Bash Input/Output

Internal

• bash

Echo Multiple Lines to stdout

cat <<EOF
  blah
  ${some_var}
  blah
EOF

• It will perform variable substitution. If you don't want that, escape \${some_var}.
• `...` sequences should be escaped as shown here: \`...\` otherwise they will executed before the output is sent to stdout.
• \ will join lines. If you want "\" in the output, then you should escape it:

      ...
      blah blah \\
      ...

Echo Multiple Lines into a File

(cat << EOF
blah
EOF
) > /tmp/test.txt

Iterating over Lines from a File in the Same bash Process

With for

Note that the following approach works better for small files. If the file is large, the content will be first cached in memory, and it may look like the command is irresponsive:

Iterating over Lines in the Same bash Process with for

With sed

This approach has the advantage that lines are read one by one and not buffered:

filename=...
line_count=$(wc -l ${filename}) || exit 1
line_count=${line_count% *}
line_count=$(echo ${line_count} | sed -e 's/ *//')

line_number=1

while [ ${line_number} -le ${line_count} ]; do

    local line
    line=$(sed -n ${line_number}p ${filename})

    # process line ...

    ((line_number++))
done

Extracting a Line Specified by Its Number from a File

With head and tail

Extract line 15 (line numbers are 1-based):

cat ./test.txt | head -15 | tail -1

With sed

Print a Line Specified by Its Number with sed