When working with command-line tools, we often need to add contextual information to each line of output. A common scenario is prefixing log lines with "[ERROR]" or timestamps for better readability and processing.

The simplest approach uses sed (stream editor):

command | sed 's/^/[ERROR] /'

Example with actual commands:

ls -l | sed 's/^/[INFO] /'

When your prefix contains special characters like forward slashes, use a different delimiter:

grep "pattern" file.txt | sed 's|^|[ERROR:404] |'

For more complex scenarios, consider these approaches:

Using awk

command | awk '{print "[DEBUG]", $0}'

Using Perl

command | perl -pe 's/^/[WARN] /'

Using while read loop

command | while read line; do echo "[TRACE] $line"; done

For large files or streams:

• sed is generally fastest
• awk offers more flexibility
• Shell loops are slowest but most readable

Adding timestamps to application logs:

tail -f /var/log/app.log | sed "s/^/$(date '+%Y-%m-%d %H:%M:%S') /"

Only prefix lines matching a pattern:

command | sed '/error/!s/^/[OK] /; /error/s/^/[ERROR] /'

Common issues and solutions:

# Problem: Prefix disappears for blank lines
command | sed 's/^$.$/[ERROR] \1/'

# Problem: Color codes interfere
command | sed 's/^/\x1b[31m[ERR]\x1b[0m /'

As developers, we frequently need to process log files or command output
where we want to add a prefix to every line. Common use cases include:

- Adding severity tags like [ERROR] or [INFO] to log entries
- Adding timestamps to each line of output
- Marking processed lines with a prefix for debugging


The simplest solution is using sed (stream editor):

command | sed 's/^/[ERROR] /'

Example with actual output:
$ echo -e "line1\nline2" | sed 's/^/[ERROR] /'
[ERROR] line1
[ERROR] line2


For more complex scenarios, awk provides better flexibility:

command | awk '{print "[ERROR] " $0}'

Benefits of using awk:
- Better handling of special characters
- Can conditionally add prefixes
- Allows more complex formatting

Example with conditional prefixing:
$ echo -e "error: crash\nwarning: memory\ndebug: trace" | awk '/error/ {print "[CRITICAL] " $0; next} {print "[LOG] " $0}'
[CRITICAL] error: crash
[LOG] warning: memory
[LOG] debug: trace


For large files (GBs of data), consider these optimizations:

1. Use sed for simplest cases (fastest)
2. For complex transformations, use awk's efficient text processing
3. Avoid piping through multiple commands when possible

Benchmark example:
# Test with 1 million lines
$ time seq 1000000 | sed 's/^/PREFIX/' > /dev/null
real    0m0.347s

$ time seq 1000000 | awk '{print "PREFIX" $0}' > /dev/null 
real    0m0.521s


When working with variables in the prefix:

prefix="[$(date +%T)] "
command | awk -v prefix="$prefix" '{print prefix $0}'

Example output:
[14:23:45] Server started
[14:23:45] Listening on port 8080


When your prefix contains special regex characters:

# Escape special chars for sed
prefix="[ERROR\\] "
command | sed "s/^/${prefix//\\/\\\\}/"

Or safer with awk:
prefix="[ERROR] "
command | awk -v p="$prefix" '{print p $0}'