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7.3 uniq: Uniquify files

uniq writes the unique lines in the given input, or standard input if nothing is given or for an input name of ‘-’. Synopsis:

     uniq [option]... [input [output]]

By default, uniq prints its input lines, except that it discards all but the first of adjacent repeated lines, so that no output lines are repeated. Optionally, it can instead discard lines that are not repeated, or all repeated lines.

The input need not be sorted, but repeated input lines are detected only if they are adjacent. If you want to discard non-adjacent duplicate lines, perhaps you want to use sort -u. See sort invocation.

Comparisons honor the rules specified by the LC_COLLATE locale category.

If no output file is specified, uniq writes to standard output.

The program accepts the following options. Also see Common options.

-f n
--skip-fields=n
Skip n fields on each line before checking for uniqueness. Use a null string for comparison if a line has fewer than n fields. Fields are sequences of non-space non-tab characters that are separated from each other by at least one space or tab.

For compatibility uniq supports an obsolete option syntax -n. New scripts should use -f n instead.

-s n
--skip-chars=n
Skip n characters before checking for uniqueness. Use a null string for comparison if a line has fewer than n characters. If you use both the field and character skipping options, fields are skipped over first.

On older systems, uniq supports an obsolete option syntax +n. This obsolete behavior can be enabled or disabled with the _POSIX2_VERSION environment variable (see Standards conformance), but portable scripts should avoid commands whose behavior depends on this variable. For example, use ‘uniq ./+10’ or ‘uniq -s 10’ rather than the ambiguous ‘uniq +10’.

-c
--count
Print the number of times each line occurred along with the line.
-i
--ignore-case
Ignore differences in case when comparing lines.
-d
--repeated
Discard lines that are not repeated. When used by itself, this option causes uniq to print the first copy of each repeated line, and nothing else.
-D
--all-repeated[=delimit-method]
Do not discard the second and subsequent repeated input lines, but discard lines that are not repeated. This option is useful mainly in conjunction with other options e.g., to ignore case or to compare only selected fields. The optional delimit-method specifies how to delimit groups of repeated lines, and must be one of the following:
none
Do not delimit groups of repeated lines. This is equivalent to --all-repeated (-D).
prepend
Output a newline before each group of repeated lines. With --zero-terminated (-z), use a zero byte (ASCII NUL) instead of a newline as the delimiter.
separate
Separate groups of repeated lines with a single newline. This is the same as using ‘prepend’, except that no delimiter is inserted before the first group, and hence may be better suited for output direct to users. With --zero-terminated (-z), use a zero byte (ASCII NUL) instead of a newline as the delimiter.

Note that when groups are delimited and the input stream contains two or more consecutive blank lines, then the output is ambiguous. To avoid that, filter the input through ‘tr -s '\n'’ to replace each sequence of consecutive newlines with a single newline.

This is a GNU extension.

--group[=delimit-method]
Output all lines, and delimit each unique group. With --zero-terminated (-z), use a zero byte (ASCII NUL) instead of a newline as the delimiter. The optional delimit-method specifies how to delimit groups, and must be one of the following:
separate
Separate unique groups with a single delimiter. This is the default delimiting method if none is specified, and better suited for output direct to users.
prepend
Output a delimiter before each group of unique items.
append
Output a delimiter after each group of unique items.
both
Output a delimiter around each group of unique items.

Note that when groups are delimited and the input stream contains two or more consecutive blank lines, then the output is ambiguous. To avoid that, filter the input through ‘tr -s '\n'’ to replace each sequence of consecutive newlines with a single newline.

This is a GNU extension.

-u
--unique
Discard the first repeated line. When used by itself, this option causes uniq to print unique lines, and nothing else.
-w n
--check-chars=n
Compare at most n characters on each line (after skipping any specified fields and characters). By default the entire rest of the lines are compared.
-z
--zero-terminated
Delimit items with a zero byte rather than a newline (ASCII LF). I.e., treat input as items separated by ASCII NUL and terminate output items with ASCII NUL. This option can be useful in conjunction with ‘perl -0’ or ‘find -print0’ and ‘xargs -0’ which do the same in order to reliably handle arbitrary file names (even those containing blanks or other special characters).

An exit status of zero indicates success, and a nonzero value indicates failure.