echo
$ echo Hello there
cat
$ cat /etc/passwd
displays the contents of the /etc/passwd system information file and then returns your shell prompt
$ cat file1 file2 …
cat prints the contents of file1, file2, and any other files that you specify (denoted by …), and then exits.
- Standard Input and Standard Output
Unix processes use I/O streams to read and write data.
Pressing CTRL-D on an empty line stops the current standard input entry from the terminal (and often terminates a program). Don’t confuse this with CTRL-C, which terminates a program regardless of its input or output.
Standard output is similar. The kernel gives each process a standard output stream where it can write its output.
The cat command always writes its output to the standard output.
There is a third standard I/O stream called standard error.
- Basic Commands
ls
The ls command lists the contents of a directory.
Use ls -l for a detailed (long) listing and ls -F to display file type information
cp
In its simplest form, cp copies files. For example, to copy file1 to file2, enter this:
$ cp file1 file2
To copy a number of files to a directory (folder) named dir, try this instead:
$ cp file1 … fileN dir
mv
$ mv file1 file2
$ mv file1 … fileN dir
touch
The touch command creates a file. If the file already exists, touch does not change it, but it does update the file’s modification time stamp printed with the ls -l command
$ touch file
rm
To delete (remove) a file, use rm.
$ rm file
echo
$ echo Hello again
- Navigating Directories
cd
The cd command changes the shell’s current working directory:
$ cd dir
mkdir
The mkdir command creates a new directory dir:
$ mkdir dir
rmdir
The rmdir command removes the directory dir:
$ rmdir dir
If dir isn’t empty, this command fails. You can use rm -rf dir to delete a directory and its contents. Don’t use the -rf flags with globs such as a star (*). And above all, always double-check your command before you run it
- Shell Globbing (Wildcards)
The shell can match simple patterns to file and directory names, a process known as globbing.
he following comm and prints a list of files in the current directory:
$ echo *
- at* expands to all filenames that start with at.
- *at expands to all filenames that end with at.
- *at* expands to all filenames that contain at.
the question mark (?), instructs the shell to match exactly one arbitrary character. For example, b?at matches boat and brat.
- Intermediate Commands
grep
The grep command prints the lines from a file or input stream that match an expression
$ grep root /etc/passwd
print the lines in the /etc/passwd file that contain the text root,
less
The less command comes in handy when a file is really big or when a command’s output is long and scrolls off the top of the screen. To quit, type q
$ grep ie /usr/share/dict/words | less
pwd
The pwd (print working directory) program simply outputs the name of the current working
directory
pwd -P for full get the true full path of the current working directory
diff
$ diff file1 file2
To see the differences between two text files, use diff
file
If you see a file and are unsure of its format, try using the file command to see if the system can guess:
$ file file
find and locate
Run find to find file in dir:
$ find dir -name file -print
head and tail
To quickly view a portion of a file or stream of data, use the head and tail commands
head /etc/passwd shows the first 10 lines of the password file, and
tail /etc/passwd shows the last 10 lines.
To change the number of lines to display, use the -n option, where n is the number of lines you want to see (for example, head -5 /etc/passwd). To print lines starting at line n, use tail +n
sort
The sort command quickly puts the lines of a text file in alphanumeric order. If the file’s lines start with numbers and you want to sort in numerical order, use the -n option. The -r option reverses the order of the sort.
Reference: How Linux Works: What Every Superuser Should Know by Brian Ward
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