I am looking to compare file1.txt contents with the last n file contents of file2.txt Can someone help in identifying this logic using anything in shell script.
Example if file1.txt has 10 lines, the last 10 lines of file2.txt should be compared for difference.
With bash's process substitution <() and command substitution $().
diff file1.txt <(tail -n $(wc -l < file1.txt) file2.txt)
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This question already has answers here:
How can I use a file in a command and redirect output to the same file without truncating it?
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Closed 3 years ago.
I'm testing basic bash command on Ubuntu, when I put on terminal
wc -l < file1.txt > file2.txt
all lines from file1 are read by wc and the number is saved to file2.txt.
However if I use the similar command wc -l < file1.txt > file1.txt but I try to save it in the same > file1 the output is always 0.
What is the reason of this behavior ?
When you write the command wc -l < file1.txt > file1.txt, your shell will first read the command as a whole and redirect the standard input, output from/to file1.txt.
Then when a file is open for writing using >, if a file exists it will be overwritten and an empty file is created. the process (your wc) will then start its execution and it will write in it until the end of the execution.
Your wc -l will however have to read the file and count the number of lines in it, as the file is empty (overwritten) it will just dump in it:
0 file1.txt
To prove this look at what happens in appending mode using >>
$ cat file1.txt
a
b
b
$ wc -l file1.txt
3 file1.txt
$ wc -l file1.txt >> file1.txt
$ cat file1.txt
a
b
b
3 file1.txt
Your file content is intact and wc does properly read the number of lines before appending the result to the file.
Note:
In general, it is never recommended to write and read in the same file except if you know exactly what you are doing. Some commands have an inline option to modify the file during the run of the command and it is highly recommended to use them, as the file will not be corrupted even if the command crash in the middle of the execution.
Its because redirections have higher priority than other commands.
Here the file1 will be emptied before any operations can be performed on it
Try >> on file1.txt instead of >. >> will append to file unlike > overwriting whole file. Try -
wc -l < file1.txt >> file1.txt
I want to copy lines 10 to 15 of a file into another file in Unix.
I am having files file1.txt and file2.txt.
I want to copy lines 10 to 15 from file1.txt to file2.txt.
Open a terminal with a shell then
sed -n '10,15p' file1.txt > file2.txt
Simple & easy.
If you want to append to the end instead of wiping file2.txt, use >> for redirection.
sed -n '10,15p' file1.txt >> file2.txt
^^
AWK is also a powerful command line text manipulator:
awk 'NR>=10 && NR<=15' file1.txt > file2.txt
In complement to the previous answer, you can use one of the following 3 solutions.
sed
Print only the lines in the range and redirect it to the output file
sed -n '10,15p' file1.txt > file2.txt
head/tail combination
Use head and tail to cut the file and to get only the range you need before redirecting the output to a file
head -n 15 file1.txt | tail -n 6 > file2.txt
awk
Print only the lines in the range and redirect it to the output file
awk 'NR>=10 && NR<=15' file1.txt > file2.txt
I am running a script that has been working fine. However, yesterday, I got a couple errors. These errors are after several loops of the script:
sed: cant read file3.txt: No such file or directory
grep: file3.txt: No such file or directory
grep: file3.txt: No such file or directory
sed: cant read file3.txt: No such file or directory
grep: file3.txt: No such file or directory
Keep in mind, these errors do not happen consistently. It's occurring once in a while somewhere near this part of the script. File3.txt is the file not being found:
cat file1.txt | while read LINE; do grep -m 1 $LINE file2.txt >> file3.txt; done
sed -i 's/string//g' file3.txt
grep 'string' file3.txt | cut -d '|' -f1-2 > file4.txt
grep -v 'string' file3.txt | cut -d '|' -f1-2 >> file5.txt
sed -i 's/string//' file3.txt
grep -Fvf file3.txt file1.txt > file6.txt
Now, I'm thinking that since file3.txt is being appended, or later operated on by SED, sometimes the next command starts too soon and it can't find the file? Should I put a wait command in between?
I have looked up many pages with this error, but was unable to find anything:
cat file_name | grep "something" results "cat: grep: No such file or directory" in shell scripting
Pipe multiple commands to a single command with no EOF signal wait
grep command works in command line, but not in bash script: get no such file or directory erro
https://serverfault.com/questions/169539/sed-cant-find-a-file-that-obviously-exists
"No such file or directory" but it exists
If you think that putting a wait or sleep command will help, please let me know. Or, if you think there's a better solution, that would be great too. I'm running on Cygwin terminal. Any insight is greatly appreciated.
Instead of redirecting to file3.txt inside the while loop, redirect the whole loop. Then the file will be created even if the loop never runs because the input file is empty.
while read LINE; do
grep -m 1 $LINE file2.txt
done < file1.txt > file3.txt
If file1.txt is ever empty then file3.txt won't be created.
Also do grep -m 1 $LINE file2.txt will cause problems if there are crucial characters (space is the easiest of them).
Let's assume that the $LINE variable contains more than one word separated by spaces: hello world.
Now the command looks like this: grep -m 1 hello world file2.txt - grep interpretation will look something like this: let's find all hello in file named world and file named file2.txt in current folder.
Using "$LINE" instead of $LINE will lead you to a whole different scenario.
Look at the difference between the following two:
grep -m 1 $LINE file2.txt
grep -m 1 "$LINE" file2.txt
I'm trying to understand a shell code which includes a line like this:
grep -n data file1.txt > file2.txt
Where data is the text i want to search for.
What does this command mean?
You can have a detailled answer here: http://explainshell.com/explain?cmd=%20grep%20-n%20data%20file1.txt%20%3E%20file2.txt
To sum it up:
grep will look for the string data in file1.txt and will output both the matching lines and their line number (because of the -n flag).
You could read the manual (man grep) to have a better understanding of what grep does.
The output will be redirected into file2.txt; that's what > is used for
I have a text file file1.txt on Unix. I'd like to produce another file file2.txt, in which I replace all occurrences of apple-pie with apple_pie. What command can I use?
Use sed to do a global substitution on file1.txt and redirect the output to file2.txt:
sed 's/apple-pie/apple_pie/g' file1.txt > file2.txt
sed has better performance than awk but both will work for this search and replace. Reference
If you put the commands in a script (e.g., ksh, sh) then here is the syntax:
awk '{gsub(/apple-pie/,"apple_pie");print}' "file1.txt" > "file2.txt"
sed -e 's/apple-pie/apple_pie/g' "file1.txt" > "file2.txt"