Creating & Manipulating Files
Reading File Contents
There are a few different ways to see the contents of a file.
We already used this first example.
cd ~/Oct22Workshops
Let's look inside the file. We have several methods of viewing the content of files that we have created.
A helpful command is cat.
cat helloworld.txt
"cat" will open the entire file, so this is not the best command for long files.
In that case "head" is a good option. Head pulls the top ten lines of the file and prints them to the screen.
head helloworld.txt
It does not look any different from cat in this case because there is only one line in the file.
A third way to check file contents is by using a program called "less" (or "more").
"less" will open the file interactively, then you can scroll through it and when you are done, push "q" on your keyboard to close the file.
less helloworld.txt
Press q to close the file opened by less
There are many versions of these tools on command line, but "cat", "head" and "less" are very common.
Copying Files
Sometimes we have a file that we want to reuse.
When copying within the same directory, make sure to change the name of the file, or the original will be overwritten.
When copying to a new directory, the name can stay the same.
This command copies the file within the same directory with a new name. Both files are kept.
cp helloworld.txt helloworld1.txt
ls
These commands make a new directory, and then copies the file into the new directory with the same name.
mkdir helloworld
cp helloworld.txt helloworld
Check this with ls helloworld (lists the contents of the directory).
Moving Files
mv is an option for renaming files, but also has the potential to overwrite existing files.
For example, this command changes the name of the file and removes the original file. If helloworld2.txt already existed, it would be replaced.
mv helloworld1.txt helloworld2.txt
Check this with ls
Removing Files
rm and rmdir are permanent in shell, so make sure you are ready to delete files.
rm helloworld/helloworld.txt
Once the directory is empty, we can remove the directory.
rmdir helloworld
It will throw an error if the directory is not empty.
If you are positive that you want to remove a directory and all the files within it, then add two flags, -r for recursive and -f for force.
Both commands above could have been replaced with one remove command: rm -rf helloworld
Tip
Until you are confident with file structure and bash commands, it is a good idea to copy instead of move and to
* cp -u will copy files only if they do not already exist.
* cp -r is a good command for copying directories, it means copy recursively which will copy the entire directory.
* cp -rf BE CAREFUL with this, it copies the entire directory AND forces the overwrite of any files that already exist.
* Adding the interactive flag -i on the commands rm and mv to set up a question that you answer y or n to before removing.
rm -i helloworld/helloworld.txt
Generates this question
rm: remove regular file ‘helloworld/helloworld.txt’?
mv -i only generates a question if you are in danger of overwriting an existing file.
For example:
1.) Make a new file from the original file we created
cp -u helloworld.txt helloworld1.txt
-u for the copy command will not copy the file if it already exists.
2.) Try to rename the file with mv, with the i parameter set to prevent overwriting an existing file.
mv -i helloworld.txt helloworld1.txt
mv: overwrite ‘helloworld1.txt’?
A great website to look at to understand the nuances of shell commands is: