Per-host prompt colouring

Like many people, I have shell accounts on many machines, and it's often very helpful to have a subtle reminder of which machine I'm on at any one time. I've set up a system where my prompt automatically changes colour according to what system I'm logged into, without my having to manually choose a colour for every system, and without the risk of it accidentally choosing some unreadable colour.

Here's the current code, with comments:

# We get the first 7 digits of the md5sum; if we got 8 digits, the
# hosthash might be negative on 32-bit machines, and that would mess
# things up.
HOSTHASH=$(hostname | md5sum)
HOSTHASH=${HOSTHASH:0:7}

# Map into the range of available colours
if [ $(tput colors) -ge 256 ]; then
    # All the colours with brightness > 25% in the default xterm
    # palette
    BRIGHT_COLORS=(2 3 4 5 6 7 8 9 10 11 12 13 14 15 22 23 24 25 26 27
    28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49
    50 51 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77
    78 79 80 81 82 83 84 85 86 87 94 95 96 97 98 99 100 101 102 103 104
    105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121
    122 123 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144
    145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 165 166
    167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183
    184 185 186 187 188 189 190 191 192 193 194 195 198 199 200 201 202
    203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219
    220 221 222 223 224 225 226 227 228 229 230 231 238 239 240 241 242
    243 244 245 246 247 248 249 250 251 252 253 254 255)
elif [ $(tput colors) -ge 16 ]; then
    BRIGHT_COLORS=(2 3 4 5 6 7 8 9 10 11 12 13 14 15)
else
    BRIGHT_COLORS=(2 3 4 5 6 7)
fi

COLOR=${BRIGHT_COLORS[$(( 0x$HOSTHASH % ${#BRIGHT_COLORS[@]} ))]}

C="\[$(tput setaf $COLOR)\]" # color
B="\[$(tput bold)\]" # bold
N="\[$(tput sgr0)\]" # normal
PS1="$C$B[$N$C\\u@\\h$B\$TMP_DELIM$N$C\$TMP_PWD_VALUE$B]\\\$$N "
unset HOSTHASH BRIGHT_COLORS COLOR C B N          # cleanup

If you're wondering about $TMP_DELIM and $TMP_PWD_VALUE, see How to limit the length of your bash prompt. Feel free to replace those variables with the standard \w or \W if you want.

changed October 25, 2009