How to Remove the Last Character of the Last Line of a File

How can I remove the last character of a file in unix?

A simpler approach (outputs to stdout, doesn't update the input file):

sed '$ s/.$//' somefile

$ is a Sed address that matches the last input line only, thus causing the following function call (s/.$//) to be executed on the last line only.
s/.$// replaces the last character on the (in this case last) line with an empty string; i.e., effectively removes the last char. (before the newline) on the line.

^{. matches any character on the line, and following it with $ anchors the match to the end of the line; note how the use of $ in this regular expression is conceptually related, but technically distinct from the previous use of $ as a Sed address.}

Example with stdin input (assumes Bash, Ksh, or Zsh):

  $ sed '$ s/.$//' <<< $'line one\nline two'
  line one
  line tw

To update the input file too (do not use if the input file is a symlink):

sed -i '$ s/.$//' somefile

^Note:

On macOS, you'd have to use -i '' instead of just -i; for an overview of the pitfalls associated with -i, see the bottom half of this answer.
If you need to process very large input files and/or performance / disk usage are a concern and you're using GNU utilities (Linux), see ImHere's helpful answer.

How to remove the last characters of each line in a file?

Open your file with open("fileName.ext") method and then iterate each line. on each line you can perform the slicing function [:]

eachLineData = 'username1:password1:dd/mm/yy'
expectedResult = eachLineData[:-9]
print(expectedResult)

Full Code Example:

# Using readlines()
file1 = open('myfile.txt', 'r')
Lines = file1.readlines()

for line in Lines:
    expectedResult = line[:-9]
    print(expectedResult)

Delete last character from line that contains specific word

This would do it:

sed '/in_bytes/ s/,$//'

Where /in_bytes/ is a search pattern ensuring only matching lines will execute the following command s/,$//, which is a standard substitution to remove a trailing comma.

Example: https://ideone.com/3hEXBY

Remove last character from a file in unix

quick and dirty

sed 's/.$//' YourFile

a bit secure

sed 's/[|]$//' YourFile

allowing space

sed 's/[|][[:space:]]*$//' YourFile

same for only last char of last line (thansk @amelie for this comment) :
add a $in front so on quick and dirty it gives sed '$ s/.$//' YourFile

Remove last character of previous line under condition

Using sed

Using GNU sed:

$ sed -z 's/&\n AA/\n AA/g' file
 F = 2 * 3 * a * b * 7&
& * 3 * b * c
 AA = ...

To keep this command simple, we use the -z option to read in the whole file at once. (Technically, -z reads in NUL-separated input. Since no valid Fortran file contains a NUL, this has the effect of reading in the whole file.)

s/&\n AA/\n AA/g does the substitution that we want. Any place where the file contains & followed by newline followed by space followed by AA, this substitution removes the &.

Reading the whole file in at once is not a good approach if the file is too big to fit in memory. This should not be a problem for Fortran files.

For non-GNU sed (BSD, OSX), we need to add code to replace the -z flag:

sed 'H;1h;$!d;x;  s/&\n AA/\n AA/g' file

Using awk

$ awk '{if (/^ AA/) sub(/[&]$/, "", last); if (NR>1) print last; last=$0} END{print last}' file
 F = 2 * 3 * a * b * 7&
& * 3 * b * c
 AA = ...

How it works:

This script uses one variable last which contains the contents of the previous line. If the current line starts with AA, then we remove, if present, the final & from last. In more detail:

if (/^ AA/) sub(/&$/, "", last)
If the current line starts with AA, then remove the final & from the previous line.
if (NR>1) print last
If we are not on the first line, then print the previous line.
last=$0
Save the current line as last.
END{print last}
After we reach the end of the file, print last.

Changing files in-place

With GNU sed:

sed -zi.bak 's/&\n AA/\n AA/g' file

With other sed:

sed -i.bak 'H;1h;$!d;x;  s/&\n AA/\n AA/g' file

With recent GNU awk:

awk -i inplace '{if (/^ AA/) sub(/&$/, "", last); if (NR>1) print last; last=$0} END{print last}' file

With older awk or non-GNU awk:

awk '{if (/^ AA/) sub(/&$/, "", last); if (NR>1) print last; last=$0} END{print last}' file >file.tmp && mv file.tmp file

How to efficiently remove last line in a file and append new text for large file?

For this code below, the routine is pretty simple since you already know that the last character in your file is a ]. Thus, all you have to do is read the last character of a file and if it is the char ] then you got that file. If that happens, you truncate the last byte from the file and append text to that file. Then you add the char ] to preserve the format. Note that this is for ASCII encoding if your last char is something else that is bigger than a byte then you would have to fix the code a little bit.

using System;
using System.IO;
using System.Text;

public class FSSeek
{
    public static void Main()
    {
        string fileName = "test.txt";
        char lastChar = ']';
        string toBeAppend = "d\ne\n";

        using (FileStream fs = new FileStream(fileName, FileMode.Open, FileAccess.ReadWrite))
        {
            fs.Seek(-1, SeekOrigin.End);
            if ( Convert.ToChar(fs.ReadByte()) == lastChar ){
                fs.SetLength(fs.Length - 1);
                fs.Write(Encoding.ASCII.GetBytes(toBeAppend));
                fs.WriteByte(Convert.ToByte(lastChar));
            }            

        }
    }
}

test.txt content:

[
a
b
c
]

How to remove last character of Nth line linux

A simple solution using wc to count lines and sed to do the editing:

sed "$(( $(wc -l <file) - 2))s/,$//" file

That will output the edited file on stdout; you can edit inplace with sed -i.