UNIX sed命令未删除cat输出中的空格

Question

I have a file as below.

Hi this is first line
    this is second line
    this is third line

Expected is:

Hi this is first line
this is second line
this is third line

What i used is

cat file.txt | sed 's/ //g'

returns,

Hi this is first line
    this is second line
    this is third line

Answer 1

For a portable sed command use this:

sed 's/^[[:blank:]]*//' file

[[:blank:]] matches space or tab.

EDIT: To remove all spaces using awk:

awk '{$1=$1}1' OFS= file

OR sed:

sed 's/[[:blank:]]*//g' file

Answer 2

 cat file.txt | sed -e 's/^[ \t]*//'

OR

 sed 's/^[ \t]*//' file.txt 

OR if you want to modify file.txt and remove the white spaces in the beginning of the line :

sed -i 's/^[ \t]*//' file.txt

Answer 3

您的示例中的sed将替换所有空格

sed 's/^[ \t]*//' file.txt

Answer 4

try this line

sed -r 's/^\s*//' file

to remove all spaces(tabs): sed -r 's/\\s//g' file

kent$ echo "Hi this is first line
    this is second line
    this is third line"|sed -r 's/\s//g' 
Hithisisfirstline
thisissecondline
thisisthirdline

Answer 5

The most efficient way to remove all blanks from your input is probably not using sed at all, but

tr -d '[:blank:]' < file.txt

That's different from what you have originally asked for, though (removing initial whitespace only).