Producing, isolating, combining, and transforming text

Producing text

date

Prints dates and times in various formats. Formatting starts with plus sign (+) and uses special expressions %X:

Format String	Example Output	Description
`%Y-%m-%d`	2025-03-29	ISO 8601 (Year-Month-Day)
`%d-%m-%Y`	29-03-2025	Day-Month-Year (common in Europe)
`%m/%d/%Y`	03/29/2025	Month/Day/Year (common in the US)
`%Y/%m/%d`	2025/03/29	Year/Month/Day
`%A, %B %d, %Y`	Saturday, March 29, 2025	Full weekday, month name, day, year
`%a, %b %d`	Sat, Mar 29	Abbreviated weekday and month
`%H:%M:%S`	14:30:15	24-hour time (HH:MM:SS)
`%I:%M:%S %p`	02:30:15 PM	12-hour time with AM/PM
`%Y-%m-%d %H:%M:%S`	2025-03-29 14:30:15	Full timestamp in ISO 8601 format
`%s`	1743456615	Unix timestamp (seconds since epoch)

seq

Prints a sequence of numbers:

requires 2 args, low and high value of sequence
if you add a third arg, it is seq <low> <increment> <high>
negative number to produce descending sequence
decimal increment for floating point numbers
-s option changes the separator. By default, separated by newline
-w makes each value the same width with leading zeroes

seq <low> [<increment>] <high>

seq 1 5             # 1 to 5
seq 1 2 10          # 1 to 10, increments of 2
seq 2 2 20          # 2 to 20, evens only
seq 10 -1 -10       # countdown from 10 to -10
seq 2.1 .01 2.5     # 2.10 to 2.50, increments of .01
seq -sx 1 10        # 1x2x3x4x5x6x7x8x9x10
seq -w 2 2 100      # 2 to 100, evens 002 - 100

Brace expansion

Shell feature that prints a sequence of numbers or characters:

numbers and letters
produces a single line separated by a space
change piping output with tr (translate)
separate the low and high values with 2 dots ..
add a third value as the increment: {x..y..z}

echo {1..5}                 # 1 2 3 4 5
echo {1..10..2}             # 1 3 5 7 9
echo {2..10..2}             # 2 4 6 8 10
echo {10..-10..-1}          # 10 9 8...-8 -9 -10
echo {a..z}                 # a b c d...w x y z
echo {A..Z..2}              # A C E G I K M O Q S U W Y
echo {A..Z} | tr -d ' '     # ABCDEFGHIJKLMNOPQRSTUVWXYZ
echo {A..Z} | tr ' ' '\n'   # replace delimiter
A
B
...
Z
alias nth="echo {A..Z} | tr -d ' ' | cut -c"    # find nth letter of alphabet

find

Prints file paths recursively:

pipe to sort to list alpha
use -exec echo rm {} for safe deletes

exec executes a linux command for each file path in the output:

Create the find command
Append -exec and then the command to execute. In the command, substitue {} for the file path
End with quoted or escaped semicolon: ";" or \;

find /etc -type f -name "*.conf"                        # quote patterns
find /etc -type f -name "*.conf" -exec ls -l {} \;      # run 'ls -l' on each file that matches the name pattern
find /etc -type f -name "*.conf" -ls                    # built-in option

# safe deletes
find ~/toolbox/ -type f -name "*~" -exec echo rm {} \;  # 1. test run
find ~/toolbox/ -type f -name "*~" -delete              # 2. execute with built-in option

yes

Prints the same line repeatedly. Prints y by default:

can pipe to commands that require you answer “yes” to interactive prompts
pipe to head to print string specific number of times, especially when generating test files

yes 'some string' | head -10

Isolating text

Primarily use these commands:

grep: prints lines that match a string
cut: prints columns from a file
head: prints first lines of file
tail: prints last lines of file

grep and regex

grep -l his dir/*           # find all files that contain 'his' in dir
grep '^[A-Z]' file          # all lines that begin with capital
grep -v '^$' file           # nonblank lines (find blank lines and ignore)
grep 'one\|two' file        # either one or two
grep '......' file          # at least 6 chars long
grep '<.*>' file            # less than before greater than, such as HTML code
grep 'w\.' file             # w followed by period

# fixed strings
fgrep w\. file              # ignore regex
grep -F w\. file            # ignore regex

# match against set of strings
cut -d: -f7 /etc/passwd | sort -u | grep -f /etc/shells -F  # match shells in passwd file w shells on computer
cat first | sort -u | grep -f second                        # simpler version

With regex:

Syntax	Example	Description
`.`	`...`	Any single character except newline
`X*`	`_*`	Zero or more occurrences of `_`
[characters]	[aeiouAEIOU]	Any characters in the set
[^characters]	[^aeiouAEIOU]	Any non-vowel
[c1-c2]	[0-4]	Any characters in the given range
[^c1-c2]	[0-4]	Any characters not in the given range
E1\| E2	[one\|two]	Either of two expressions (either ‘one’ or ’two’)
E1\| E2	[0-4]	Any characters not in the given range

tail

Prints last 10 lines of file, can change number of lines like head:

tail -n+3 frost                 # print starting at the third line
tail +3 frost                   # print starting at the third line

head -4 tempfile | tail -2      # print 3rd and 4th lines in file

awk

Can extract columns from a file in ways that cut cannot:

use when columns have arbitrarily sized delimiters
refer to each field with $<fieldNum>
- multi-digit columns, use parentheses: $(11)
final field: $NF (number of fields)
entire line: $0
use commas to include whitespace in output

awk '{print $2}' /etc/hosts                         # print second column
echo one two three four | awk '{print $2 $3}'       # twothree
echo one two three four | awk '{print $2, $3}'      # two three

df | awk '{print $4}'
df | awk 'FNR>1 {print $4}'                         # print only line numbers greater than 1
echo first:::::second | awk '-F':* '{print $2}'     # change delimiter. Prints 'second'

