How to append one file to another in Linux from the shell

Question

I have two files  file1 and file2  How do I append the contents of file2 to file1 so that contents of file1 persist the process

User · Answer

Note  if you need to use sudo  do this   sudo bash -c  cat file2  gt  gt  file1   The usual method of simply prepending sudo to the command will fail  since the privilege escalation doesn t carry over into the output redirection

User · Answer

You can also do this without cat  though honestly cat is more readable    gt  gt  file1  lt  file2  The  gt  gt  appends STDIN to file1 and the  lt  dumps file2 to STDIN

User · Answer

Use bash builtin redirection  tldp    cat file2  gt  gt  file1

User · Answer

Try this command   cat file2  gt  gt  file1

User · Answer

cat file2  gt  gt  file1  The  gt  gt  operator appends the output to the named file or creates the named file if it does not exist   cat file1 file2  gt  file3  This concatenates two or more files to one   You can have as many source files as you need   For example   cat   txt  gt  gt  newfile txt  Update 20130902 In the comments eumiro suggests  don t try cat file1 file2  gt  file1    The reason this might not result in the expected outcome is that the file receiving the redirect is prepared before the command to the left of the  gt  is executed   In this case  first file1 is truncated to zero length and opened for output  then the cat command attempts to concatenate the now zero-length file plus the contents of file2 into file1   The result is that the original contents of file1 are lost and in its place is a copy of file2 which probably isn t what was expected   Update 20160919 In the comments tpartee suggests linking to backing information sources   For an authoritative reference  I direct the kind reader to the sh man page at linuxcommand org which states      Before a command is executed  its input and output may be redirected   using a special notation interpreted by the shell    While that does tell the reader what they need to know it is easy to miss if you aren t looking for it and parsing the statement word by word   The most important word here being  before    The redirection is completed  or fails  before the command is executed   In the example case of cat file1 file2  gt  file1 the shell performs the redirection first so that the I O handles are in place in the environment in which the command will be executed before it is executed   A friendlier version in which the redirection precedence is covered at length can be found at Ian Allen s web site in the form of Linux courseware   His I O Redirection Notes page has much to say on the topic  including the observation that redirection works even without a command   Passing this to the shell      gt out      creates an empty file named out   The shell first sets up the I O redirection  then looks for a command  finds none  and completes the operation

User · Answer

Another solution   cat file1   tee -a file2   tee has the benefit that you can append to as many files as you like  for example   cat file1   tee -a file2 file3 file3   will append the contents of file1 to file2  file3 and file4   From the man page   -a  --append        append to the given FILEs  do not overwrite

User · Answer

cat can be the easy solution but that become very slow when we concat large files  find -print is to rescue you  though you have to use cat once   amey xps   work python tmp   ls -lhtr total 969M -rw-r--r-- 1 amey amey 485M May 24 23 54 bigFile2 txt -rw-r--r-- 1 amey amey 485M May 24 23 55 bigFile1 txt   amey xps   work python tmp   time cat bigFile1 txt bigFile2 txt  gt  gt  out txt  real    0m3 084s user    0m0 012s sys     0m2 308s   amey xps   work python tmp   time find   -maxdepth 1 -type f -name  bigFile   -print0   xargs -0 cat --  gt  outFile1  real    0m2 516s user    0m0 028s sys     0m2 204s

User · Answer

Just for reference  using ddrescue provides an interruptible way of achieving the task if  for example  you have large files and the need to pause and then carry on at some later point   ddrescue -o   wc --bytes file1   awk    print  1     file2 file1 logfile   The logfile is the important bit  You can interrupt the process with Ctrl-C and resume it by specifying the exact same command again and ddrescue will read logfile and resume from where it left off  The -o A flag tells ddrescue to start from byte A in the output file  file1   So wc --bytes file1   awk    print  1    just extracts the size of file1 in bytes  you can just paste in the output from ls if you like    As pointed out by ngks in the comments  the downside is that ddrescue will probably not be installed by default  so you will have to install it manually  The other complication is that there are two versions of ddrescue which might be in your repositories  see this askubuntu question for more info  The version you want is the GNU ddrescue  and on Debian-based systems is the package named gddrescue   sudo apt install gddrescue   For other distros check your package management system for the GNU version of ddrescue

[linux] How to append one file to another in Linux from the shell?

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