Remove Fixed Character Columns Safely with colrm

You have a fixed-width dump and you want columns 9 to 16 gone from every line: colrm does exactly that, and nothing more. This guide removes a numbered range from each input line, writes the result to a separate file, and checks that the source is unchanged. The examples use the installed colrm from util-linux 2.39.3, provided by the Ubuntu bsdextrautils package version 2.39.3-9ubuntu6.6.

Time: about ten minutes.
You need: a shell, a readable text file, and permission to create a new file in the destination directory.
Quirk: the command reads standard input and writes standard output. It does not accept an input filename as a positional argument, so redirect the file or use a pipeline.

1. Check the installed command

Confirm which binary your shell will run and check its version. These commands are read-only and need no elevated privileges:

$ command -v colrm
/usr/bin/colrm
$ colrm --version
colrm from util-linux 2.39.3
$ colrm --help
Usage:
 colrm [startcol [endcol]]

Filter out the specified columns from standard input.

The manual's syntax is colrm first last. Column numbering starts at 1.

One number. Removal starts at that column and runs to the end of each line.
Two numbers. Only that inclusive range is removed.

Checkpoint: if command -v colrm points somewhere unexpected, stop and decide whether that is the version you intend to use. Different util-linux releases can be installed side by side, and the command in your PATH is the one that will run.

2. Try it without touching a file

Start with literal input so the column positions are obvious. This removes columns 2 through 4 from both lines:

$ printf '%s\n' 'abcdef' '123456' | colrm 2 4
aef
156

The first column is a, columns 2 through 4 are bcd, and columns 5 and 6 are ef. The same rule is applied independently to each input line. The trailing newline is kept because it is a line separator, not a removable column.

To remove everything from column 4 onwards, leave out the second number:

$ printf '%s\n' 'abcdef' '123456' | colrm 4
abc
123

There is no persistent state to undo here. The process reads the pipe and exits, and neither command changes a file or a service.

3. Remove columns from a real file into a new file

Use input redirection and choose a new destination. This example removes columns 9 through 16 from /path/to/input.txt:

$ colrm 9 16 < /path/to/input.txt > /path/to/output.txt

Run it as your normal user. You need read permission on the input and write permission on the destination directory, but not root. A successful run is normally silent, so check both the exit status and the resulting file:

$ printf 'exit status: %s\n' "$?"
exit status: 0
$ test -s /path/to/output.txt && echo 'output is non-empty'
output is non-empty
$ sed -n '1,3p' /path/to/output.txt

The sed command is only a preview. Compare a few known lines with the source before using the output in a later command. If a line has fewer than 16 columns, colrm removes only the columns that exist on that line.

4. Protect an existing destination

Warning: shell redirection with > truncates an existing destination before colrm starts. A failed conversion can therefore destroy the old output. Write to a temporary name in the same directory, inspect it, then replace the destination only when you are satisfied:

$ colrm 9 16 < /path/to/input.txt > /path/to/output.txt.new
$ test -s /path/to/output.txt.new
$ sed -n '1,3p' /path/to/output.txt.new
$ mv /path/to/output.txt.new /path/to/output.txt

The final mv changes the destination name. If validation fails, leave the original output alone and remove the temporary file explicitly:

$ rm /path/to/output.txt.new

Recovery: that rm is destructive, so check the exact path before running it. If a direct redirection already overwrote the old output, colrm has no recovery facility. Restore from your backup or version-control system.

5. Treat columns as display positions

Column positions are not necessarily bytes in a file. Tabs and multibyte text can make visual counting differ from counting bytes in a hex dump. A tab occupies a tab stop when the command processes it, and the installed util-linux build handles multibyte characters as characters rather than blindly splitting their encoded bytes.

When exact alignment matters, inspect the input with a tool that makes whitespace visible, and try a small representative sample first:

$ sed -n '1,5l' /path/to/input.txt
$ sed -n '1,5l' /path/to/output.txt

The $ at the end of each sed -n l line marks the newline, while tabs are shown visibly. Do not infer a safe column range from a text editor's proportional display.

Tip: if your data is delimited rather than fixed-width, use a delimiter-aware tool instead of deleting character columns.

6. Diagnose the common failures

A blank result can be correct. Removing from column 1 to a number beyond the longest line removes all visible characters but leaves the line endings. Test the input and the range independently:

$ test -r /path/to/input.txt && echo 'input is readable'
input is readable
$ wc -L /path/to/input.txt
      42 /path/to/input.txt

Empty output. Check whether the first column is too small or the input lines are shorter than you expected.
Cannot open input or destination. Inspect the path and directory permissions.
Do not add sudo automatically. Elevated access can hide an ownership problem and create root-owned output that your normal account cannot later replace.

The -h and --help options display help. The -V and --version options print the version. Neither processes standard input, so use them to check the installation, not as part of a transformation.

Done means

Command checked. You confirmed the version and binary path.
Range chosen properly. Column 1 is the first position, and you tested the input shape.
Source untouched. Input came through standard input and the source file was not modified.
Output verified. You wrote to a new or temporary path and checked it before any replacement.
Width considered. You thought about whitespace, tabs and multibyte text before applying the range to a larger dataset.