diff --git a/src/main.rs b/src/main.rs
index 6f52d61..791a66e 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -45,6 +45,9 @@ pub enum Commands {
         csvfile: PathBuf,
     },
     /// Get LVDS TX skew from slave log _clk file.
+    ///
+    /// PSB ID is extracted from `logfile` name and the corresponding QAQC run is retrieved by
+    /// searching the n-th run with the same PSB ID in chronological order.
     AddSkew {
         /// Slave _clk log file.
         logfile: PathBuf,
diff --git a/src/skew.rs b/src/skew.rs
index a47e8e0..73bf381 100644
--- a/src/skew.rs
+++ b/src/skew.rs
@@ -6,6 +6,27 @@ use std::{
     path::PathBuf,
 };
 
+use crate::PsbId;
+
+/// Extract PsbId and runid from filename.
+///
+/// format:
+/// `PSBID_RUNID_clk.txt`
+pub fn get_psbid_from_clklogfile(path: PathBuf) -> Result<(PsbId, u32)> {
+    let filename = path
+        .file_stem()
+        .context("path is not a file")?
+        .to_str()
+        .context("Invalid utf-8")?;
+    let mut parts = filename.split('_');
+    let psbid: PsbId = {
+        let id: u32 = parts.next().context("No psbid")?.parse()?;
+        PsbId::new(id)
+    };
+    let runid = parts.next().context("No run id")?.parse()?;
+    Ok((psbid, runid))
+}
+
 fn parse_single_line(l: String) -> Result<(f64, u32, u32)> {
     let mut splitted = l.split_whitespace();
     let delay: f64 = splitted.next().context("No value found")?.parse()?;
@@ -38,7 +59,22 @@ pub fn parse_count_file(filename: PathBuf) -> Result<f64> {
 
 #[cfg(test)]
 mod test {
+    use crate::{skew::get_psbid_from_clklogfile, PsbId};
+
     use super::parse_counts;
+
+    #[test]
+    fn test_get_psbid_from_clklog() {
+        assert_eq!(
+            get_psbid_from_clklogfile("120_1234.log".into()).unwrap(),
+            (PsbId::new(1020), 1234)
+        );
+        assert_eq!(
+            get_psbid_from_clklogfile("1024_124.log".into()).unwrap(),
+            (PsbId::new(1024), 124)
+        );
+    }
+
     #[test]
     fn test_parse_counts1() {
         let file = "