This is more shell dependent than OS dependent.

Depending on the version, ksh read the script on demand by 8k or 64k bytes block.

bash read the script line by line. However, given the fact lines can be of arbitrary lenght, it reads each time 8176 bytes from the beginning of the next line to parse.

This is for simple constructions, i.e. a suite of plain commands.

If shell structured commands are used (a case the accepted answer miss to consider) like a for/do/done loop, a case/esac switch, an here document, a subshell enclosed by parentheses, a function definition, etc. and any combination of the above, shell interpreters reads up to the end of the construction to first make sure there is no syntax error.

This is somewhat inefficient as the same code can be read again and again a large number of times but mitigated by the fact this content is normally cached.

Whatever the shell interpreter, it is very unwise to modify a shell script while it is being executed as the shell is free to read again any portion of the script and this can lead to unexpected syntax errors if out of sync.

Note too that bash might crash with a segmentation violation when it is unable to store an overly large script construction ksh93 can read flawlessly.

This is more shell dependent than OS dependent.

Depending on the version, ksh read the script on demand by 8k or 64k bytes block.

bash read the script line by line. However, given the fact lines can be of arbitrary lenght, it reads each time 8176 bytes from the beginning of the next line to parse.

This is for simple constructions, i.e. a suite of plain commands.

If shell structured commands are used (a case the accepted answer misses to consider) like a for/do/done loop, a case/esac switch, an here document, a subshell enclosed by parentheses, a function definition, etc. and any combination of the above, shell interpreters reads up to the end of the construction to first make sure there is no syntax error.

This is somewhat inefficient as the same code can be read again and again a large number of times but mitigated by the fact this content is normally cached.

Whatever the shell interpreter, it is very unwise to modify a shell script while it is being executed as the shell is free to read again any portion of the script and this can lead to unexpected syntax errors if out of sync.

Note too that bash might crash with a segmentation violation when it is unable to store an overly large script construction ksh93 can read flawlessly.

This is more shell dependent than OS dependent.

ksh read the script on demand by 8192 bytes block.

bash read the script line by line. However, given the fact lines can be of arbitrary lenght, it reads each time 8176 bytes from the beginning of the next line to parse.

This is for simple constructions, i.e. a suite of plain commands.

If shell structured commands are used, like a for/do/done loop, a case/esac switch, an here document, a subshell enclosed by parentheses, a function definition, etc. and any combination of the above, the shell reads up to the end of the construction to first make sure there is no syntax error.

This is somewhat inefficient as the same code can be read again and again a large number of times but mitigated by the fact this content is normally cached.

Whatever the shell interpreter, it is very unwise to modify a shell script while it is being executed as the shell is free to read again any portion of the script and this can lead to unexpected syntax errors if out of sync.

This is more shell dependent than OS dependent.

Depending on the version, ksh read the script on demand by 8k or 64k bytes block.

bash read the script line by line. However, given the fact lines can be of arbitrary lenght, it reads each time 8176 bytes from the beginning of the next line to parse.

This is for simple constructions, i.e. a suite of plain commands.

If shell structured commands are used (a case the accepted answer miss to consider) like a for/do/done loop, a case/esac switch, an here document, a subshell enclosed by parentheses, a function definition, etc. and any combination of the above, shell interpreters reads up to the end of the construction to first make sure there is no syntax error.

This is somewhat inefficient as the same code can be read again and again a large number of times but mitigated by the fact this content is normally cached.

Whatever the shell interpreter, it is very unwise to modify a shell script while it is being executed as the shell is free to read again any portion of the script and this can lead to unexpected syntax errors if out of sync.

Note too that bash might crash with a segmentation violation when it is unable to store an overly large script construction ksh93 can read flawlessly.