Dfyjkt

I want to know how to remove the first occurrence of a word in a file.

Example: I want to remove the admin, from the first occurrence only.

account required pam_opendirectory.so
account sufficient pam_self.so
account required pam_group.so no_warn group=admin,wheel fail_safe
account required pam_group.so no_warn deny group=admin,wheel ruser fail_safe

I have tried:

sudo sed -i.bak "s/admin,//1" /etc/pam.d/screensaver

But that removes both cases, any ideas?

Using sed

To remove the first occurrence in the file:

$ sed -e ':a' -e '$!N;ba;' -e 's/admin,//1' file
account required pam_opendirectory.so
account sufficient pam_self.so
account required pam_group.so no_warn group=wheel fail_safe
account required pam_group.so no_warn deny group=admin,wheel ruser fail_safe

(The above was tested with GNU sed.)

How it works

• 
  

  -e ':a' -e '$!N;ba;'

  
  
  

  This reads the whole file in at once to the pattern space. (If your file is too large for memory, this is not a good approach.)

  
  


• 
  

  -e 's/admin,//1'

  
  
  

  This performs the substitution on the first occurrence of admin and only the first occurrence.

  
  

Using BSD (OSX) sed

Wintermute reports that BSD sed cannot do branch instructions in one-liners and suggests this alternative for changing the file in place:

sed -i.bak -n '1h; 1!H; $ x; s/admin,//; p; ' file

This reads the whole file in at once and then does the substitution once the last line has been read.

In more detail:

• 
  

  -n

  
  
  

  By default, sed would print each line. This turns that off.

  
  


• 
  

  1h

  
  
  

  This places the first line in the hold buffer.

  
  


• 
  

  1!H

  
  
  

  For all subsequent lines, this appends them to the hold buffer.

  
  


• 
  

  $ x; s/admin,//; p;

  
  
  

  For the last line ($), this exchanges the hold and pattern buffer so that the pattern buffer now has the complete file. s/admin,// does the substitution and p prints the result.

  
  

Using awk

$ awk '/admin/ && !fsub(/admin,/, ""); f=1 1' file >file.tmp && mv file.tmp file

This results in:

account required pam_opendirectory.so
account sufficient pam_self.so
account required pam_group.so no_warn group=wheel fail_safe
account required pam_group.so no_warn deny group=admin,wheel ruser fail_safe

How it works

• 
  

  /admin,/ && !fsub(/admin,/, ""); f=1

  
  
  

  For each line, check to see if it contains the word admin, and if the flag f still has its default value of zero. If so, remove the first occurrence of admin, and set the flag to one.

  
  


• 
  

  1

  
  
  

  Print each line. 1 is awk's cryptic shorthand for print $0.

  
  

Using sed, without reading the whole file:

sed 's/admin,//;tl;b;:l;n;bl' file

This waits for a successful substitution and then goes into a loop of printing the rest of the file.

• s/admin,//;tl;b;: substitute admin with empty string, if substitution occurs jump to label l otherwise print the line and exit (next line will be processed the same way).




• :l;n;bl: define label l, read line, jump to l.