= read -ra nums < nums.txt

$ printf '%d\n' "${nums[@]}"
1234

$ IFS=\\0 read -ra nums < nums.txt

$ printf '%d\n' "${nums[@]}"
1234

It seems even using IFS=\\0 or IFS='\0' don’t work. Let’s use this to learn more about both read and mapfile, shall we‽

Note

readarray is an alias to mapfile

Understanding mapfile vs read#

So, why does read -a -d '' does not work, while mapfile -d '' does‽

First let’s see what read and mapfile are supposed to do.

help read says:

“Read a line from the standard input and split it into fields.”

Reads a single line from the standard input, or from file descriptor FD if the -u option is supplied. The line is split into fields as with word splitting, and the first word is assigned to the first NAME, the second word to the second NAME, and so on, with any leftover words assigned to the last NAME. Only the characters found in $IFS are recognized as word delimiters.”

—help read

And help mapfile says:

“Read lines from the standard input into an indexed array variable.

Read lines from the standard input into the indexed array variable ARRAY, or from file descriptor FD if the -u option is supplied. The variable MAPFILE is the default ARRAY.”

—help mapfile

The important bits for our case is that read reads a single line, and mapfile reads reads lines. Note the plural on “lines” is a very important detail here.

Now let’s scrutinize the -d option of both commands.

help read says:

-d delim  continue until the first character of SELIM is
read, rather than newline

help mapfile says:

-d delim  Use DELIM to terminate lines, instead of newline

OK, so -d does the same thing for both commands. They use the delimiter in -d DELIM to indicate what character should be used to indicate line termination, rather than \n.

That means read -d '' will read the first value of our NUL-separated input and consider it a line of input, and be done with it (after all, read “reads a single line of input”).

mapfile will also read the first value of our NUL-separated input and consider it a line of input, but rather than be done with it at this point, it will continue reading more likes (after all, mapfile “reads multiple lines of input”).

IFS= and \0#

One thing to consider is that variables cannot hold NUL Also, in

But:

$ printf a\\0b | od -A n -tac
a nul   b
a  \0   b

It wouldn’t be particularly useful if variables could store NUL since the point of a variable is usually to be used in the environment or as an argument to a command, where NULs are not accepted either.

printf interprets \0 but IFS=\\0 is something different.

The spec says: “Variables shall be initialized from the environment”.

And we can’t have NUL in the environment.

Note

This topic is hard and has tormented me for a long time 😅.

Fernando-Basso About “variables cannot hold NUL”, 2.5 Paramater and Variables states that “A parameter is set if it has an assigned value (null is a valid value).” And “a variable is a parameter denoted by name”. The more I try to understand, the more I do not understand.

Soliton they mean empty string with null there.

phogg Fernando-Basso: a variable set to an empty string is indistinguishable from a variable set to null.

Soliton i think they should just write empty string but…

phogg probably should, but if you write a lot of C it can be hard to notice mistakes like that I assume that in the implementation there is a struct for the variable and it can be allocated (var defined) or not (var undefined), even if the allocated struct has a NUL where the value would go. From that POV it makes sense. none of it means that the user can put a NUL into a variable

field separator vs terminator#

We also have to be clear on the fact that field separator is different than terminator. A terminator could indicate the end of input, end of a line, etc. An input could be separated into multiple fields, and each field could be an entire line, so multiple lines would mean multiple fields.

Some people say that it makes more sense to use \n as field separator and \0 (NUL) as terminators rather than the other way around.

Serialize a NUL-separated list of files to a variable#

$ mapfile -td '' files < <(find ... -print0)
$ printf %s\\0 "${files[@]}"