Combining text

cat

cat is short for “concatenate”, so it combines files from top to bottom:

cat file1 file2 file3

tac

Combines files bottom-to-top. Its the opposite of cat, which is why its cat backwards:

great for processing files that are in chronological order but not reversible with sort -r, such as a web server log

tac /var/log/apache2/access.log.1           # reverse web server log

paste

Combines files side-by-side in columns separated by a tab character:

-d to change delimiter

paste title-words1 title-words2             # tab-delimited
paste -d, title-words1 title-words2         # comma-delimited
paste -d "\n" title-words1 title-words2     # interleave lines from two or more files

diff

Interleaves text from two files by printing their differences:

1c1 means “line 1 in file 1 is different than line 1 in file 2”
2a3 means “file 2 has a third line not present in file 1”

diff file1 file2
1c1                                         # line 1 in file1 is different than line 2 in file2
< Linux is all about efficiency.            # line from file1
---                                         # separator
> MacOS is all about efficiency.            # line from file2
2a3                                         # addition: file2 has third line not present in file1                          
> Have a nice day.

diff file1 file2 | grep '^[<>]'             # grep for just the differences
< Linux is all about efficiency.
> MacOS is all about efficiency.
> Have a nice day.

diff file1 file2 | grep '^[<>]' | cut -c3-  # same grep, remove leading </>
Linux is all about efficiency.
MacOS is all about efficiency.
Have a nice day.

Transforming text

tr

Translates characters into other characters:

takes two sets of characters: translates the first set into the second set
-d: deletes whitespace and tabs

echo one two      three | tr -d ' '     # onetwothree

rev

Reverses characters on a line. Useful if you need to extract field in a file where each line has a different number of columns. For example, if a file is a list of names, and you need the last name, the number of words on each line varies:

rev file | cut -d ' ' -f1 | rev

awk

Transforms lines of text from files or stdin into any other text, using a sequence of instructions called an awk program:

-f option accepts awk program files
always enclose the awk program in single or double quotes when provided on the command line
-F changes the input separator

awk <program> <input-files>
awk -f <program-file1> -f <program-file2> <input-files>
awk 'FNR<=7' nums.file                  # print first 7 lines of file
echo "one two" | awk '{print $2, $1}'   # two one (switch cols)

awk '{print $NF}' file                  # print last word on each line

awk '/^word/{print $4}' file            # if the line begins with 'word', print the fourth field

# if field 3 starts with 201, perform awk program on that line
awk -F'\t' '$3~/^201/{print $4, "(" $3 ").", "\"" $2 "\""}' animals.txt

awk -F'\t' \    # same command with intro and ending
'BEGIN {print "Recent books:"} \
$3~/^201/{print $4, "(" $3 ").", "\"" $2 "\""} \
END {print "For more books, search the web"}' \
animals.txt 

Recent books:
...
For more books, search the web

seq 1 100 | awk '{s+=$1} END {print s}'     # sum numbers 1 - 100

Arrays and loops

An array in awk stores a collection of values. Each value in the array is an element, which consists of the array name and stored value: counts["f44323234453400"]. The syntax to access an element is a key. (It seems to act more like a map than an array.)

md5sum *.jpg | awk '{counts[$1]++}'   # store all md5sums in array, where the element is count[<md5sum>] and key is the number of occurrences

The for loop steps through an array and processes each element in sequence. It uses this syntax:

for (var in array) <action> array[var]

If this is used in the same command that creates the array, run the for loop after an END instruction so it operates on the completed array:

md5sum *.jpg \
| awk '{counts[$1]++} \
END {for (key in counts) print counts[key] " " key}'
3 5bbf5a52328e7439ae6e719dfe712200
1 c193497a1a06b2c72230e6146ff47080
1 febe6995bad457991331348f7b9c85fa

sed

Transforms text from files or stdin into other text with a sequence of instructions called a sed script (which is a string):

most common sed script replaces strings with another string s/regexp/replacement/
- replaces the first occurence only. Use g global option to replace all occurences
-e lets you use a sequence of scripts on a single input file
-f lets you use sed scripts stored in files
replace the / with any other character, such as _

sed <script> <input-files>
sed -e <script1> -e <script2> -e <script3> <input-files>
sed -f <script-file1> -f <script-file2> -f <script-file3> <input-files>

echo Ricky Henderson | sed 's/Henderson/Ricardo/'   # Ricky Ricardo

sed 's/.* //' celebrities               # replace all chars up to last space with nothing
sed 7q nums.file                        # print first 7 lines of file
sed 's/\.jpg/\.png/'                    # replace .jpg by .png
echo Case Insensitive | sed 's/case/how/i'  # case insensitive
echo one one two | sed 's/one/yes/g'    # global replace

Deletion

You can delete by line number, or with regex matches:

seq 1 10 | sed 5d                       # delete 5th line
seq 1 10 | sed '/[2468]/d'              # delete even numbers

Subexpressions

Lets you manipulate and change strings:

each subexpression is numbered \1, \2, and so on

Create a regex that matches what you want to change
Isolate with parenteses and backslashes ((\ and )\) the portion of the string that you want to manipulate

image\.jpg\.[1-3]                               # 1. create reges
image\.jpg\.\([1-3]\)                           # 2. isolate the portion you want to manipulate
sed "s/image\.jpg\.\([1-3]\)/image\1.jpg/"      # 3. move the string with \1