Debugging Scripts

PS4 Prefix

Shape Trace Lines

PS4 controls the prefix Bash uses for execution traces. Modeling the prefix helps explain set -x output without printing noisy traces.

Program

Play the script to choose a trace prefix and build one modeled trace line.

prefix
ps4_prefix.sh
Replay: real traced execution (multi-file project)
#!/usr/bin/env bash

prefix="+ "
command="echo ready"
trace_line="${prefix}${command}"
echo "$trace_line"

#!/usr/bin/env bash

prefix="TRACE> "
command="echo ready"
trace_line="${prefix}${command}"
echo "$trace_line"

  1. prefix ← + 

    3prefix="+ "4command="echo ready"
    values this step+ prefix

  2. command ← echo ready

    3prefix="+ "4command="echo ready"5trace_line="${prefix}${command}"
    values this stepecho readycommand

  3. trace_line ← + echo ready

    4command="echo ready"5trace_line="${prefix}${command}"6echo "$trace_line"
    values this step+ echo readytrace_line+ prefixecho readycommand

  4. echo "$trace_line"

    5trace_line="${prefix}${command}"6echo "$trace_line"
    output+ echo ready
    values this step+ echo readytrace_line

  1. prefix ← TRACE> 

    3prefix="TRACE> "4command="echo ready"
    values this stepTRACE> prefix

  2. command ← echo ready

    3prefix="TRACE> "4command="echo ready"5trace_line="${prefix}${command}"
    values this stepecho readycommand

  3. trace_line ← TRACE> echo ready

    4command="echo ready"5trace_line="${prefix}${command}"6echo "$trace_line"
    values this stepTRACE> echo readytrace_lineTRACE> prefixecho readycommand

  4. echo "$trace_line"

    5trace_line="${prefix}${command}"6echo "$trace_line"
    outputTRACE> echo ready
    values this stepTRACE> echo readytrace_line
PS4 `PS4` is the prompt prefix Bash prints before traced commands.
set -x `set -x` emits commands as Bash expands and runs them.
modeled trace This example prints one trace-shaped line instead of enabling global tracing.