Sed is a non-interactive stream editor. It receives text input,
whether from stdin or from a file, performs certain operations on
specified lines of the input, one line at a time, then outputs the
result to stdout or to a file. Within a shell script, sed is
usually one of several tool components in a pipe.
Sed determines which lines of its input that it will operate on from
the address range passed to it. [2] Specify this address range either
by line number or by a pattern to match. For example, 3d
signals sed to delete line 3 of the input, and /Windows/d tells sed
that you want every line of the input containing a match to "Windows"
deleted.
Of all the operations in the sed toolkit, we will focus
primarily on the three most commonly used ones. These are printing (to
stdout), deletion, and substitution.
Print [specified address range]
Delete
Delete [specified address range]
Substitute
Substitute pattern2 for first instance of pattern1 in a line
Substitute
Substitute pattern2 for first instance of pattern1 in a line, over address-range
Transform
Replace any character in pattern1 with the corresponding character in
pattern2, over address-range (equivalent of tr)
Insert
Insert pattern at the address indicated
Global
Unless the g (global) operator is appended to a substitute
command, the substitution operates only on the first instance of a
pattern match within each line.
From the command-line and in a shell script, a sed operation may require quoting and certain options.
sed -e '/^$/d' $filename
# The -e option causes the next string to be interpreted as an editing instruction.
# (If passing only a single instruction to sed, the "-e" is optional.)
# The "strong" quotes ('') protect the RE characters in the instruction
#+ from reinterpretation as special characters by the body of the script.
# (This reserves RE expansion of the instruction for sed.)
#
# Operates on the text contained in file $filename.In certain cases, a sed editing command will not work with single quotes.
filename=file1.txt pattern=BEGIN
sed "/^$pattern/d" "$filename" # Works as specified. # sed '/^$pattern/d' "$filename" has unexpected results. # In this instance, with strong quoting (' ... '), #+ "$pattern" will not expand to "BEGIN".
Note: Sed uses the -e option to specify that the following
string is an instruction or set of instructions. If there is only a
single instruction contained in the string, then this may be omitted.
sed -n '/xzy/p' $filename # The -n option tells sed to print only those lines matching the pattern. # Otherwise all input lines would print. # The -e option not necessary here since there is only a single editing instruction.
Examples of sed operators
# Delete 8th line of input. sed -e 8d
# Delete all blank lines. sed -e '/^$/d'
# Delete from beginning of input up to, and including first blank line. sed -e '1,/^$/d'
# Print only lines containing "Jones" (with -n option). sed -e '/Jones/p'
# Substitute "Linux" for first instance of "Windows" found in each input line. sed -e 's/Windows/Linux/'
# Substitute "Linux" for first instance of "Windows" found in each input line. sed -e 's/BSOD/stability/g'
# Delete all spaces at the end of every line. sed -e 's/ *$//'
# Compress all consecutive sequences of zeroes into a single zero. sed -e 's/00*/0/g'
# Prints "How far are you along?" as first line, "Working on it" as second. echo "Working on it." | sed -e '1i How far are you along?'
# Inserts 'Linux is great.' at line 5 of the file file.txt. sed -e 5i 'Linux is great.' file.txt
# Delete all lines containing "GUI". sed -e '/GUI/d'
# Delete all instances of "GUI", leaving the remainder of each line intact. sed -e 's/GUI//g'
An address range followed by one or more operations may require open and closed curly brackets, with appropriate newlines.
/[0-9A-Za-z]/,/^$/{
/^$/d
}This deletes only the first of each set of consecutive blank lines. That might be useful for single-spacing a text file, but retaining the blank line(s) between paragraphs.
Note: The usual delimiter that sed uses is /. However, sed
allows other delimiters, such as %. This is useful when / is
part of a replacement string, as in a file pathname.