Ben Chuanlong Du's Blog

It is never too late to learn.

Programming in Shell

String

Regular Expression Comparison

https://stackoverflow.com/questions/229551/how-to-check-if-a-string-contains-a-substring-in-bash

Bash Programming

  1. case ... esac is different from switch(){} in C/C++. You do not have to return/exit manually after each branch.

not condition

if [ ! -d "$1" ]; then
fi

Shell Programing Trick

  1. Generate 00, 01, ..., to 25.

     for i in {00..25}; do echo $i; done

    or

     seq -f %02g 0 25

Shell Scripting

  1. It is recommended that you use single quotes when defining aliases unless you have good reasons to use double quotes. The reason is that shell variables in aliases defined in single quotes are not expanded until runtime. This makes your alias more robust (shell variables do not have to defined before aliases) and flexible (you can change definition of shell variables later as needed).

  2. In terminal, wild cards are expanded before passed to functions. This means that if you pass wild cards from a function to nested functions, it usually will not work. Instead, you should pass the whole argument list, which is represented by $@.

if a function has the same name as a script, the function comes first, but if you prefix it with sudo, then the script is run as a function is invisible in sub shells

Programming

  1. Dash is faster than bash and thus is often used as login shell while bash has more features and is often used as interactive shell .profile is intend to be read by shell script you should not use any bash specific feature in it.

  2. You do not have to escape quotes in sub shells (command in $()).

  3. advance use of [[ ]] wild cards ...

  1. alias, function versus scripts, function is more robust, convenient, but you have to source them in every time, which decreases performance you should think about things similar to python and ruby, use them in both ways! alias similar to function, prefer script than alias and function, generally speaking counter example cs Vim !: has no access to shell alias or functions another reason to use script function versus script: function need to refreshed (fun is cached) while scripts need not after modified so I think scripts is more suitable to use I vote for separate fun and script

  2. shell scripts are more flexible than shell functions, because you can use shabang, which gives flexibility to use any script language shell function is must sometimes, e.g., cs example, shell scripts are run in sub shell () ... $() ... use $0 and $BASH_SOURCE to write ... your example

  1. avoid using "global" bash variables in sub shell, many things are sub shell, very dangerous, and be careful!!!, duplicity, aerofs, dropbox examples ...

  2. bash array index is 0-based

-b file = True if the file exists and is block special file.

-c file = True if the file exists and is character special file.

-d file = True if the file exists and is a directory.

-e file = True if the file exists.

-f file = True if the file exists and is a regular file

-g file = True if the file exists and the set-group-id bit is set.

-k file = True if the files' "sticky" bit is set.

-L file = True if the file exists and is a symbolic link.

-p file = True if the file exists and is a named pipe.

-r file = True if the file exists and is readable.

-s file = True if the file exists and its size is greater than zero.

-s file = True if the file exists and is a socket.

-t fd = True if the file descriptor is opened on a terminal.

-u file = True if the file exists and its set-user-id bit is set.

-w file = True if the file exists and is writable.

-x file = True if the file exists and is executable.

-O file = True if the file exists and is owned by the effective user id.

-G file = True if the file exists and is owned by the effective group id.

file1 –nt file2 = True if file1 is newer, by modification date, than file2.

file1 ot file2 = True if file1 is older than file2.

file1 ef file2 = True if file1 and file2 have the same device and inode numbers.

-z string = True if the length of the string is 0.

-n string = True if the length of the string is non-zero.

string1 = string2 = True if the strings are equal.

string1 != string2 = True if the strings are not equal.

!expr = True if the expr evaluates to false.

expr1 –a expr2 = True if both expr1 and expr2 are true.

expr1 –o expr2 = True is either expr1 or expr2 is true.

  1. In a case ... esac clause, you have to espace spaces in matching patterns, e.g., You should use *Linux\ Mint* instead of *Linux Mint* in the following code.

    case "$dist" in
    *Debian* | *LMDE* | *Ubuntu* | *Linux\ Mint* )
        # install minimum texlive and some extra fonts
        wajig install texlive texlive-latex-extra texlive-fonts-extra dvipng texstudio
        return 0;;
    * )
        echo "This script does not support ${dist:13}."
        return 1;;
esac

  2. loop throught English letters

    for letter in {a..z} ; do
echo $letter
done

for letter in {c..f} ; do
echo $letter
done

  1. Direct output and error message

     command1 > out.txt 2> err.txt

 command2 -f -z -y > out.txt 2> err.txt

Environment Variables

using envrionment variables in script that doesn't exists is really dangerous!!! came across this multiple times. how should solve the problem? Check whether a variable exists before you use it?

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