Advanced (but Common) Concepts

Bashrc

Bash can be configured with the bashrc file, located at $HOME/.bashrc.

Bashrc file contains bash scripts that will be executed when the interactive bash shell is launching.

To see it work, append this line to the end of ~/.bashrc

echo "I will become a great programmer!"

When opening a new terminal, you will see I will become a great programmer printed before the first prompt.

After updating .bashrc the current session will not automatically load the new configurations. You can also manually load the configurations by source ~/.bashrc.

Prompt

Bash prompt is stored in the variable PS1. You can change its value by running the following line:

PS1="\
\[\033[01;32m\]\u@\h\[\033[00m\]\
:\
\[\033[01;34m\]\w\[\033[00m\]\
:\033[02m\d \t\033[00m\n\$ "

A different prompt will be shown. If you like it, append this line onto .bashrc.

For more prompt customisation, see wiki and prompt generator.

Alias

Alias command in bash is defined this way

alias ll="ls -lar --sort t --color"  # append to .bashrc for persistant change

Now ll will be automatically expanded into ls -lar --sort t --color. To ignore the alias and use the original command, prepend with slash: \ll.

Path and Executable

In Linux, an executable file can be directly executed by entering its path.

Let us create a bash file and make it executable:

$ cd ~; mkdir executable_demo; cd executable_demo;
$ echo "echo hi executable" > hi.bash
$ chmod +x hi.bash  # make it executable
$ ~/executable_demo/hi.bash  # execute it by entering its full path
hi executable
$ ./hi.bash  # execute it by entering its relative path
hi executable

You can append the directory into PATH envrionment variable. (For persistant change modify the bashrc).

$ PATH="$PATH:$HOME/executable_demo"
$ hi.bash  
hi executable

When entering a bash command, bash will search all executable under $PATH directories and execute the first matched one.

Check your current PATH variable this way:

$ echo $PATH  # it shall look like this
/home/virtus/.local/bin:/home/virtus/bin:/usr/local/bin:/usr/bin:/home/virtus/.cargo/bin/
$ echo $PATH | tr ':' '\n'  # substitute : for \n, which is carriage return
/home/virtus/.local/bin
/home/virtus/bin
/usr/local/bin
/usr/bin
/home/virtus/.cargo/bin/

Glob pattern

Strings in bash containing *, ?, or [ are glob patterns, which are wildcard that can match strings following these rules (among others):

* matches any strings including the empty string;
? match exactly one character of any sort;
[, ] match any single character contained in the brackets

Glob pattern is different from regular expression, or regex.

See following examples:

$ touch a.txt b.txt abc.txt ab.md ac.md a.md  # create some files for experiment
$ ls *.txt  # list any files that end with .txt
abc.txt  a.txt  b.txt
$ ls ??.*  # list files of any extension that have two letters before .
ab.md  ac.md
$ ls [ab].txt
a.txt b.txt
$ ls [ab]*.txt
abc.txt  a.txt  b.txt
$ rm *  # rm all files

Stdin, Stdout, Stderr are abbreviations for standard input, standard output, and standard error. These are stdio (standard input and output) streams used in operating system as a abstract layer to handle the inputs and outputs of all programs.

For simlicity:

Stdin, stdout, stderr are represented by fd 0, 1, 2, respectively. Fd means file descriptor.
If a bash command runs successfully, it may a return some information through stdout, which will be printed on the terminal;
If a command failed, it may return the informations through stderr, which will be printed on the terminal and likely have no difference from stdout for users’s perspective;
Many commands can handle inputs from command arguments and from stdin;
Bash has many tools to handle stdio, including | (pipeline), > (output redirection), < (input redirection), etcs.

Check stdio(3)

Symbols controlling input and output are the most exploited features in bash. They are used to produce obfuscated code like this:

$ for i in $(echo -e 'G\nM\nK'); do du -hsx *  2>/dev/null | grep '[0-9]'$i | sort -rn; done

This code enumerate files in the current directory in order of size.

The following sesssion is a gental introduction for bash io symbols.

Pipeline, `|`

Pipeline redirect the stdout of the previous command to the stdin of the next command. This symbol is used with commands that accept input from the stdin.

Here is a naive example:

$ echo "hello" | wc -c  # count the number of characters in the string

Many useful and creative commands depend on pipeline.

$ grep -r "pattern" . | wc -l  # count the number of lines that contain "pattern" in the current directory
$ ls -l | grep "drwx"  # list all directories in the current directory
$ grep -r "pattern" . | sort | uniq  # sort and remove duplicates
$ find . -type f | less  # list all files recursively and open in less

Most commands will treat stdin and command arguments differently.

One example is to count the total number of lines in all files under the current directory, by using find ., wc, xargs, and pipeline.

wc -l <filename> counts the number of lines in a file; find . -type f lists all files in the current directory. find . -type f | wc -l will, however, only counts the number of lines in the output of find . -type f, i.e., how many files there are in the current directory.

We need, instead, pipe the output of find to the arguments of wc. xargs is the commad used for this.

$ find . -type f | xargs wc -l # count total number of lines in files of current directory

Bash History, `!`, arrow keys, and `Ctrl-r`.

history command shows the bash command history.

Here is an example:

$ history
  (omitted)
  928  a
  929  history 
  930  man history 
  931  ls
  933  man history
  934  clear

You can use up and down arrow keys to substitute current command with history commands.

!<n> is the shortcut to execute the nth command in the history.

Chech man history for more.

Yetin Guide for Coding