Using Files & Commands

Files and commands are the backbone of a getting work done on a terminal. Knowing how to effectively manipulate them is key, so we’ll need to learn the commands that make manipulating these files possible.

Remotely Transfer Files (scp)

SCP (for “secure copy”) is a program for copying files from one machine to another. It uses the same authentication and provides the same security as ssh. scp will ask for passwords if they are needed for authentication.

scp [-r] <source> <destination>

$ scp school/notes.txt andrew:notes_sept_2.txt
# Copies school/notes.txt from your computer to Andrew and renames it

$ scp andrew:~/private/myfile.txt projects/
# Copies ~/private/myfile.txt from Andrew to your computer and puts it
# in the projects directory.

$ scp -r school/projects andrew:~/private/
# Copies the whole school/projects folder to Andrew and places it in
# the ~/private/ folder

Working with PuzzlePacks

PuzzlePacks are distributed as a tar file. This is basically like a zipped (.zip) folder. To “untar” the file from the command line, you can run this:

$ ls
my-tar-file.tar

$ tar xvf my-tar-file.tar

$ ls
jezimmer/ my-tar-file.tar

Once you’ve unpacked the tar file’s contents, you’ll notice a folder in the current directory named your Andrew ID. All PuzzlePacks are named this way; you’ll want to rename them to keep things organized:

$ mv jezimmer week1

This renames the folder jezimmer to week1. Next week when you get a tar file for the PuzzlePack and untar it, you’ll again get a folder named according to your Andrew ID. You’ll want to rename this folder along the lines of week2, etc.

Current Working Directory (pwd, cd)

On most systems that use a command line, there’s something called your “current working directory.” The current working directory is used as the default directory for many commands if you don’t specify a directory.

There are two commands commonly used to work with the current working directory:

• pwd (“print working directory”) tells you what your current working directory is
• cd (“change directory”) takes a directory as an argument and changes your working directory to that directory.

Examples

$ pwd
/afs/andrew.cmu.edu/usr10/jezimmer
$ cd private
$ pwd
/afs/andrew.cmu.edu/usr10/jezimmer/private

Tip: Lines that don’t begin with $ when $’s are present in a code block usually mean that those lines are the output from running a particular command.

Listing Files (ls)

The program ls allows you to list files and folders within a directory. It can be passed many different options (or “flags”) that control the output it gives.

Examples

# Initial contents of the current folder
$ ls
file1 folder1

# The -l flag tells ls to give you more information
$ ls -l
total 2
-rw-r--r-- 1 jezimmer    0 Aug 17 18:20 file1
drwxr-xr-x 2 jezimmer 2048 Aug 17 18:21 folder1

# If you specify a path, ls will print in that path instead of the
# current working directory
$ ls folder1
file2

# The -a flag tells ls to list hidden and non-hidden files (i.e. 'all'
# files) The -R flag is for recursively listing the contents of a
# directory
$ ls -alR
.:
total 6
drwxr-xr-x 3 jezimmer 2048 Aug 17 18:21 .
drwx------ 5 jezimmer 2048 Aug 17 18:20 ..
-rw-r--r-- 1 jezimmer    0 Aug 17 18:21 .hidden-file1
-rw-r--r-- 1 jezimmer    0 Aug 17 18:20 file1
drwxr-xr-x 2 jezimmer 2048 Aug 17 18:21 folder1/

./folder1:
total 4
drwxr-xr-x 2 jezimmer 2048 Aug 17 18:21 .
drwxr-xr-x 3 jezimmer 2048 Aug 17 18:21 ..
-rw-r--r-- 1 jezimmer    0 Aug 17 18:21 file2

Tip: The # is a comment character in bash (to be discussed later!). These lines are purely annotations.

Managing Files (cat, less, cp, mv, rm, mkdir)

There are many commands you can use to work with files on UNIX. Here are some of the more common ones.

cat <filename> - print files

To quickly dump the contents of a file to the console, use cat.

$ cat file1
The quick brown fox jumped over the lazy dog.

less <filename> - display and scroll through files

The program less is useful if you want to view the contents of a long file that doesn’t entirely fit on one screen. To exit less after running it, press q.

You can do tons of other things in less, but one useful thing is to be able to search. You can search with /banana to find all instances of “banana” in the file.

cp <source> <destination> - copy files

# Copy existing file to new file
$ cp file1 file3
$ ls
file1 file3 folder1/
# The contents of the new file are the same
$ cat file3
The quick brown fox jumped over the lazy dog.

# You can also use the -r option to copy directorys
$ cp folder1 folder2
$ ls
file1 file3 folder1/ folder2/

mv <source> <destination> - move and rename files

# Moving a file "into" another file is how you rename files
$ mv file1 file4
$ ls
file3 file4 folder1/ folder2/

# Move file into directory
$ mv file3 folder1
$ ls -R
.:
file4

folder1:
file2 file3

folder2:
file2

rm <filename> - PERMANENTLY delete files

$ rm file4
$ ls
folder1/ folder2/

# use the -r flag to recursively remove a directory
$ rm -r folder2/
$ ls
folder1/

mkdir <directory> - make directories

$ mkdir folder3
$ ls -R
.:

folder1:
file2 file3

folder3:

Running Executables (chmod)

Very frequently, you’ll want to run executables that you’ve either written or compiled. For example, if you have an executable file named a.out in your current directory, you’d run ./a.out (. stands for your current working directory).

The ./ is necessary because if you don’t specify a path to the executable, bash only checks in the directories listed in the PATH variable (more on this later), and the directory . is in is almost certainly not in your PATH. (System utilities, like the commands we taught you about above, are in your PATH, usually in the standard directories /bin and /usr/bin. You can also modify your PATH to tell bash to look in other places for executables.)

Fixing “Permission denied” Error

Sometimes, you’ll attempt to run an executable and get an error that looks something like -bash: ./a.out: Permission denied. This means that for some reason, you’re not allowed to execute the file. If you’re the owner of the file, you can run

$ chmod u+x ./a.out

to give yourself permissions to run the executable. (chmod is a utility that lets you change the file permissions [or “mode”] of a file, and u+x means to take the permissions for the user who owns the file and add the execute permission).