Shell Data Processing - Cat command (short for concatenate)

About

cat is a core utility that generates a stream of data from one or more files (ie file descriptor)

It therefore can:

• concatenate files.
• read any file (or file descriptor ie tcp connection, …)

Example

Concatenate files

file 1 - line 1
file 1 - line 2

file 2 - line 1
file 2 - line 2

• Output file1.txt then file2.txt contents and redirect and create the file fileAll.txt

cat file1.txt file2.txt > fileAll.txt

• Content of fileAll.txt

cat fileAll.txt

file 1 - line 1
file 1 - line 2
file 2 - line 1
file 2 - line 2

Concatenate Output files selected with a glob pattern

Output all files with the pgn extension

cat *.pgn

where: *.pgn is a glob pattern

Concatenate Output files and standard input

Concatenate the output of file and of standard input

cat f - g

where:

• f is a file
• - is standard input
• g is a file

Example:

• As a prerequisite, we create two files f and g that contains the text Hello World with the help of tee that redirect the output of the echo command to this files.

echo 'Hello World' | tee f g

Copy standard input to standard output

Without any argument, cat will copy standard input to standard output

cat

Example:

• As prerequisites, we create of a input file

echo 'Hello World' > helloIn.txt

• We use then the redirection operator:
  • < to set helloIn.txt as the standard input of cat
  • > to redirect the standard output into the file helloOut.txt

cat < helloIn.txt > helloOut.txt

• Result: showing the content of helloOut.txt with cat

cat helloOut.txt

Hello World

Read a file descriptor

# create file descriptor 3 to a file (ie an alias)
exec 3<>/tmp/mycat
# cat works but return nothing
cat /tmp/mycat
# Writes to it
echo "Hello" >&3
# Return Hello
cat /tmp/mycat

Read a named pipe

cd /tmp
# Creates the pipe in the current directory
mkfifo named_pipe
# Reads from the pipe (blocking) until it can read data
cat named_pipe

• In another terminal

cd /tmp
# Writes to the pipe (blocking) until the FIFO is emptied
echo "Hi" > named_pipe