How to remove leading whitespace from each line in a file

sed "s/^[ \t]*//" -i youfile

Warning: this will overwrite the original file.

For this specific problem, something like this would work:

$ sed 's/^ *//g' < input.txt > output.txt

It says to replace all spaces at the start of a line with nothing. If you also want to remove tabs, change it to this:

$ sed 's/^[ \t]+//g' < input.txt > output.txt

The leading "s" before the / means "substitute". The /'s are the delimiters for the patterns. The data between the first two /'s are the pattern to match, and the data between the second and third / is the data to replace it with. In this case you're replacing it with nothing. The "g" after the final slash means to do it "globally", ie: over the entire file rather than on only the first match it finds.

Finally, instead of < input.txt > output.txt you can use the -i option which means to edit the file "in place". Meaning, you don't need to create a second file to contain your result. If you use this option you will lose your original file.

You can use AWK:

$ awk '{$1=$1}1' file
for (i = 0; i < 100; i++)
for (i = 0; i < 100; i++)
for (i = 0; i < 100; i++)
for (i = 0; i < 100; i++)
for (i = 0; i < 100; i++)
for (i = 0; i < 100; i++)
for (i = 0; i < 100; i++)

sed

$ sed 's|^[[:blank:]]*||g' file
for (i = 0; i < 100; i++)
for (i = 0; i < 100; i++)
for (i = 0; i < 100; i++)
for (i = 0; i < 100; i++)
for (i = 0; i < 100; i++)
for (i = 0; i < 100; i++)
for (i = 0; i < 100; i++)

The shell's while/read loop

while read -r line
do
    echo $line
done <"file"

You can remove leading white space by below any command:

 - sed -i "s/\s*//" filename
 - sed -i "s/[[:space:]]*//"  filename        
 - sed -i "s/^\s*//"   filename
 - sed -i "s/[ \t]*//"  filename

If you want to remove blank line you can try any one of the following command:

 - sed "/^$/d"  filename

If you want to delete any leading white space, tab or empty line in that case you can use below command:

sed  -i -e "/^$/d"   -i -e "s/^\s*//"  filename

Here you go:

user@host:~$ sed 's/^[\t ]*//g' < file-in.txt

Or:

user@host:~$ sed 's/^[\t ]*//g' < file-in.txt > file-out.txt

For what it's worth, if you are editing this file, you can probably highlight all the lines and use your un-tab button.

• In Vim, use Shift + V to highlight the lines, then press <<
• If you're on a Mac, then you can use Atom, Sublime Text, etc., then highlight with your mouse and then press Shift + Tab

I am not sure if there is some requirement that this must be done from the command line. If so, then :thumbs-up: to the accepted answer! =)