seq 1 2 echo "---------------------------------" a=$(seq 1 2) echo $a
1 2 --------------------------------- 1 2
Bash Manual 的
Command Substitution 章节
Command Substitution ....... Bash performs the expansion by executing command and replacing the command substitution with the standard output of the command, with any trailing newlines deleted. *Embedded newlines are not deleted, but they may be removed during word splitting.* The command substitu‐ tion $(cat file) can be replaced by the equivalent but faster $(< file). ....... *,If the substitution appears within double quotes, word splitting and pathname expansion are not performed on the results.*
Word Splitting 的说明如下:
Word Splitting The shell scans the results of parameter expansion, command substitu‐ tion, and arithmetic expansion that *did not occur within double quotes for word splitting*. The shell treats each character of IFS as a delimiter, and splits the results of the other expansions into words on these characters. If IFS is unset, or its value is exactly <space><tab><newline>, the default, then sequences of <space>, <tab>, and <newline> at the beginning and end of the results of the previous expansions are ignored, and any sequence of IFS characters not at the beginning or end serves to delimit words. If IFS has a value other than the default, then sequences of the whitespace characters space and tab are ignored at the beginning and end of the word, as long as the whitespace character is in the value of IFS (an IFS whitespace char‐ acter). Any character in IFS that is not IFS whitespace, along with any adjacent IFS whitespace characters, delimits a field. A sequence of IFS whitespace characters is also treated as a delimiter. If the value of IFS is null, no word splitting occurs. Explicit null arguments ("" or '') are retained. Unquoted implicit null arguments, resulting from the expansion of parameters that have no values, are removed. If a parameter with no value is expanded within double quotes, a null argument results and is retained. Note that if no expansion occurs, no splitting is performed.
a="$(seq 1 2)" echo "$a"
IFS='' a=$(seq 1 2) echo $a