Parsing the text that sets colors in a bash shell

Question

Could someone either point me to some documentation on the web that would explain how a bash shell would pars the following lines that sets up colors or explain from the parsers view, how the following lines would be read?

• I know that 32 cause dark green to be displayed
• 34 causes dark blue to be displaied

but what is the first \033 used for and what dose the 'm' after the 32 and 33 do?

green ="\[\033[0;32m\]"
blue="\[\033[0;34m\]" 

and eventually export these and other items to PS1

Answer 1

The best place to start reading for bash is the Bash Prompt HOWTO, specifically the section Bash Prompt Escape Sequences.

The characters \033 (called esc or escape) followed by a left square bracket [ is the way 7-bit characters form the ECMA-48 control sequence initiator (CSI). That starts a control sequence, which continues until the final character (has to be from the range 64-126) which in this case is m. The final character is one of the things that determines what type of control sequence it is — this one is the SGR (set graphic rendition).

While most final characters are alphabetic (A-Z or a-z), a few are not. XTerm Control Sequences lists some for instance:

CSI Pm `  Character Position Absolute  [column] (default = [row,1])
          (HPA).
CSI Pt; Pl; Pb; Pr $ {
          Selective Erase Rectangular Area (DECSERA), VT400 and up.
            Pt; Pl; Pb; Pr denotes the rectangle.
CSI Ps ' |
          Request Locator Position (DECRQLP).
CSI Pm ' }
          Insert Ps Column(s) (default = 1) (DECIC), VT420 and up.
CSI Pm ' ~
          Delete Ps Column(s) (default = 1) (DECDC), VT420 and up.

For your example

green ="\[\033[0;32m\]"

Taking it apart:

• The \[ and \] tell bash to discount the enclosed characters (do not count them as printable characters with a width). This is bash-specific. If you used zsh, there is a different way to do the same thing. Most shells (such as ksh) do not provide this feature.
• The 0 in the SGR sequence resets any attribute (such as color) which an SGR sequence might have set.
• The ; semicolon separates that from the next parameter, which is a particular color.
• ECMA-48 defines 8 foreground and 8 background colors (often referred to as "ANSI colors" although it has been a long time since there was a corresponding ANSI standard).