Copy, move, find, grep, and sed

Copy, move, and link

cp makes a copy; a directory needs -r or -R. cp -a copies recursively while keeping permissions, owners, and timestamps, which suits backups. mv both moves and renames. If the destination exists, behavior depends on whether it is a file or directory, so inspect it before overwriting. -i asks before overwriting and -n never overwrites.

ln -s /opt/app/current app creates a symbolic link, a pointer to another path. Deleting the link leaves the target intact; deleting the target leaves a broken link.

flowchart LR
  F["find /var/log<br/>-name '*.log'"] --> X["filenames"]
  X --> G["grep -i error<br/>search content"]
  G --> P["sort, uniq -c<br/>pipe and count"]
  P --> S["sed 's/a/b/'<br/>transform text"]
  S --> O["output<br/>or file"]
  %% step F: find searches by name, size or time and prints matching paths.
  %% step X: The result is a list of filenames, not their content.
  %% step G: grep searches inside files for a pattern.
  %% step P: Pipes pass one command's output to the next: sort, uniq -c, wc -l, head.
  %% step S: sed edits text in a stream; add -i only after the preview looks right.
  %% step O: Print to the screen or redirect to a new file.

Search for the right thing

find can filter by more than names: -type f or -type d, -size +100M, and -mtime -1 (modified within the last day). find walks the current filesystem; locate consults a database that may be stale. Quote a glob so find receives the pattern instead of the shell expanding it early.

grep has a few flags worth memorising: -r searches a directory tree, -n prints line numbers, -v inverts the match, -c counts matches, and -E enables extended regular expressions such as 'error|fail'.

Pipes and small text tools

The pipe | sends one command's output into the next, so small tools combine into a quick report:

grep 'ERROR' app.log | cut -d' ' -f1 | sort | uniq -c | sort -rn | head

cut picks columns, sort orders lines, uniq -c counts adjacent duplicates (sort first), and awk '{print $2}' prints a field. tee writes output to a file while still showing it.

Edit with sed

sed does not edit a file without -i; preview output before choosing in-place editing. sed -i.bak 's/8080/9090/' app.conf edits in place and keeps app.conf.bak as a backup. sed -n '10,20p' file prints only lines 10 to 20.