bash Cookbook

Splitting Output Only When Necessary

Problem

You want to split output only if the input exceeds your limit, but the split command always creates at least one new file.

Solution

# cookbook filename: func_split

#+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
# Output fixed-size pieces of input ONLY if the limit is exceeded
# Called like: Split <file> <prefix> <limit option> <limit argument>
# e.g. Split $output ${output}_ --lines 100
# See split(1) and wc(1) for option details
function Split {
    local file=$1
    local prefix=$2
    local limit_type=$3
    local limit_size=$4
    local wc_option

    # Sanity Checks
    if [ -z "$file" ]; then
        printf "%b" "Split: requires a file name!\n"
        return 1
    fi
    if [ -z "$prefix" ]; then
        printf "%b" "Split: requires an output file prefix!\n"
        return 1
    fi
    if [ -z "$limit_type" ]; then
        printf "%b" "Split: requires a limit option (e.g. --lines), see 'man split'!\n"
        return 1
    fi
    if [ -z "$limit_size" ]; then
        printf "%b" "Split: requires a limit size (e.g. 100), see 'man split'!\n"
        return 1
    fi

    # Convert split options to wc options. Sigh.
    # Not all options supported by all wc/split on all systems
    case $limit_type in
        -b|--bytes)      wc_option='-c';;
        -C|--line-bytes) wc_option='-L';;
        -l|--lines)      wc_option='-l';;
    esac

    # If whatever limit is exceeded
    if [ "$(wc $wc_option $file | awk '{print $1}')" -gt $limit_size ]; then
        # actually do something
        split --verbose $limit_type $limit_size $file $prefix
    fi
} # end of function Split

Discussion

Depending on your system, ...