Replace text of a specific line number using sed

Question

I have a html file users.html that i want to replace the first line. The first line looks like this

<template name="AccountSettings">

I want the first line to read

<template name="Users">

Solution 1

Commands in sed accept an address range. In this case, to limit the replacement to just the first line, the range should be the single address 1 i.e.

sed '1 s/AccountSettings/Users/' users.html

Solution 2

Perl

$ perl -pi -e 's/AccountSettings/Users/ if $. == 1' input.txt

• -pi allows to make changes for input file in-place, i.e. alter the actual file. If you want to see result before overwriting the file, use -p instead
• s/AccountSettings/Users/ perform first substitution for pattern.
• $. is processed line count

AWK

$ awk 'NR==1{sub("AccountSettings","Users")};1' input.txt                
<template name="Users">
AccountSettings

• NR==1{sub("AccountSettings","Users")} is Pattern or Condition {actions} structure typical of awk. Action is performed when condition is true.
• NR==1{} is number of processed lines, specifically we're looking if number of lines is equal to 1.
• sub("AccountSettings","Users") performs replacement for the first match of the pattern.
• 1 is pattern with omitted action. Default action for pattern that is true is to print, and making pattern to be 1 ensures we print resulting lines.

Python

One liner with reading stdin:

$ python -c 'import sys;print("\n".join([ l.strip().replace("AccountSettings","Users") if i==0 else l.strip()  for i,l in enumerate(sys.stdin)]))' < input.txt

Script version:

#!/usr/bin/env python
from sys import argv;
with open(argv[1]) as f:
    for index,line in enumerate(f):
        text = line.strip().replace("AccountSettings","User") if index == 0 else line.strip()
        print(text)

• core idea is to either redirect stdin stream and read lines from there or provide file as argv[1] - first command-line argument to script
• We use variable = value_1 if condition else value_2 structure to build up a list of lines with appropriate values in one-liner, while in script version assignment to text variable and immediate printing of that variable is performed. The condition for the replacement is of course line number

Ruby

$ ruby -pe '$_=$_.sub(/AccountSettings/,"Users") if $. == 1' input.txt                                                                             
<template name="Users">
AccountSettings

• Similar idea to Perl, -p switch allows us to print $_ (current line) variable each time.
• $_=$_.sub(/AccountSettings/,"Users") if $. == 1 is action if condition structure,where we use .sub() function to perform pattern replacement and assign it back to $_ buffer if line number is 1

Solution 3

Use following command:

sed   's/name="AccountSettings"/name="Users"/g' users.html

Comments

[fiatux]

It is prudent to use an XML parser to parse XML.

[Ravexina]

Nice answer ...