[unix] How to remove leading whitespace from each line in a file

I have a file that looks something like this:

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++)

I want it to look like this (remove indentations):

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++)

How can this be done (using sed maybe?)?

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.

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! =)

Use:

sed -e **'s/^[ \t]*//'**  name_of_file_from_which_you_want_to_remove_space > 'name _file_where_you_want_to_store_output'

For example:

sed -e 's/^[ \t]*//'  file1.txt > output.txt

Note:

s/: Substitute command ~ replacement for pattern (^[ \t]*) on each addressed line

^[ \t]*: Search pattern ( ^ – start of the line; [ \t]* match one or more blank spaces including tab)

//: Replace (delete) all matched patterns

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"

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

This Perl code edits your original file:

perl -i -ne 's/^\s+//;print' file

The next one makes a backup copy before editing the original file:

perl -i.bak -ne 's/^\s+//;print' file

Notice that Perl borrows heavily from sed (and AWK).