from the desk of stinkpot

if you find that sed isn’t matching \t, the easiest workaround is to not use sed at all. instead, install ‘gsed’, which can handle tabs.

i’ve got a bunch of files with one line — a number — and i’d like to sort those filenames on the command line based on that number. to that in one line of shell code: for i in `ls *.*`; do echo “`head -n1 $i` $i” ; done | sort -n

to  send the standard output and standard error streams from your shell script somewhere, place the following at the top of your script: #!/bin/bash exec &>  output.file; [the rest of your code]

to measure how much time a program takes to execute from the command-line, use the ‘time’ function. for instance, to time the ‘date’ command: >> time date Wed Oct 31 14:12:41 EDT 2007 real    0m0.003s user    0m0.000s sys     0m0.002s

to count the number of times you match a substring in a larger text file, you can use the following shell one-liner: >> sed “s/[sub_str]/[sub_str]\n/g” [text_file] | wc -l

to spread a one-line command over multiple lines in a shell script, end each line with a slash and indent the next line.  for instance: cat $1 | sed ‘s/>FALF/F_FAL/g’  \ | sed ‘s/>FAL1/1_FAL/g’  \ | sed ‘s/>FAL5/5_FAL/g’  \ | sed ‘s/>FALZ/Z_FAL/g’  \ | sed ‘s/>OUTGROUP/5_OUTGROUP/’ > $2

turns out floating point division (or any other type of floating point math) isn’t built-into the bash shell.  instead, you’ll have to use the bc utility: echo “6.0/10″ | bc -l

file existence can be tested for in a conditional using the -a operator: if [ -a filename.txt ] then echo “file here!” fi

let’s say you’ve got: $j=”foo” and you’d like to tack on its good partner ‘bar’. then, use the dollar sign and curly braces to delineate variable from string: $k=”${j}bar”

say you’ve got a stream of filenames that you’d like to remove.  for instance, i’ve got a bunch of filenames that pop out of this command: grep -v “,” `search “boot0.txt”` | sed ‘s/boot.*//’ turns out i can’t directly pipe this to rm.  instead, i’ve got to use the xargs command, as so: grep -v [...]